Research on Enhancing the Yield and Quality of Oat Forage: Optimization of Nitrogen and Organic Fertilizer Management Strategies

Abstract

In the context of the increasingly serious issues of resource waste, soil degradation, and environmental pollution caused by excessive nitrogen fertilizer application worldwide, this study conducted a two-year field experiment in Qinghai Province to explore suitable nitrogen fertilizer management strategies for the region. Ten fertilization levels were set, incorporating varying ratios of conventional nitrogen fertilizer and organic fertilizer, as well as the proportion of base fertilizer and topdressing. The focus was on monitoring the forage yield, quality, and related physiological indicators of oats during the flowering and milk stages. The use of correlation analysis and the multi-criteria decision-making model TOPSIS was applied for comprehensive data evaluation to determine the optimal fertilization strategy. After systematic data collection and analysis, the results showed that when 75% conventional nitrogen fertilizer was combined with 4500 kg·hm⁻² of organic fertilizer (F4), the oat yield during the milking stage reached its peak at 14,722.48 kg·hm⁻². Additionally, the yield effect was optimal (13,677.34 kg·hm⁻²) when using 30% base fertilizer and 70% jointing fertilizer (D2). Regarding nutritional quality, the fertilization strategy combining 75% conventional nitrogen fertilizer with 4500 kg·hm⁻² of organic fertilizer, along with 30% base fertilizer and 70% jointing fertilizer (F4D2), significantly reduced the content of acid detergent fiber (ADF), neutral detergent fiber (NDF), and coarse fiber (CF) in oats, while increasing the content of EE (crude fat) and CP (crude protein). This significantly improved the nutritional value of oats. Correlation analysis further revealed the positive effect of fertilization amount and fertilization period on oat yield, as well as a negative correlation with fiber content. Finally, through comprehensive evaluation using the multi-criteria decision-making model TOPSIS, we verified the superiority of the fertilization strategy.

1. Introduction

The oat (Avena sativa L.), a widely cultivated grain, is renowned for its adaptability, resistance, and nutritional richness [1]. In China, oat cultivation is extensive and long-standing. It serves both as a key food crop and an essential feed crop, vital for enhancing animal production [2]. The oat nourishes livestock with essential nutrients like protein, energy, fiber, minerals, and vitamins, ensuring their healthy growth [3]. It thrives in cool, dry environments and tolerates poor soil, completing its lifecycle even at 4000 m altitudes. Its low-cost cultivation and high yield offer excellent economic returns [4], making it a crucial winter fodder source for livestock, especially in pastoral areas. The geographically diverse Qinghai–Tibetan Plateau, with its mixed agricultural and pastoral regions, is an ideal location for crop diversification. The oat’s dual functionality as a grain and feed crop supports sustainable agricultural and livestock practices. Additionally, it plays a pivotal role in ecological management, enhancing soil conservation, water retention, and ground cover, and ameliorating saline–alkali soil, thus improving the land’s replanting index [5].

Enhancing the production and quality of oats is crucial, as it can meet the growing market demand for oats, bring substantial economic returns to farmers, and promote sustainable agricultural development [6]. In addition, this will help optimize the nutritional structure of the population and promote the development of related industrial chains. However, we also need to recognize that relying solely on nitrogen fertilizers to ensure food security is not a long-term solution. Although the extensive use of nitrogen fertilizer in the last century has resulted in significant yield increases [7], it has also led to excessive depletion of soil nutrients and disruption of the balance of soil elements [8,9]. Therefore, to achieve genuine sustainable development, we not only need to increase the yield and quality of oats but also explore and implement more scientific and environmentally friendly farming strategies and management methods. In this process, considering the integration of organic and conventional farming will provide us with a more comprehensive perspective and facilitate innovative management of nitrogen fertilizer use, ultimately leading to the sustainability of agricultural production [10]. The widespread dissemination of the concept of green agriculture and the transformation and upgrading of agricultural production methods have posed a new challenge to agriculture in Qinghai Province and the whole country: how to use fertilizers scientifically and rationally to reduce potential threats to the ecological environment while ensuring stable agricultural development. Qinghai Province, owing to its unique geographical and climatic conditions, has relatively fragile agroecosystems and more stringent demands and management requirements for fertilizer use [11]. Therefore, how to effectively reduce the burden of fertilizers on the environment while meeting the needs of agricultural production in Qinghai Province and the country as a whole has become a central issue in promoting sustainable and healthy agricultural development. Nitrogen fertilizer management plays a crucial role in this, and it is a powerful response to the challenges of agricultural production. This requires careful planning of nitrogen fertilizer distribution to ensure that crops are supplied with the right amount of nitrogen fertilizer at the right time when they need it. At the same time, adopting scientific fertilizer application methods, such as combining basal and follow-up fertilizers, not only enhances the efficiency of N fertilizer utilization but also reduces N fertilizer losses. Moreover, the synergistic use of other fertilizers is also an important approach to improving nitrogen fertilizer utilization efficiency. For instance, the combination of nitrogen fertilizer and biochar can effectively reduce nitrate leaching, mitigate water pollution, enhance nitrogen utilization efficiency, and support crop growth. The combined application of green manure and nitrogen fertilizer has achieved remarkable yield-increasing effects in wheat, comparable to or even surpassing the use of urea, thus demonstrating the promising prospects of organic agriculture [12,13,14]. But relying solely on organic fertilizers may also lead to excessive accumulation of phosphorus and other elements, resulting in environmental pollution [15,16]. Therefore, we need to explore the reasonable ratio and application of chemical and organic fertilizers to achieve green development and ecological balance in agriculture. Organic fertilizers, chemical fertilizers, and their combined application were effective in increasing wheat, corn, and rice (Triticum aestivum L., Zea mays L., Oryza sativa L.) yields in 32 long-term trials in China. In particular, combining organic and inorganic fertilizers is the most productive fertilization strategy [17]. The rational combination of organic and chemical fertilizers provides a comprehensive and balanced supply of nutrients to crops. Organic fertilizers are rich in organic matter and trace elements, which can improve soil structure, enhance soil fertility, and provide long-lasting nutrient support for crop growth [18]. Chemical fertilizers, however, can quickly replenish large amounts of elements in the soil to meet the nutrient needs of crops at different stages of growth [19]. Such a combination not only overcomes the limitations of single-application fertilizers, but also achieves nutrient complementarity and synergism, and creates an ideal soil environment for crop growth. It is essential for achieving zero growth in fertilizer use and maintaining long-term soil fertility [20]. To maximize the utility of nitrogen and organic fertilizers, scholars have conducted in-depth studies on the types of fertilizers, the rate of fertilizers applied, and the period of fertilizer [21,22,23], which not only enriched the theoretical knowledge of agricultural production but also provided farmers with scientific guidance on the actual operation, which has significantly contributed to the improvement of crop yield and quality [24]. Although the pre-sowing application of N fertilizer helps crops accumulate more dry matter before the jointing stage, basal fertilizer alone may not be able to meet the N demand of crops throughout the growth cycle [25]. Therefore, experts recommend that supplementary fertilizers should be applied at the jointing and flowering stages of the crop [26], especially at the jointing stage of oats, which can significantly improve the yield and quality [27]. In addition, fertilization strategies also have a positive effect on the photosynthetic characteristics of crops, and photosynthesis is the cornerstone of crop growth and yield formation. Fertilization, as an important part of agricultural production, has the core purpose of optimizing the crop growth environment and promoting photosynthesis [28]. As the main organ of photosynthesis, the structure and function of leaves have a direct impact on photosynthetic efficiency. Reasonable fertilization strategies can promote leaf growth and development, increase leaf area and chlorophyll content, and enhance the photosynthetic capacity of leaves. At the same time, fertilization can also regulate the opening of leaf stomata, optimize the gas exchange process, and further improve photosynthetic efficiency [29]. This helps the crop to make better use of light energy for photosynthesis, increases the synthesis and accumulation of photosynthetic products, and lays a solid foundation for high-yield and high-quality crops. Therefore, scientifically arranging the period and mode of fertilizer application is of profound significance for enhancing the yield and quality of oats, as well as achieving sustainable agricultural development [30]. With the continuous progress of society and the steady improvement in people’s living standards, the pursuit of a better quality of life and health awareness has also been growing. Against this backdrop, the scientific use of nitrogen fertilizer management is crucial not only for enhancing crop yields but also for improving the quality of agricultural products, ensuring food safety, and promoting sustainable agricultural development [31,32]. Especially with the increasing recognition and demand for organic agriculture, this study hypothesizes that the combined use of nitrogen fertilizer management and organic fertilizer applications will significantly improve the yield and quality of oat pasture. By investigating the specific effects of this combined approach on Qinghai oat (Avena sativa L. Qinghai), we aim to develop more effective and sustainable fertilization methods, reduce nitrogen fertilizer usage, and thereby contribute to the development of environmentally friendly agricultural practices and the sustainable development of agriculture.

2. Materials and Methods

2.1. Experimental Site

The experimental site was located in Shang Xinzhuang Town (101°59′ E, 36°42′ N), Huang Zhong District, Qinghai Province, which is the main oat cultivation area in the province, with an altitude of 2630 m. The prevailing climate exhibits cold and humid characteristics and no absolute frost-free periods. It has an average annual temperature of 4.6 °C and receives an annual precipitation ranging between 500 and 650 mm. Buckwheat has been cultivated as the preceding crop in this area. The experiment was conducted on the same plot in both 2020 and 2021, to ensure the consistency and comparability of the results.

The average nutrient content of the experimental plot over two years is shown in

Table 1. The basic nutrient content of the experimental plot.

Soil Type	Total Nitrogen g·kg−1	Total Phosphorus g·kg−1	Total Potassium g·kg−1	Available Nitrogen mg·kg−1	Available Phosphorus mg·kg−1	Available Potassium mg·kg−1	Organic Matter g·kg−1	pH
loam	1.87	2.46	23.40	106.00	27.57	290.00	34.39	8.18

.

2.2. Experimental Material

The experimental material for both years was ‘Avena sativa L. Qinghai’, provided by the College of Animal Husbandry and Veterinary Science, Qinghai University. The fertilizers used were urea (46% N) and organic fertilizer (mainly sheep manure, with a minimum effective colony count of 0.2 billion·g⁻¹, organic matter ≥ 40.0%, N ≥ 0.6%, p ≥ 0.3%, K ≥ 0.2%). This experiment employed a randomized block design with three replicates and a plot size of 15 m² (3 m × 5 m). Two fertilization strategies (D1 and D2) were set: basal fertilizer (100% N) and basal fertilizer (30% N) + jointing fertilizer (70% N). For each fertilization period, we used five different fertilization levels: conventional N application rate (F1, conventional N application consultation in the study by Guoling Liang [33] et al.), 75% conventional N application rate (F2), 50% conventional N application rate (F3), 75% conventional N application rate with 4500 kg·hm⁻² organic fertilizer (F4), and 50% conventional N application rate with 4500 kg·hm⁻² organic fertilizer (F5). The types and application rates of organic fertilizer were referred to in a study by Ma Xiang [34] et al.

Table 2. Application rates for different treatments.

Treatments	Basal Fertilizer (kg·hm−2)		Jointing Fertilizer N (kg·hm−2)
	Organic Fertilizer	N
F1D1	0	69	0
F2D1	0	51.75	0
F3D1	0	34.5	0
F4D1	4500	51.75	0
F5D1	4500	34.5	0
F1D2	0	20.7	48.3
F2D2	0	15.525	36.225
F3D2	0	10.35	24.15
F4D2	4500	15.525	36.225
F5D2	4500	10.35	24.15

lists the fertilizer application rates for different treatments. The trials were broadcast on 24 April 2020, and 1 May 2021, with a row spacing of 25 cm and a seeding rate of 240 kg·hm⁻². Manual weeding was performed twice during the growth period and no irrigation was applied.

2.3. Determination Indices and Methods

During the flowering and milk stages, we removed the side rows and selected a 1 m² area from each plot for oat sampling which was conducted by drying naturally and recording the weight.

During the flowering and milk stages: choose a sunny morning between 9:00 and 11:00 and carefully select three iconic flag leaves from each plot. Using the LI-6800 photosynthesis meter, accurately measure the net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular carbon dioxide concentration (Ci), and transpiration rate (Tr) of the oat leaves. In addition, take five additional flag leaves from each plot back to the laboratory and determine the chlorophyll content in these leaves using spectrophotometry [35].

During the flowering and milk stage: in each plot, 15 well-grown and pest-free plants were selected. They were put into air-dry bags and brought back to the laboratory. First, they were dried in an oven at 110 °C for one hour and then dried at 80 °C until the mass became constant. After cooling, they were ground into fine particles using a grinder and sieved through a 2 mm sieve. The acid detergent fiber (ADF), neutral detergent fiber (NDF), and coarse fiber (CF) contents were determined by the Van Soest method of washing fiber analysis. The crude fat (EE) and crude protein (CP) contents were determined by the Soxhlet extraction method [36].

2.4. Statistical Analysis

Microsoft Excel 2019 was used to collate the data from both years. Differences between different fertilizer application periods and years were analyzed using t-tests in SPSS 21.0. One-way analysis of variance (ANOVA) with multiple comparisons (Duncan) in one-way ANOVA was used to analyze the photosynthetic characteristics, forage yield, and quality of the treatments, and the data were expressed as mean ± standard error. To further reveal the intrinsic associations between fertilizer application rate, fertilizer application period, photosynthetic characteristics, and oat yield and nutrient quality indicators, a meta-analysis of correlations was conducted using R 4.3.2. In addition, the Plyr data package of R 4.0.2 was used in combination with a multi-criteria decision-making model, TOPSIS (Technique for Order Preference by Similarity to an Ideal Solution), to comprehensively evaluate the photosynthetic characteristics, yield, and quality characteristics of each treatment to determine the best fertilization strategy. Origin 2018 was used to map forage yield.

3. Results

3.1. Effect of Different Fertilizer Treatments on Photosynthetic Characteristics of Oats

The fertilizer-rate and fertilizer-period interaction had a significant effect (p < 0.001) on oat Gs and Chl (

Table 3. Effects of different fertilizer treatments on photosynthetic characteristics of oats at the flowering stage.

Years	Treatments	Chl/mg·g−1	Pn/μmol·m−2·s−1	Tr/μmol·mol−1	Ci/mmol·m−2·s−1	Gs/mol·m−2·s−1
2020	F1D1	4.88 ± 0.13d	15.82 ± 0.35bc	9.93 ± 0.17bc	285.20 ± 7.91f	334.32 ± 10.62d
	F2D1	3.31 ± 0.42f	12.68 ± 0.57efg	7.11 ± 0.58ef	334.62 ± 9.71bc	282.76 ± 5.69fg
	F3D1	2.83 ± 0.06g	11.01 ± 0.65g	6.49 ± 0.13f	378.53 ± 4.00a	268.89 ± 11.22g
	F4D1	4.03 ± 0.05e	15.31 ± 0.22cd	9.06 ± 0.69cd	301.47 ± 8.79e	303.22 ± 6.28e
	F5D1	2.89 ± 0.02g	12.01 ± 0.65fg	7.05 ± 1.08ef	348.53 ± 12.94b	275.38 ± 6.99fg
	F1D2	7.42 ± 0.04a	21.68 ± 2.94a	12.08 ± 0.85a	239.27 ± 5.23h	481.41 ± 2.78a
	F2D2	5.27 ± 0.14c	15.90 ± 0.22bc	10.36 ± 1.05b	277.38 ± 3.76f	358.89 ± 9.66c
	F3D2	3.78 ± 0.24e	13.49 ± 0.43def	7.90 ± 0.52de	328.53 ± 16.07cd	285.47 ± 6.87f
	F4D2	6.18 ± 0.09b	17.35 ± 0.38b	11.08 ± 0.59ab	259.02 ± 4.13g	383.17 ± 8.31b
	F5D2	4.08 ± 0.11e	14.01 ± 0.65cde	8.06 ± 0.69de	314.80 ± 5.94de	302.47 ± 7.36e
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
F × D		p < 0.001	p = 0.023	p = 0.102	p = 0.244	p < 0.001
2021	F1D1	5.73 ± 0.06e	20.08 ± 0.11c	16.06 ± 0.36e	245.07 ± 0.84e	409.43 ± 8.07d
	F2D1	4.39 ± 0.01i	15.23 ± 0.44f	10.74 ± 0.38h	271.41 ± 1.54b	333.91 ± 2.40h
	F3D1	4.05 ± 0.05j	14.14 ± 0.61g	9.37 ± 0.25i	307.28 ± 1.86a	319.78 ± 3.10i
	F4D1	6.97 ± 0.11c	22.90 ± 0.81b	18.41 ± 0.30c	229.02 ± 4.13g	456.50 ± 8.40b
	F5D1	5.42 ± 0.06f	19.19 ± 0.96cd	15.63 ± 0.68e	244.47 ± 3.00e	393.19 ± 0.73e
	F1D2	6.15 ± 0.03d	22.88 ± 0.08b	17.46 ± 0.27d	239.27 ± 5.23f	434.80 ± 4.48c
	F2D2	5.06 ± 0.11g	18.31 ± 0.59d	14.14 ± 0.61f	252.73 ± 2.83d	380.22 ± 6.11f
	F3D2	4.89 ± 0.01h	16.44 ± 0.48e	12.16 ± 0.22g	263.60 ± 1.45c	356.53 ± 5.68g
	F4D2	7.85 ± 0.12a	24.35 ± 0.38a	21.08 ± 0.25a	210.60 ± 2.34i	481.41 ± 2.78a
	F5D2	7.33 ± 0.20b	23.78 ± 0.70ab	19.63 ± 0.52b	218.05 ± 2.67h	462.47 ± 6.83b
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
F × D		p < 0.001	p = 0.003	p < 0.001	p < 0.001	p < 0.001

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

), but not on Tr and Ci (p > 0.05). In 2020, the F1D2 treatment exhibited the best performance for Chl, Fn, Tr, and Gs. Compared with the F1D1 treatment, the F1D2 treatment significantly (p < 0.05) increased Chl, Fn, Tr, and Gs by 34.23%, 27.03%, 17.80%, and 30.55%, respectively. Compared with the F1D1 treatment, the F4D2 treatment resulted in a significant increase (p < 0.05) in Chl, Fn, Tr, and Gs values by 21.04%, 8.82%, 10.38%, and 12.75%, respectively. However, in 2021, there was a change in the patterns of photosynthetic characteristics. The Fn, Tr, and Gs values were the highest under the F4D2 treatment, followed by the F5D2 treatment. Compared to the F1D1 treatment, the F4D2 treatment resulted in a significant increase (p < 0.05) of 27.01% in Chl, 17.54% in Fn, 23.81% in Tr, and 14.95% in Gs. Similarly, the F5D2 treatment resulted in a significant increase (p < 0.05) of 21.83% in Chl, 15.56% in Fn, 18.19% in Tr, and 11.47% in Gs.

At the flowering stage, as the amount of N fertilizer increased, the Chl, Fn, Tr, and Gs of oats showed an upward trend (

Table 4. Effects of different fertilizer application rates and application periods on photosynthetic characteristics of oats at the flowing stage.

Treatments	Chl/mg·g−1	Pn/μmol·m−2·s−1	Tr/mmol·m−2·s−1	Ci/μmol·mol−1	Gs/mol·m−2·s−1
F1	6.04 ± 0.05b	20.12 ± 0.84a	13.88 ± 0.16b	252.20 ± 0.80c	414.99 ± 4.22a
F2	4.51 ± 0.07d	15.33 ± 0.25c	10.59 ± 0.22d	284.04 ± 2.64b	338.94 ± 1.65d
F3	3.89 ± 0.04e	13.77 ± 0.40d	8.98 ± 0.14e	319.49 ± 2.23a	307.66 ± 3.10e
F4	6.26 ± 0.06a	19.98 ± 0.35a	14.91 ± 0.06a	250.03 ± 3.42c	406.08 ± 3.32b
F5	4.93 ± 0.06c	17.25 ± 0.34b	12.59 ± 0.32c	281.46 ± 4.16b	358.38 ± 1.94c
D1	4.44 ± 0.02b	15.84 ± 0.11b	10.98 ± 0.16b	294.56 ± 2.62a	337.74 ± 5.37b
D2	5.80 ± 0.04a	18.82 ± 0.44a	13.40 ± 0.13a	260.33 ± 0.81b	392.68 ± 1.23a

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

). Among all fertilizer rate treatments, the Chl and Tr values of the F4 treatment were the highest, increasing by 3.51% and 6.91%, respectively, compared to those of the F1 treatment. However, the differences between Pn, Ci, and F1 were insignificant (p > 0.05). During the different fertilization periods, the performances of Chl, Pn, Tr, Gs, and Ci varied. Chl, Pn, Tr, and Gs were higher in D2 than in D1, whereas Ci was higher in D1 than in D2.

The changes in Chl, Pn, Tr, Ci, and Gs during milk were similar to those observed during flowering (

Table 5. Effects of different fertilizer treatments on photosynthetic characteristics of oats at the milk stage.

Years	Treatments	Chl/mg·g−1	Pn/μmol·m−2·s−1	Tr/mmol·m−2·s−1	Ci/μmol·mol−1	Gs/mol·m−2·s−1
2020	F1D1	3.87 ± 0.13d	13.41 ± 0.40bc	10.16 ± 0.91ab	314.02 ± 4.90bc	282.88 ± 7.85b
	F2D1	2.53 ± 0.42g	11.58 ± 0.27ef	6.41 ± 0.48c	331.99 ± 2.34a	275.38 ± 6.99c
	F3D1	2.05 ± 0.06h	10.89 ± 0.44f	5.11 ± 0.88d	337.47 ± 7.32a	268.89 ± 11.22d
	F4D1	3.25 ± 0.05e	13.16 ± 0.62bc	9.37 ± 0.61b	317.11b ± 6.52b	358.89 ± 9.66b
	F5D1	2.11 ± 0.02h	11.29 ± 0.39ef	5.14 ± 0.01d	332.74 ± 2.58a	303.22 ± 6.28c
	F1D2	6.32 ± 0.04a	15.63 ± 1.18a	11.01 ± 0.74a	307.09 ± 1.94c	306.04 ± 1.80a
	F2D2	4.17 ± 0.14c	13.71 ± 0.36bc	10.47 ± 0.39a	313.29 ± 4.09bc	298.70 ± 4.31a
	F3D2	2.68 ± 0.24fg	12.02 ± 0.28de	6.62 ± 0.40c	332.14 ± 2.83a	275.68 ± 14.27b
	F4D2	5.08 ± 0.09b	14.08 ± 0.41b	10.78 ± 0.11a	307.68 ± 6.30c	303.83 ± 2.29a
	F5D2	2.97 ± 0.11ef	12.71 ± 0.94cd	7.17 ± 0.57c	321.23 ± 4.54b	276.47 ± 1.75b
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
F × D		p < 0.001	p = 0.264	p = 0.001	p = 0.160	p < 0.001
2021	F1D1	4.70 ± 0.02d	15.10 ± 0.36d	10.37 ± 0.25e	275.68 ± 2.42e	364.71 ± 11.45d
	F2D1	3.76 ± 0.02f	10.20 ± 0.82g	6.92 ± 0.45g	316.63 ± 5.05c	296.34 ± 3.20g
	F3D1	3.33 ± 0.11g	8.53 ± 0.09h	5.18 ± 0.27h	327.88 ± 3.63b	268.47 ± 7.14h
	F4D1	5.55 ± 0.10c	17.77 ± 0.32c	12.08 ± 0.76c	257.21 ± 1.99f	389.13 ± 7.49c
	F5D1	4.35 ± 0.28e	14.35 ± 0.51d	9.89 ± 0.12e	288.88 ± 1.92d	344.92 ± 3.09e
	F1D2	5.37 ± 0.26c	17.26 ± 0.46c	11.19 ± 0.18d	266.19 ± 7.47f	372.36 ± 9.05d
	F2D2	4.32 ± 0.03e	13.20 ± 0.06e	8.98 ± 0.12f	297.37 ± 3.89d	338.56 ± 1.30e
	F3D2	4.17 ± 0.02e	11.70 ± 0.63f	7.66 ± 0.03g	314.47 ± 4.29c	320.05 ± 5.57f
	F4D2	7.00 ± 0.12a	21.14 ± 0.90a	14.74 ± 0.96a	215.68 ± 12.64h	472.47 ± 10.29a
	F5D2	6.18 ± 0.11b	19.11 ± 0.60b	13.26 ± 0.49b	239.27 ± 5.23g	455.08 ± 3.39b
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
F × D		p < 0.001	p = 0.009	p = 0.002	p < 0.001	p < 0.001

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

). N fertilization and fertilization period had significant effects (p < 0.001) on Pn, Tr, Ci, and Gs. Interactions had significant effects on Tr, Chl, and Gs (p ≤ 0.001). In the 2020 experiment, the F1D2 treatment significantly increased (p < 0.05) Chl, Fn, Tr, and Gs. Compared with the F1D1 treatment, the F1D2 treatment significantly increased (p < 0.05) Chl, Fn, Tr, and Gs by 38.77%, 14.20%, 7.72%, and 7.57%, respectively. In addition, the F4D2 treatment also had a positive effect on Chl, Fn, Tr, and Gs. Compared with the F1D1 treatment, Chl, Fn, Tr, and Gs significantly increased (p < 0.05) by 23.82%, 4.76%, 5.75%, and 6.90%, respectively. In 2021, the effect of F4D2 treatment on Chl, Fn, Tr, and Gs was the most pronounced. Compared to the F1D1 treatment, the F4D2 treatment significantly increased (p < 0.05) Chl, Fn, Tr, and Gs by 32.86%, 28.57%, 29.65%, and 22.81%, respectively. Following F5D2 treatment, Chl, Fn, Tr, and Gs significantly increased (p < 0.05) by 23.95%, 20.98%, 21.79%, and 19.86%, respectively.

At the milk stage, among all treatments, the F4 treatment had the highest values of Chl, Pn, Tr, and Gs, which significantly increased (p < 0.05) by 2.87%, 7.19%, 9.03%, and 8.43%, respectively, compared to the F1 treatment (

Table 6. Effects of different fertilizer application rates and application periods on photosynthetic characteristics of oats at the milk stage.

Treatments	Chl/mg·g−1	Pn/μmol·m−2·s−1	Tr/mmol·m−2·s−1	Ci/μmol·mol−1	Gs/mol·m−2·s−1
F1	5.07 ± 0.11b	15.35 ± 0.23b	10.68 ± 0.37b	290.74 ± 2.76c	331.50 ± 6.66b
F2	3.69 ± 0.07d	12.18 ± 0.18d	8.20 ± 0.28d	314.82 ± 0.82b	290.67 ± 0.33c
F3	3.06 ± 0.03e	10.79 ± 0.15e	6.14 ± 0.16e	327.99 ± 1.78a	267.58 ± 4.82d
F4	5.22 ± 0.05a	16.54 ± 0.35a	11.74 ± 0.33a	274.42 ± 5.50d	362.04 ± 5.79a
F5	3.90 ± 0.09c	14.37 ± 0.21c	8.86 ± 0.01c	295.53 ± 3.42c	325.75 ± 2.97b
D1	3.55 ± 0.03b	12.63 ± 0.15b	8.06 ± 0.24b	309.96 ± 0.77a	289.09 ± 2.95b
D2	4.83 ± 0.05a	15.06 ± 0.13a	10.18 ± 0.03a	291.44 ± 2.78b	341.92 ± 2.11a

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

). Chl, Pn, Tr, Gs, and Ci varied across different fertilization periods. The Chl, Pn, Tr, and Gs values in D2 were higher than those in D1, whereas the Ci value in D1 was higher than that in D2.

3.2. Effect on Oat Forage Yield of Different Fertilizer Treatments

During the flowering period, forage yield was significantly influenced (p < 0.001) by the fertilizer rate, period, and year (

Table 7. Effect of fertilizing rate and period on oat forage yield.

Period of Fertility	Items	df	Forage Yield
			F-Value	p-Value
Flowering stage	Fertilizer rate (F)	4	1053.956	p < 0.001
	Fertilizer period (D)	1	1916.710	p < 0.001
	Years (Y)	1	249.825	p < 0.001
	F × D	4	47.274	p < 0.001
	F × Y	4	444.437	p < 0.001
	D × Y	1	1.460	p = 0.234
Milk stage	Fertilizer rate (F)	4	1882.990	p < 0.001
	Fertilizer period (D)	1	2426.165	p < 0.001
	Years (Y)	1	46.888	p < 0.001
	F × D	4	41.886	p < 0.001
	F × Y	4	623.328	p < 0.001
	D × Y	1	232.622	p < 0.001

). Additionally, the interaction between fertilizer rate and fertilizer period, as well as between fertilizer application and year, had a significant effect (p < 0.001) on forage yield. Similarly, at the milk stage, the interaction between the three parameters was equally significant (p < 0.001).

Changes in the amount of fertilizer applied and the choice of fertilizer application period had a significant effect on oat pasture yield, which is visually demonstrated in Figure 1a–d. Each year, there were different patterns of change in oat forage yield, and it is particularly noteworthy that the patterns of change in yield at the flowering and milk stages were consistent. Specifically, in 2020, the F1D2 treatment showed the best yield, with 12,200 kg·hm⁻² at flowering and 15,966.67 kg·hm⁻² at the milk stage, followed by the F4D2 treatment at 12,089.56 kg·hm⁻² at the flowering and 15,022.56 kg·hm⁻² at the milk stage; however, in 2021, the yield ranking changed. In 2021, there was a significant change in the yield ranking, with the F4D2 treatment jumping to the top, followed by the F5D2 treatment.

Oat forage yield was enhanced with the increase in fertilizer application (

Table 8. Average yield of oat forage under different fertilization amounts or periods in different years.

Items	Treatments	Forage Yield/kg·hm−2
		Flowering Stage	Milk Stage
Fertilizer rate	F1	10,921.78 ± 77.68b	14,428.00 ± 145.44b
	F2	9449.08 ± 36.61d	11,593.47 ± 66.97d
	F3	8356.61 ± 40.07e	9919.31 ± 118.19e
	F4	12,426.42 ± 90.02a	14,722.48 ± 70.60a
	F5	10,347.83 ± 21.81c	12,642.54 ± 22.06c
Fertilizer period	D1	9375.27 ± 31.18b	11,644.98 ± 20.92b
	D2	11,225.42 ± 18.21a	13,677.34 ± 81.51a
Years	2020	9966.37 ± 31.18b	10,934.98 ± 50.59b
	2021	10,634.32 ± 51.78a	13,257.34 ± 75.00a

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

), and the F4 treatment had the highest yield at the flowering stage, which was significantly increased (p < 0.05), by 13.78% compared with the F1 treatment; the fertilizer application period also had a significant effect (p < 0.05) on the yield, and the yield was the highest at the D2 fertilizer application period, which was significantly increased (p < 0.05), by 16.48% compared with D1. The changing yield pattern at the milk stage was similar to that at the flowering stage. Under F4 nitrogen application, the oat forage yield significantly increased (p < 0.05), by 2.00% over F1. When the fertilization period was D2, the yield increased significantly (p < 0.05), by 14.86% compared with D1. In addition, the oat forage yield peaked at the milk stage in 2021, with a significant increase (p < 0.05) of 17.52% compared to 2020.

3.3. Effect of Different Fertilizer Treatments on Oat Forage Quality

The flowering stage, N application rate, and fertilization period exhibited significant effects (p < 0.001) on the NDF, ADF, CF, and CP contents of oats (

Table 9. Effect of various fertilization treatments on oat forage quality at the flowering stage.

Years	Treatments	CP%	ADF/%	NDF/%	CF/%	EE/%
2020	F1D1	10.59 ± 0.23d	22.17 ± 0.35g	37.36 ± 0.49g	22.70 ± 0.42g	1.30 ± 0.17abc
	F2D1	8.75 ± 0.22g	29.27 ± 0.21c	47.84 ± 0.41c	32.68 ± 0.76c	1.27 ± 0.25abc
	F3D1	7.68 ± 0.07i	34.41 ± 0.24a	53.06 ± 0.70a	36.78 ± 0.64a	0.95 ± 0.35bc
	F4D1	9.87 ± 0.08e	24.04 ± 0.55f	39.63 ± 0.29f	24.98 ± 0.15f	1.03 ± 0.23bc
	F5D1	8.16 ± 0.14h	31.87 ± 0.18b	49.44 ± 0.90b	35.76 ± 0.37b	1.02 ± 0.25bc
	F1D2	13.28 ± 0.16a	16.18 ± 0.69j	30.70 ± 0.40j	16.01 ± 0.66j	1.77 ± 0.54a
	F2D2	11.82 ± 0.03c	20.03 ± 0.44h	35.85 ± 0.34h	20.65 ± 0.85h	1.53 ± 0.41ab
	F3D2	9.25 ± 0.09f	28.46 ± 0.18d	44.12 ± 0.60d	30.45 ± 0.31d	0.90 ± 0.43c
	F4D2	12.83 ± 0.14b	18.65 ± 0.44i	33.21 ± 0.80i	18.89 ± 0.66i	1.54 ± 0.27ab
	F5D2	9.32 ± 0.08f	26.61 ± 0.46e	41.96 ± 0.76e	27.31 ± 0.80e	0.99 ± 0.08bc
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p = 0.067
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p = 0.020
F × D		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p = 0.424
2021	F1D1	13.65 ± 0.05d	19.25 ± 0.22f	38.87 ± 0.09f	21.86 ± 0.05f	1.74 ± 0.34d
	F2D1	9.67 ± 0.06g	28.97 ± 0.09b	50.62 ± 0.14b	30.71 ± 0.13b	1.32 ± 0.17d
	F3D1	8.31 ± 0.09h	31.42 ± 0.10a	53.84 ± 0.23a	33.44 ± 0.13a	1.32 ± 0.18d
	F4D1	14.28 ± 0.60bc	16.85 ± 0.34h	32.37 ± 0.27h	16.21 ± 0.05h	2.56 ± 0.10bc
	F5D1	11.50 ± 0.61e	22.19 ± 0.49e	40.56 ± 0.25e	23.10 ± 0.13e	1.48 ± 0.20d
	F1D2	14.06 ± 0.09cd	17.42 ± 0.37g	35.12 ± 0.19g	19.44 ± 0.27g	2.21 ± 0.12c
	F2D2	10.51 ± 0.15f	23.36 ± 0.19d	42.88 ± 0.08d	25.19 ± 0.21d	1.43 ± 0.22d
	F3D2	9.98 ± 0.19g	25.84 ± 0.34c	47.53 ± 0.42c	27.83 ± 0.13c	1.33 ± 0.25d
	F4D2	14.92 ± 0.12a	15.51 ± 0.24i	28.57 ± 0.26j	14.69 ± 0.43j	3.77 ± 0.45a
	F5D2	14.59 ± 0.03ab	16.68 ± 0.02h	30.14 ± 0.17i	15.61 ± 0.06i	2.99 ± 0.06b
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
F × D		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

). Moreover, there was a significant interaction (p < 0.001) between the N application rate and fertilization period concerning the NDF, ADF, CF, and CP contents of oats. However, no significant effects (p > 0.05) were observed on the crude fat content of oats concerning the N application rate. Similarly, the interaction between the N application rate and fertilization period did not yield any significant effect (p > 0.05) on the crude fat content of oats.

Among the various fertilizer application rates and application period combinations in the past two years, the F1D2 treatment had the lowest NDF, ADF, and CF content in 2020. The CP and EE contents were the highest, with a significant increase (p < 0.05) in CP content of 20.26% compared with the F1D1 treatment. Notably, there was no significant difference (p > 0.05) in crude fat content between the two treatments. However, the quality of oats had shown different patterns of change by 2021. In 2021, the F4D2 treatment had the lowest NDF, ADF, and CF content, followed by the F5D2 treatment. Compared to the F1D1 treatment, the NDF, ADF, and CF contents of the F4D2 and F5D2 treatments significantly decreased (p < 0.05), by 26.50%, 24.11% and 22.46%, and 13.35%, 32.80% and 28.59%, respectively. In addition, the EE content in both treatments significantly increased (p < 0.05), by 53.85% and 41.81%, respectively, and the CP content significantly increased (p < 0.05), by 8.51% and 6.44%, respectively.

Among the different fertilizer treatments, the F4 treatment had the lowest oat NDF and CF contents, which were reduced by 5.83% and 6.60%, respectively, compared to treatment F1 (

Table 10. Effect of different fertilization rates and periods on oat quality at the flowering stage.

Treatments	CP/%	ADF/%	NDF/%	CF/%	EE/%
F1	12.89 ± 0.04a	18.76 ± 0.33d	35.51 ± 0.10d	20.01 ± 0.26d	1.75 ± 0.02b
F2	10.19 ± 0.06b	25.41 ± 0.12b	44.29 ± 0.20b	27.31 ± 0.27b	1.39 ± 0.13c
F3	8.80 ± 0.05c	30.03 ± 0.10a	49.64 ± 0.09a	32.12 ± 0.22a	1.12 ± 0.06c
F4	12.97 ± 0.16a	18.76 ± 0.17d	33.44 ± 0.25e	18.69 ± 0.17e	2.22 ± 0.19a
F5	10.89 ± 0.11d	24.34 ± 0.22c	40.53 ± 0.07c	25.45 ± 0.33c	1.62 ± 0.14b
D1	10.25 ± 0.12b	26.04 ± 0.10a	44.36 ± 0.16a	28.82 ± 0.13a	1.40 ± 0.08b
D2	12.06 ± 0.02a	20.87 ± 0.10b	37.00 ± 0.22b	21.94 ± 0.10b	1.85 ± 0.11a

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

). Meanwhile, the F4 treatment had the highest EE and CP contents, and the EE content was significantly increased, by 21.17%. However, the oat NDF, ADF, and CF contents in treatment D2 were lower than those in treatment D1 during different fertilization periods.

During the milk stage, an increase in N application rate led to a significant decrease (p < 0.05) in the NDF, ADF, and CF content of oats (

Table 11. Effect of various fertilizer treatments on forage quality of oats at the milk stage of maturity.

Years	Treatments	CP%	ADF/%	NDF/%	CF/%	EE/%
2020	F1D1	9.69 ± 0.21d	37.46 ± 0.22e	52.81 ± 0.56g	27.68 ± 0.29g	1.76 ± 0.33abc
	F2D1	8.42 ± 0.09h	41.75 ± 1.05c	62.36 ± 0.99c	38.70 ± 0.42c	1.10 ± 0.42d
	F3D1	7.25 ± 0.05j	45.28 ± 0.52a	68.17 ± 0.80a	42.01 ± 0.49a	1.80 ± 0.17abc
	F4D1	9.49 ± 0.04e	34.97 ± 0.12f	54.75 ± 0.26f	30.03 ± 0.37f	1.62 ± 0.21bcd
	F5D1	8.06 ± 0.09i	43.17 ± 0.23b	66.12 ± 0.42b	40.78 ± 0.81b	1.50 ± 0.17bcd
	F1D2	12.67 ± 0.09a	25.89 ± 0.05i	45.96 ± 0.85j	21.23 ± 0.51j	2.30 ± 0.53a
	F2D2	11.40 ± 0.06c	30.46 ± 0.20g	51.26 ± 0.54h	25.79 ± 0.25h	1.77 ± 0.46abc
	F3D2	8.80 ± 0.11g	40.59 ± 0.32d	58.87 ± 0.92d	35.55 ± 0.36d	1.40 ± 0.43bcd
	F4D2	12.37 ± 0.13b	27.51 ± 0.24h	48.06 ± 0.43i	23.75 ± 0.62i	2.02 ± 0.27ab
	F5D2	9.04 ± 0.11f	37.67 ± 0.41e	57.18 ± 0.20e	32.20 ± 0.80e	1.26 ± 0.43cd
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p = 1.161
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p = 0.029
F × D		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p = 0.064
2021	F1D1	11.87 ± 0.21e	23.83 ± 0.19f	41.19 ± 0.39f	25.82 ± 0.10e	2.33 ± 0.38d
	F2D1	7.81 ± 0.18i	32.33 ± 0.20b	53.82 ± 0.14b	32.31 ± 0.20b	1.53 ± 0.08ef
	F3D1	6.38 ± 0.17j	36.49 ± 0.12a	56.56 ± 0.29a	35.37 ± 0.28a	1.08 ± 0.13g
	F4D1	13.10 ± 0.35c	21.57 ± 0.45g	35.79 ± 0.20h	21.59 ± 0.34g	3.01 ± 0.10c
	F5D1	10.07 ± 0.58f	27.04 ± 0.06e	43.45 ± 0.39e	25.40 ± 0.19e	1.93 ± 0.20e
	F1D2	12.57 ± 0.10d	21.87 ± 0.96g	38.61 ± 0.49g	23.22 ± 0.39f	2.66 ± 0.12cd
	F2D2	9.38 ± 0.10g	28.63 ± 0.19d	45.35 ± 0.50d	27.83 ± 0.06d	1.88 ± 0.22e
	F3D2	8.71 ± 0.16h	30.39 ± 0.34c	50.48 ± 0.39c	29.34 ± 0.23c	1.78 ± 0.25e
	F4D2	14.35 ± 0.03a	20.34 ± 0.35h	31.45 ± 0.10j	17.43 ± 0.26i	4.22 ± 0.45a
	F5D2	13.70 ± 0.05b	20.05 ± 1.70h	33.38 ± 0.33i	19.34 ± 0.29h	3.44 ± 0.06b
Fertilizer rate (F)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
Fertilizer period (D)		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
F × D		p < 0.001	p < 0.001	p < 0.001	p < 0.001	p = 0.001

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

). However, the CF and CP contents exhibited the opposite trends. The N application rate and fertilization period had a significant impact on the nutrient content of the oats (p < 0.05). According to the 2020 experimental results, the ADF, NDF, and CF contents of the oat samples in the F1D2 treatment group were the lowest, whereas the CP content was the highest. In the 2021 experiment, the oat samples treated with F4D2 also exhibited a similar trend. This group also had the lowest ADF, NDF, and CF contents and the highest EE and CP contents. In contrast, F5D2 was second. The ADF, NDF, and CF contents of oats were significantly influenced by the N application rate and fertilization period, with a significant interaction effect.

Further research showed that, as shown in

Table 12. Effect of various fertilizer rates and periods on the quality of oat milk stage.

Treatments	CP/%	ADF/%	NDF/%	CF/%	EE/%
F1	11.70 ± 0.09b	27.27 ± 0.24d	44.64 ± 0.29d	24.49 ± 0.20d	2.26 ± 0.16b
F2	9.25 ± 0.04d	33.29 ± 0.21b	53.20 ± 0.22b	31.16 ± 0.18b	1.57 ± 0.07c
F3	7.78 ± 0.03e	38.19 ± 0.12a	58.52 ± 0.25a	35.57 ± 0.14a	1.52 ± 0.11c
F4	12.33 ± 0.09a	26.10 ± 0.13e	42.51 ± 0.07e	23.20 ± 0.14e	2.72 ± 0.05a
F5	10.22 ± 0.14c	31.98 ± 0.46c	50.03 ± 0.16c	29.43 ± 0.37c	2.04 ± 0.11b
D1	9.21 ± 0.05b	34.39 ± 0.04a	53.50 ± 0.19a	32.67 ± 0.09a	1.77 ± 0.09b
D2	11.30 ± 0.05a	28.34 ± 0.08b	46.06 ± 0.08b	25.57 ± 0.21b	2.27 ± 0.07a

Note: Lowercase letters in the table represent significant differences (p < 0.05) between different treatments within the same column.

, in addition to the significant effect of fertilizer application on oat quality, different fertilization periods also affected oat quality. When the fertilization period was D1, the ADF, NDF, and CF contents were the highest, whereas the EE and CP contents were the lowest.

3.4. Correlation Analysis of Different Fertilizer Treatments with Various Indicators of Oats

The correlation analysis of Figure 2a,b indicates that the rate and period of fertilization were positively correlated with forage yield, Chl, CP, and EE content, but negatively correlated with Ci, ADF, NDF, and CF. Furthermore, Chl, Pn, Tr, and Gs were positively correlated with forage yield, CP, and EE content, but negatively correlated with ADF, NDF, and CF. Conversely, Ci was positively correlated with ADF, NDF, and CF. As shown in Figure 2c,d, during the milk stage, the correlations between fertilization rate and period, oat forage yield, photosynthetic characteristics, and forage quality were consistent with those observed during the flowering period.

3.5. Multi-Criteria Decision-Making TOPSIS Model Evaluation

The analysis of the TOPSIS comprehensive evaluation model showed that the F4D2 treatment had the highest comprehensive evaluation value, which was 0.69 in both periods (Figure 3a,b). Therefore, the F4D2 treatment was the best choice for oat cultivation in the Haidong area of Qinghai.

4. Discussion

4.1. Analysis of the Impact of Fertilizer Application Rate and Period on Forage Yield and Their Annual Variations

Through analysis, we found that fertilizer application rate and fertilization period had significant effects on forage yield, although there were also some differences in the experimental results between different years. Firstly, regarding the rate of fertilizer application, the results of the study showed that an appropriate increase in the rate of fertilizer application had a significant effect on increasing forage yield [37,38]. Oat flowering and milk stage are two key stages in the growth process [39]; they show the same changing pattern under the treatment of 75% conventional N application combined with organic fertilizer, and the forage yield of oats reached the highest values of 12,426.42 kg·hm⁻² and 14,722.48 kg·hm⁻², respectively. However, when the fertilizer was only the conventional N application, the forage yield decreased by 13.78% and 2.04% compared to the 75% conventional N application combined with organic fertilizer treatment. We found that by applying 75% conventional N fertilizer combined with organic fertilizer, we could achieve better yield increases. This is different from previous studies [40,41]. This could be because organic fertilizers contain large amounts of essential and trace elements required for plant growth, which can meet the nutrient requirements of plants. Simultaneously, the nutrient release rate of organic fertilizers matches the plant absorption rate, which enables plants to better absorb and utilize nutrients and promotes their healthy growth. Therefore, compared with applying N fertilizer alone, the combination of N fertilizer and organic fertilizer can more effectively increase forage production. In addition to the rate of fertilizer application, the period of fertilization also affects the forage yield of oats [42]. Choosing the right time for fertilization can significantly increase the yield of oat forage [43]. Considering the different nutrient requirements of oats at different growth stages, mastering the correct fertilization period is crucial to maximizing yield [44,45]. During the jointing and booting stages, oats have a high demand for nutrients, and fertilization at this time can meet the growth needs of oats and increase forage production [46]. This is because these two stages are critical for oat growth, requiring a large amount of nutrients to support their rapid growth. By fertilization during these two stages, sufficient nutrients can be provided to oats to promote healthy growth, thereby increasing forage production. By comparing different fertilization periods, this study revealed a significant impact of the N fertilizer application period on forage yield. Specifically, topdressing with N fertilizer increased the yield of flowering forage by 16.48% and milk stage forage yield by 14.86%. These data demonstrate the effectiveness of N fertilizer application in increasing forage yield. This is similar to previous results [47,48]. The year is also an important factor that affects forage yield [49]. In 2020, the oat yield treated with F1D2 was the highest, reaching 12,200 kg·hm⁻². However, in 2021, the oat feed yield treated with F4D2 was the highest, reaching 14,444.44 kg·hm⁻². This result is similar to previous findings [50,51]. The variation pattern of forage yield in different years is different, which may be affected by various factors such as organic fertilizer release rate, excessive or insufficient rainfall, and drought. It is noteworthy that, in addition to the significant impact of individual factors on oat forage yield mentioned above, this experiment also found an interaction effect between these factors. Specifically, using the same amount of fertilizer in different fertilization periods can make the results more complex and variable. If the year factor is considered, this interaction effect is further enhanced.

4.2. Effect of Different Nitrogen Fertilizer Management on Photosynthetic Properties

Photosynthesis is a crucial process for the accumulation of dry matter in plants, and significantly affects crop yield [52]. The influence of N and organic fertilizers on photosynthesis in wheat has shown that fertilization can significantly improve photosynthetic characteristics [53]. Additional studies have also demonstrated that reducing N fertilizer and supplementing it with organic fertilizer can significantly enhance photosynthetic characteristics in spring maize [54]. Similarly, the present study also found that reduction in N fertilizer dosage and the application of organic fertilizer significantly improved photosynthetic characteristics. Furthermore, it has been shown that reducing N by 50% and supplementing it with an equal amount of N in organic fertilizer does not significantly differ from applying N fertilizer alone in terms of net photosynthetic rate and stomatal conductance in oat leaves, which may be related to the application period of N fertilizer [55]. This study showed that in the 2020 experiment, the conventional N application treatment performed best in terms of Chl, Fn, Tr, and Gs. The application of 75% N fertilizer with organic fertilizer significantly increased the values of Fn, Tr, and Gs. In the 2021 experiment, the application of 75% N fertilizer with organic fertilizer performed best in terms of Fn, Tr, and Gs, while the application of 50% N fertilizer with organic fertilizer also significantly increased Fn, Tr, and Gs. The reasonable matching of fertilization rate and application period was the main reason for this difference, which significantly increased Chl, Fn, Tr, and Gs. Compared with the application of N fertilizer at the beginning of growth, the optimization of N fertilization application can significantly improve the efficiency of photosynthesis, thereby promoting the accumulation of dry matter and increasing yield. Zhang [29] found that when N fertilizer was applied in batches, the photosynthetic efficiency of peanuts was significantly improved. Deng [56] showed that compared with applying N fertilizer at the beginning, delaying the application of N fertilizer to the silking stage can enhance the photosynthetic rate and chlorophyll content of ear leaves at different growth stages in corn. These results further confirmed the fact that combining organic and inorganic fertilizers can promote photosynthesis more effectively. Fertilization during key growth stages can promote crop absorption and nutrient utilization, thereby optimizing photosynthesis.

4.3. A Comprehensive Study on the Improvement of the Nutritional Quality of Oats by Combining Nitrogen and Organic Fertilizers

Nitrogen is an essential element for plant growth and has a crucial effect on the nutritional quality of forage [57,58]. Numerous studies have shown that the rational application of nitrogen fertilizers can effectively improve the nutritional value of forage. In oat production, with an increase in nitrogen application rate, the EE and CP contents showed a significant growth trend. The ADF and NDF contents exhibit an opposite trend to EE and CP [59,60]. This means that by increasing the application of nitrogen fertilizer, the fat content of oats can be improved, thereby enhancing their feeding value. However, it is worth noting that in a study of nitrogen fertilizer combined with organic fertilizer, compared with the treatment of a single nitrogen fertilizer application, the treatment of nitrogen fertilizer combined with organic fertilizer had lower contents of NDF, CF, and ADF, and higher contents of CP and EE, indicating that the application of N fertilizer combined with organic fertilizer has a positive effect on improving the quality of oats [27]. To further explore the impact of N fertilizer on oat growth, we designed a two-year field experiment. In this study, we systematically studied the effects of different combinations of fertilization rates and fertilization periods on oat growth. We found that when N fertilizer was applied in stages and combined with organic fertilizer, the contents of NDF, ADF, and CF in oats were the lowest, while the EE and CP contents were the highest. This finding has important practical significance, because reducing the content of these fiber components can improve the digestibility of forage and facilitate the absorption of nutrients by animals. In addition, we observed that CF and CP contents reached their highest values under the same fertilization conditions. CP is one of the main nutrients in plants and is essential for animal growth [61]. Therefore, through reasonable application of N fertilizer, not only can the fat content of oats be improved, but their CP content can also be increased, thereby comprehensively improving the nutritional value of forage. It is worth mentioning that we found a strong interaction between the nitrogen application rate and fertilization period. This means that selecting the appropriate fertilization period is crucial for maximizing the effectiveness of N fertilizers. In actual production, an inappropriate fertilization period may lead to growth retardation or nutritional imbalance in oats.

4.4. Studies on the Combined Effects of Fertilizer Application on Forage Yield and Quality and Their Relationship with Photosynthetic Characteristics

Fertilizer application has a significant effect on forage yield [62]. Appropriate fertilization can improve the photosynthetic characteristics of oats, such as by increasing the photosynthetic pigment content and photosynthetic rate of leaves, thereby promoting forage yield [63]. In addition, appropriate fertilization can improve the quality of forage by increasing protein, fat content, and other nutrients [64]. The fertilization period is crucial for crop growth [65]. The fertilization period ensures that the fertilizers are fully absorbed, improving the photosynthetic capacity, growth rate, and yield. Basic fertilizer provides a good foundation for crop growth, which is beneficial for later ground growth and yield formation. Topdressing supplements soil nutrients at different growth stages, promotes crop growth and development, and enhances photosynthetic capacity and yield [66]. Through correlation analysis, we found that there are complex and variable relationships between fertilizer application rate, fertilizer application period, photosynthetic characteristics, forage yield, and forage quality. These relationships are not single or linear, but vary according to different factors. First, the impact of fertilizer application rate on forage yield is obvious. The data showed that the increase in fertilizer application rate and the reasonable selection of a fertilizer application period were positively correlated with the increase in forage yield. This means that fertilization at appropriate times can effectively promote forage growth, thereby increasing forage yield. This is consistent with previous research [67]. However, a negative correlation exists between the fertilizer application rate and period and the forage ADF, NDF, and CF. This is consistent with previous research results [68], indicating that fertilization strategies have a significant impact on forage quality. Appropriate adjustment of fertilizer application rate and timing can effectively improve forage quality and reduce ADF, NDF, and CF content in forage, making it easier to digest and utilize, which is of great significance for animal husbandry. In addition, Chl, Pn, Tr, and Gs as photosynthetic characteristics are positively correlated with forage yield CP and EE content [69]. This further confirms the fact that the enhancement of photosynthesis helps increase forage yield, CP, and EE. However, similar to fertilizer application rate and period, there was also a negative correlation between these photosynthetic characteristics and ADF, NDF, and CF, emphasizing the trade-off relationship between fiber composition and forage quality indicators. Conversely, there was a positive correlation between Ci and ADF, NDF, and CF. This suggests that an increase in Ci may lead to an increase in fiber components in the forage, affecting its quality. This discovery provides a potential pathway for optimizing forage quality by regulating carbon dioxide concentration.

4.5. The Profound Impact of Strategies on Sustainable Development: Farmers’ Practical Exploration and Research Prospects

Our study has revealed the significant impact of different fertilization strategies on oat growth, especially the role of staged nitrogen application in enhancing photosynthetic efficiency, forage yield, and quality. This not only provides farmers with scientific fertilizer application guidelines but also opens up a new path for the sustainable development of agriculture. Staged nitrogen application achieves efficient resource utilization, reducing chemical fertilizer waste. Optimized fertilization strategies reduce environmental pressure, such as water resource protection and greenhouse gas emission reduction. A rational combination of nitrogen fertilizer and organic fertilizer promotes soil health and biodiversity protection.

Farmers can incorporate research findings into agricultural production, developing personalized fertilization plans to improve fertilization effectiveness and reduce costs. They can draw on the staged nitrogen application strategy to reasonably arrange fertilization time and quantity, enhancing oat yield and quality. At the same time, adding organic fertilizer improves soil structure and maintains soil health. Relevant departments should organize training activities, establish demonstration fields, and promote the popularization and application of optimized fertilization strategies.

Future research will focus on the long-term impact of fertilization strategies on agroecosystems, including changes in soil fertility, evolution of soil microbial communities, and the occurrence of crop diseases and pests.

5. Conclusions

The effects of different fertilization strategies on photosynthesis, forage yield, and quality of oats were significant. Under the condition of the same fertilizer application rate, the phased application of nitrogen fertilizer could significantly improve the photosynthetic efficiency of oats, which in turn promoted the forage yield and quality. Meanwhile, the fertilizer application rate and fertilizer application period were positively correlated with the yield, chlorophyll (Chl) content, transpiration rate (Tr), stomatal conductance (Gs), crude protein (CP), and crude fat (EE) content of oats. In addition, chlorophyll content, net photosynthetic rate (Pn), transpiration rate, and stomatal conductance were also positively correlated with yield, crude protein, and crude fat content. When comparing different fertilizer treatments, we found that the staged application of nitrogen fertilizer in combination with organic fertilizer further improved oat yield and quality. Through TOPSIS analysis, F4D2 treatment, which not only maintains high productivity but also improves oat forage quality, is an ideal fertilizer program for oat cultivation in Haidong.