Influence of Nitrogen Application and Planting Dates on Growth, Forage Yield and Quality of Maize

Abstract

Optimizing nitrogen fertilization and planting date is essential for improving forage maize productivity under semi-arid conditions. This study evaluated the effects of nitrogen application rates and planting dates on growth, forage yield, and quality of maize (Zea mays L.) in Upper Egypt. A two-year field experiment (2024–2025) was conducted at the Experimental Farm of Assiut University using a strip-plot design arranged in a randomized complete block design with three replications. Four planting dates (15 April, 15 May, 15 June, and 15 July) were assigned horizontally, while three nitrogen rates (167, 238, and 309 kg N ha⁻¹) were applied vertically. Growth traits, fresh and dry forage yield, dry matter percentage, crude protein content, and protein yield were recorded at 60 days after sowing. Results showed that planting date, nitrogen rate, and their interaction significantly affected most measured traits in both seasons. Sowing in mid-May consistently produced the highest plant height, chlorophyll content, fresh and dry forage yield, and protein yield. Increasing nitrogen application enhanced biomass production and forage quality, with the highest values generally recorded at 309 kg N ha⁻¹. The strongest yield response to nitrogen occurred when maize was sown at the optimal planting date, indicating that nitrogen utilization was closely linked to favorable environmental conditions. Phenotypic correlation and multivariate analyses revealed strong associations among vegetative growth traits and forage yield, with a single dominant factor explaining more than 91% of the variation in yield-related traits across seasons. Overall, the results demonstrate that synchronizing planting date with appropriate nitrogen fertilization is critical for maximizing maize forage yield and quality under semi-arid conditions. Mid-May sowing combined with adequate nitrogen supply represents an effective management strategy for forage maize production in Upper Egypt, while further research is needed to optimize nitrogen-use efficiency and long-term sustainability.

1. Introduction

Maize (Zea mays L.) is a significant multipurpose crop, serving as a major source of food for humans and feed for animals. In Egypt, maize cultivation ranks third in terms of cultivated area and total production, following wheat and rice. Over recent decades, global maize production has shown a steady increase and is projected to rise further under changing climatic conditions, particularly in regions with Mediterranean climates such as southern Europe [1]. In addition, maize represents one of the principal summer annual green forage crops in Egypt, playing a vital role in livestock and poultry nutrition. As a non-leguminous forage, maize is characterized by rapid growth and high biomass accumulation, with green forage yields reaching up to 40–50 t ha⁻¹ [2].

The increasing demand for high-quality forage has encouraged researchers to explore agronomic practices that enhance yield, nutritive value, and resource-use efficiency in forage maize systems. Owing to its importance in animal feed and industrial applications, maize remains a cornerstone crop worldwide. Nevertheless, significant yield gaps persist in many production systems, often resulting from inefficient agronomic management and suboptimal nutrient supply. Among essential nutrients, precise nitrogen (N) fertilization is particularly important, as it directly influences crop productivity while also affecting environmental sustainability.

Nitrogen fertilization is among the most influential management practices affecting forage maize productivity. Nitrogen is a fundamental component of chlorophyll, amino acids, and proteins, and it plays a central role in vegetative growth and biomass accumulation [3]. Numerous studies have demonstrated that increasing nitrogen application generally enhances plant height, leaf area index, and fresh forage yield up to an optimum level, beyond which yield responses diminish and nitrogen-use efficiency declines [4,5]. In forage maize, adequate nitrogen availability is closely associated with increased dry matter production and improved crude protein content, both of which are essential for meeting livestock nutritional requirements.

Planting data is another critical factor influencing maize growth and forage productivity. Sowing date determines the environmental conditions under which seedlings emerge and develop, including temperature regime, radiation interception, and the effective length of the growth cycle. Early planting enables maize plants to exploit more favorable thermal conditions and longer growing periods, often resulting in higher forage yield and improved nutrient accumulation [6]. In contrast, delayed planting may expose crops to heat stress, shortened photo periods, or unfavorable moisture conditions, which can negatively affect growth, yield formation, and biomass partitioning [7]. For forage production, planting time is particularly important because it influences both total dry matter accumulation and the physiological stage of the crop at harvest.

Understanding the interaction between nitrogen fertilization and planting date is therefore essential for optimizing forage maize productivity. Several studies have reported significant interactions between these factors, indicating that crop responses to nitrogen are strongly influenced by sowing time, especially under arid and semi-arid conditions [8]. Early planting combined with adequate nitrogen supply is frequently associated with superior forage yields due to enhanced canopy development and extended periods of effective growth. However, the optimum nitrogen requirement varies with environmental conditions, genotype, and production objectives.

Despite extensive research on maize agronomy, there remains a need for location-specific studies that evaluate the combined effects of planting date and nitrogen fertilization under distinct environmental conditions. This need is particularly evident in Upper Egypt, where temperature patterns, soil properties, and seasonal water availability differ markedly from those of other maize-producing regions. Evaluating forage maize performance under varying nitrogen rates and sowing dates in this region can therefore provide valuable information for improving forage productivity and sustainability.

The present study aimed to evaluate the effects of different nitrogen fertilization levels and planting dates on the growth performance of maize, to assess their influence on forage yield, dry matter production, and forage quality traits—including crude protein content—and to investigate the interaction between nitrogen rate and sowing date in order to identify the optimal management combination for maximizing biomass production and forage quality. Ultimately, the study seeks to provide practical recommendations for sustainable forage maize production under the semi-arid conditions of Assiut, Egypt.

2. Materials and Methods

2.1. Description of the Site and Weather

The field experiments were conducted during the 2024 and 2025 growing seasons at the Experimental Farm of the Agronomy Department, Faculty of Agriculture, Assiut University, Assiut, Egypt (27°08′20.8″ N, 31°19′40.5″ E). The experimental site is characterized by clay soil and represents typical semi-arid conditions of Upper Egypt. Prior to the establishment of each experiment, soil samples were collected from the surface layer (0–15 cm) to assess baseline physicochemical properties and confirm soil homogeneity across the experimental area. Soil analyses were performed according to the standard procedures described by Estefan et al. [9], and the results are presented in

Table 1. Physical and chemical properties of the experimental soil.

Property (Unit)	Season 2024	Season 2025	Property	Season 2024	Season 2025
Particle size distribution			Soluble Cations
Sand (%)	26.80	26.60	Ca2+ (meq 100 g−1)	9.40	9.50
Silt (%)	25.50	25.90	Mg2+ (meq 100 g−1)	3.00	3.00
Clay (%)	47.70	47.50	Na+ (meq 100 g−1)	6.00	5.90
Soil Texture class	Clay	Clay	K+ (meq 100 g−1)	2.00	2.10
Bulk density (g cm−3)	1.20	1.19	Soluble Anions
Field capacity (%)	40.20	40.21	Cl− (meq 100 g−1)	4.00	3.90
Wilting point (%)	21.00	20.90	HCO3− + CO32− (meq 100 g−1)	6.90	6.80
Infiltration rate (cm h−1)	0.12	0.11	SO42− (meq 100 g−1)	10.40	10.50
CaCO3 (%)	1.18	1.16	Total nitrogen (%)	0.09	0.08
pH (1:2.5)	7.78	7.75	Available phosphorous (mg kg−1)	11.20	11.10
Electrical conductivity (dS m−1)	2.03	2.01	Available potassium (mg kg−1)	235.00	233.00
Organic Matter (%)	1.72	1.70

.

Meteorological data, including minimum and maximum air temperature, relative humidity, and precipitation, were obtained from the official meteorological station in Assiut, Egypt. Mean climatic conditions prevailing during the two growing seasons are summarized in

Table 2. Means of air temperature, relative humidity, and precipitation obtained at Assiut, Egypt during the two growing seasons.

Month	Periods	Temperature (°C)			Relative Humidity (%)			Precipitation (m3)
		Min	Max	Mean	Min	Max	Mean
2024
April	11–20	16.9	31.1	24.1	21.1	57.7	36.5	0
	21–30	18.3	34.4	26.4	14.7	53.9	30.5	0
May	1–10	18.8	32.9	26.2	14.3	48.6	27.2	0
	11–20	20.8	35.6	28.4	15.1	45.0	27.4	0
	21–31	22.7	37.6	30.6	14.1	41.0	24.9	0
June	1–10	24.3	40.0	32.4	12.8	43.5	26.1	0
	11–20	25.0	40.3	33.1	14.4	50.3	28.5	0
	21–30	26.1	41.0	34.0	12.7	43.3	25.3	0
July	1–10	20.9	38.8	32.4	16.0	48.7	29.4	0
	11–20	27.1	40.9	34.5	16.0	36.4	24.3	0
	21–31	26.7	39.9	33.7	17.5	47.0	30.0	0
August	1–10	26.6	40.3	33.8	16.7	44.8	26.6	0
	11–20	26.4	38.8	32.9	18.1	44.4	29.4	0
	21–31	26.7	39.0	33.0	19.7	49.9	33.2	0
September	1–10	25.4	37.3	31.4	24.1	51.7	35.9	0
	11–20	24.4	37.8	31.2	21.9	58.1	38.1	0
2025
April	11–20	13.1	28.6	21.0	19.8	62.2	38.3	0
	21–30	19.9	34.4	27.2	15.7	42.7	27.6	0
May	1–10	17.4	32.1	25.2	17.0	43.9	28.5	0
	11–20	19.2	37.2	29.4	13.7	48.4	29.0	0
	21–31	19.4	34.8	27.4	16.7	49.6	30.1	0
June	1–10	20.6	34.9	28.0	18.8	58.3	34.7	0
	11–20	22.5	35.9	29.6	16.8	52.4	30.7	0
	21–30	23.2	36.8	30.6	16.4	47.0	28.0	0
July	1–10	24.4	37.9	31.4	15.1	47.3	28.8	0
	11–20	25.1	37.7	31.8	16.2	44.9	26.5	0
	21–31	25.9	40.1	33.7	15.9	41.5	26.2	0
August	1–10	24.5	36.4	30.9	23.1	57.8	38.1	0
	11–20	25.8	39.6	32.7	21.6	59.2	37.3	0
	21–31	23.5	37.1	30.4	22.4	57.3	36.7	0
Sepember	1–10	22.6	35.0	29.0	26.0	58.8	39.6	0
	11–20	19.5	34.1	28.0	25.4	66.1	43.8	0

Source: Meteorological authority, Assiut, Egypt.

. Rainfall during the experimental period was negligible, reflecting the arid nature of the region, and irrigation was applied as required to avoid moisture stress.

2.2. Experimental Design and Factors

The experiment was arranged as a strip-plot design within a randomized complete block design (RCBD) with three replications. The main experimental factors were planting date and nitrogen fertilization rate. Four planting dates were assigned horizontally as strip plots: 15 April (D₁), 15 May (D₂), 15 June (D₃), and 15 July (D₄). Nitrogen fertilization rates were allocated vertically as subplots and consisted of three levels: 167 (T₁), 238 (T₂), and 309 (T₃) kg N ha⁻¹. The single-cross maize hybrid SC 2031, obtained from High Tech Seeds Company, was used in both seasons. The maize hybrid used in this study was the single cross SC 2031, a widely cultivated hybrid in Egypt and recommended for forage production. It is characterized by vigorous early growth, good adaptation to high temperatures, and high biomass production. SC 2031 belongs to the medium-maturity group and shows a good response to nitrogen fertilization, making it suitable for semi-arid conditions. Each experimental plot measured 10.5 m² (3.5 m × 3.0 m). Maize was sown using a broadcasting method, with the same amount of seed (60 g per plot) applied to all treatments to ensure a uniform plant density across plots. This approach was used to facilitate consistent growth, forage yield, and quality assessment among the tested nitrogen rates and planting dates, a common forage maize practice in the region, to ensure uniform plant distribution. Standard agronomic practices recommended for forage maize production in Upper Egypt were followed throughout both growing seasons. All sowing dates were irrigated uniformly following local recommended practices for forage maize under semi-arid conditions. Crop water requirements were fully supplied through controlled irrigation. Irrigation was applied every 12 days, with a total of five irrigations per plot, including the initial irrigation at sowing. The same water quantity was applied to all treatments to ensure that observed differences were due solely to nitrogen rates and planting dates. Nitrogen fertilizer was applied in two equal split doses: the first dose was applied at 15 days after sowing, and the second dose was applied at 30 days after sowing. Phosphorus fertilizer was incorporated during land preparation at a rate equivalent to 72 kg P₂O₅ ha⁻¹, in accordance with local recommendations. Harvesting was performed 60 days after sowing for each planting date to ensure that all plots were sampled at the same growth stage.

2.3. Data Recorded

Forage yield and growth-related traits were recorded 60 days after sowing for each planting date. The following parameters were measured:

Plant height (PH, cm): Plant height was measured at harvest from the soil surface to the tip of the tallest plant. Measurements were taken from five randomly selected plants per plot, and the mean value was calculated for each treatment.

Chlorophyll content (Chl.): Leaf chlorophyll content was determined using a SPAD-502 Plus chlorophyll meter (Konica Minolta, Tokyo, Japan). Measurements were taken from the fully expanded flag leaves of five randomly selected plants per plot at 60 days after sowing, following the method described by Dash et al. [10].

Leaf-to-stem ratio (fresh and dry): A representative fresh forage sample (500 g) was collected from each plot and manually separated into leaves and stems. Fresh weights were recorded immediately to calculate the fresh leaf-to-stem ratio. The separated components were then oven-dried at 70 °C until constant weight to determine the dry leaf-to-stem ratio.

Total fresh forage yield (FFY, kg m⁻²): Fresh forage yield was determined by harvesting the entire plot area at 60 days after sowing for each treatment. The harvested biomass was weighed, and yield was expressed on an area basis (kg m⁻²).

Dry matter percentage (DMP, %): Dry matter percentage was determined from a subsample (150 g) taken from each plot. Samples were oven-dried at 70 °C until constant weight, and dry matter percentage was calculated accordingly.

Total dry forage yield (DFY, g m⁻²): Dry forage yield was calculated by multiplying total fresh forage yield by the corresponding dry matter percentage for each treatment.

Crude protein percentage (PP, %): Crude protein content was determined using the micro-Kjeldahl method as described by AOAC [11]. Total nitrogen concentration was multiplied by a conversion factor of 6.25 to estimate crude protein percentage.

Protein yield (PY, g m⁻²): Protein yield was calculated by multiplying dry forage yield by crude protein percentage and dividing by 100.

2.4. Total Growing Degree Days (GDD)

In addition, daily maximum and minimum air temperatures were recorded from sowing to harvest for each planting date in both seasons. Growing degree days (GDD) were calculated using a base temperature of 10 °C, following the method described by Saeed and Francis [12], as follows:

GDD = [(Tmax + Tmin)/2] − 10

where Tmax and Tmin represent the daily maximum and minimum temperatures, respectively. Cumulative GDD values for each planting date and season are presented in

Table 3. Total growing degree days (GDD) for each sowing date and season at Assiut where maize forage trials were conducted.

Sowing Date	Growing Degree Days (GDD)
	2024	2025
15 April	1121.8	1008.5
15 May	1308.9	1208.9
15 Jun	1391.1	1334.0
15 July	1389.0	1254.3

.

2.5. Statistical Analysis

All recorded data were subjected to analysis of variance (ANOVA) appropriate for a strip-plot design within an RCBD, following the procedures outlined by Gomez and Gomez [13]. Treatment means were compared using the least significant difference (LSD) test at the 0.05 and 0.01 probability levels, as described by Waller and Duncan [14]. Phenotypic correlation coefficients among the studied traits were calculated separately for each season across all planting dates and nitrogen treatments, following the procedures described by Gomez and Gomez [14]. Stepwise multiple regression analysis was conducted using SPSS software (version 10) [15] to identify the most important traits contributing to total fresh forage yield (FFY). Fresh forage yield was treated as the dependent variable, while plant height, chlorophyll content, fresh and dry leaf-to-stem ratios, dry matter percentage, and dry forage yield were used as independent variables. Regression models were developed separately for each growing season, and coefficients of determination (R²) were calculated for all models. Factor analysis was employed to summarize the relationships among maize forage yield components and to identify underlying latent factors influencing productivity. The analysis was conducted following established multivariate statistical procedures [16,17,18,19,20,21]. Prior to factor extraction, the suitability of the data was evaluated using the Kaiser–Meyer–Olkin (KMO) measure of sampling adequacy and Bartlett’s test of sphericity. Principal axis factoring was then applied to extract factors, and factor loadings were interpreted to identify key traits contributing to forage yield variation.

3. Results

The results of this study demonstrate clear and consistent effects of nitrogen fertilization, planting dates, and their interaction on maize growth, forage yield, and quality traits across the two growing seasons. Most measured traits responded significantly to nitrogen rate, sowing date, and their combined effects, with comparable trends observed in both 2024 and 2025 seasons.

3.1. Effects of Sowing Dates and Nitrogen Rates on Growth Traits and Fresh Forage Yield

Plant height, chlorophyll content, leaf-to-stem ratio (fresh and dry), and total fresh forage yield were significantly influenced by sowing date in both growing seasons (

Table 4. Mean of plant height cm, chlorophyll, fresh and dry leaves/stems ratio, and total fresh forage yield kg m² as affected by sowing dates, nitrogen rates fertilizer maize forage and interaction in 2024 and 2025 seasons.

Variable		Plant Height cm		Chlorophyll.		Fresh Leaves/ Stems Ratio		Dry Leaves/ Stems Ratio		Total Fresh Forage Yield kg m2
Year		2024	2025	2024	2025	2024	2025	2024	2025	2024	2025
15 April		142.1 d	144.3 d	30.8 c	33.7 d	0.641 b	0.651 b	1.069 a	1.556 a	3.14 d	3.26 d
15 May		176.4 a	179.4 a	42.3 a	44.9 a	0.682 a	0.700 a	0.687 d	0.955 d	4.47 a	4.50 a
15 June		161.4 b	165.2 b	35.6 b	38.4 b	0.653 b	0.684 a	0.797 c	1.078 c	3.56 b	3.61 b
15 July		155.1 c	158.4 c	34.7 b	35.5 c	0.635 b	0.649 b	0.948 b	1.208 b	3.42 c	3.44 c
167 N kg ha−1		152.7 c	155.6 c	34.6 c	37.0 c	0.631 b	0.671 a	0.910 a	1.296 a	3.48 c	3.57 c
238 N kg ha−1		158.9 b	161.9 b	35.9 b	38.3 b	0.657 ab	0.667 a	0.887 ab	1.207 b	3.64 b	3.70 b
309 N kg ha−1		164.7 a	167.9 a	37.1 a	39.2 a	0.670 a	0.675 a	0.828 b	1.095 c	3.82 a	3.84 a
15 April	167 N kg ha−1	132.0 j	133.7 i	29.8 a	32.7 k	0.629 a	0.653 a	1.081 a	1.787 a	2.90 i	3.13 j
	238 N kg ha−1	142.0 i	143.7 h	30.5 a	33.7 j	0.639 a	0.645 a	1.067 a	1.557 b	3.17 h	3.27 i
	309 N kg ha−1	152.3 h	155.5 g	32.0 a	34.6 i	0.654 a	0.655 a	1.058 a	1.324 c	3.34 g	3.37 h
15 May	167 N kg ha−1	171.3 c	174.5 c	40.7 a	43.3 c	0.657 a	0.699 a	0.736 a	1.063 de	4.28 c	4.30 c
	238 N kg ha−1	175.7 b	178.5 b	42.7 a	45.3 b	0.677 a	0.696 a	0.729 a	0.998 e	4.42 b	4.44 b
	309 N kg ha−1	182.3 a	185.2 a	43.7 a	46.2 a	0.712 a	0.704 a	0.595 a	0.803 f	4.72 a	4.75 a
15 June	167 N kg ha−1	156.1 g	159.8 f	34.7 a	37.4 f	0.640 a	0.677 a	0.842 a	1.109 de	3.38 fg	3.45 f
	238 N kg ha−1	162.1 e	166.1 e	35.4 a	38.4 e	0.662 a	0.686 a	0.795 a	1.069 de	3.55 e	3.65 e
	309 N kg ha−1	166.1 d	169.8 d	36.7 a	39.5 d	0.658 a	0.690 a	0.755 a	1.056 de	3.75 d	3.74 d
15 July	167 N kg ha−1	151.4 h	154.6 g	33.1 a	34.6 i	0.599 a	0.656 a	0.981 a	1.225 cd	3.38 fg	3.39 gh
	238 N kg ha−1	156.0 g	159.3 f	35.2 a	35.6 h	0.652 a	0.641 a	0.957 a	1.203 cd	3.42 fg	3.44 fg
	309 N kg ha−1	158.0 f	161.3 f	35.9 a	36.4 g	0.655 a	0.650 a	0.905 a	1.197 cd	3.46 ef	3.49 f
S. L	Sowing dates (SD)	**	**	**	**	**	*	**	**	**	**
	Nitrogen rates (NR)	**	**	**	**	*	N. S	*	**	**	**
	SD × NR	**	**	N. S	**	N. S	N. S	N. S	*	**	**

Means within the same column followed by the same letter(s) for the same treatment are not significantly different at p < 0.05 according to the least significant different (LSD) test. S.L = *, ** significant levels of 5 and 1% of probability, respectively. N.S = not significant.

). Among the evaluated planting dates, sowing on 15 May consistently produced the tallest plants, recording mean heights of 176.4 and 179.4 cm in 2024 and 2025, respectively. This sowing date also resulted in the highest chlorophyll content and fresh leaf-to-stem ratio, indicating more vigorous vegetative growth and improved canopy development. In contrast, sowing on 15 April resulted in the lowest plant height and chlorophyll values, while June and July sowings produced intermediate responses. Fresh leaf-to-stem ratio was significantly higher under mid-May sowing, whereas the dry leaf-to-stem ratio showed an inverse pattern, with the highest values observed under early (15 April) sowing. Total fresh forage yield followed a similar trend to vegetative growth traits, with 15 May producing the highest yields (4.47 and 4.50 kg m⁻² in 2024 and 2025, respectively), while the lowest yields were recorded under 15 April sowing (Table 4).

Nitrogen fertilization significantly affected all growth-related traits and fresh forage yield (Table 4). Increasing nitrogen rate from 167 to 309 kg N ha⁻¹ resulted in progressive increases in plant height, chlorophyll content, fresh leaf-to-stem ratio, and total fresh forage yield. The highest nitrogen rate (309 kg N ha⁻¹) produced the greatest fresh forage yield (3.82–3.84 kg m⁻²), whereas the lowest nitrogen rate consistently resulted in reduced growth and productivity. Differences in dry leaf-to-stem ratio among nitrogen rates were less pronounced but remained statistically significant.

The interaction between sowing date and nitrogen rate significantly affected most measured traits in both seasons (Table 4). The combination of 15 May sowing with 309 kg N ha⁻¹ consistently produced the highest plant height, chlorophyll content, fresh leaf-to-stem ratio, and total fresh forage yield. This treatment combination resulted in maximum fresh forage yields of 4.72 and 4.75 kg m⁻² in 2024 and 2025, respectively. Conversely, the lowest growth and yield values were obtained under the interaction of early sowing (15 April) combined with the lowest nitrogen rate (167 kg N ha⁻¹), reflecting the combined effects of suboptimal environmental conditions and limited nitrogen availability. These results indicate that the positive response to increased nitrogen fertilization was most pronounced when maize was sown at the optimal planting date.

3.2. Effects on Dry Matter Accumulation and Protein Yield

Sowing date significantly influenced dry matter percentage, dry forage yield, crude protein percentage, and protein yield in both growing seasons (

Table 5. Mean of dry matter percentage, dry forage yield g m², protein percentage, and protein yield g m² as affected by sowing dates, nitrogen rates fertilizer maize forage and interaction in 2024 and 2025 seasons.

Variable		Dry Matter Percentage		Dry Forage Yield g m2		Protein Percentage		Protein Yield g m2
Year		2024	2025	2024	2025	2024	2025	2024	2025
15 April		19.24 c	20.14 d	607.5 d	657.1 d	6.10 d	7.33 d	37.53 d	48.5 d
15 May		22.81 a	23.94 a	1044.1 a	1077.9 a	9.11 a	9.70 a	95.49 a	104.9 a
15 June		20.88 b	22.27 b	746.6 b	805.1 b	8.21 b	8.56 b	61.42 b	69.0 b
15 July		20.20 cb	21.78 c	689.5 c	749.6 c	7.54 c	8.47 c	52.04 c	63.5 c
167 N kg ha−1		20.16 c	21.41 c	713.0 c	768.8 c	7.31 c	7.96 c	53.9 c	62.4 c
238 N kg ha−1		20.74 b	21.99 b	768.4 b	819.7 b	7.73 b	8.58 b	61.0 b	71.5 b
309 N kg ha−1		21.45 a	22.70 a	834.5 a	878.9 a	8.18 a	9.01 a	70.0 a	80.5 a
15 April	167 N kg ha−1	18.24 k	19.14 j	531.0 i	599.4 k	5.26 j	6.67 j	27.9 k	40.0 i
	238 N kg ha−1	19.24 j	20.14 i	612.9 h	658.8 j	5.96 i	7.17 i	36.6 j	47.2 k
	309 N kg ha−1	20.24 gh	21.14 h	678.8 g	713.3 i	7.09 h	8.17 g	48.1 i	58.3 i
15 May	167 N kg ha−1	21.81 c	22.94 c	954.7 c	986.6 c	8.63 c	9.20 c	82.5 c	90.8 c
	238 N kg ha−1	22.81 b	23.94 b	1030.6 b	1063.6 b	9.10 b	9.70 b	93.9 b	103.2 b
	309 N kg ha−1	23.81 a	24.94 a	1147.0 a	1183.7 a	9.60 a	10.20 a	110.1 a	120.8 a
15 June	167 N kg ha−1	20.51 f	21.91 f	689.7 fg	755.2 fg	7.97 e	8.20 g	55.0 f	62.0 h
	238 N kg ha−1	20.75 e	22.14 e	741.5 e	807.5 e	8.30 d	8.67 f	61.6 e	70.0 e
	309 N kg ha−1	21.38 d	22.77 d	808.6 d	852.6 d	8.37 d	8.80 e	67.6 d	75.0 d
15 July	167 N kg ha−1	20.06 i	21.65 g	676.4 g	733.9 h	7.40 g	7.77 h	50.1 hi	57.0 j
	238 N kg ha−1	20.16 hi	21.75 g	688.6 fg	748.9 g	7.57 fg	8.77 e	52.1 gh	65.7 g
	309 N kg ha−1	20.36 g	21.95 f	703.5 f	766.0 f	7.67 f	8.87 d	54.0 fg	67.9 f
S. L	Sowing dates (SD)	**	**	**	**	**	**	**	**
	Nitrogen rates (NR)	**	**	**	**	**	**	**	**
	SD × NR	**	**	**	**	**	**	**	**

Means within the same column followed by the same letter(s) for the same treatment are not significantly different at p < 0.05 according to the least significant different (LSD) test. S.L = ** significant level of 1% of probability, respectively.

). Sowing on 15 May produced the highest dry matter percentage (22.81–23.94%) and dry forage yield (1044.1–1077.9 g m⁻²), as well as the greatest protein concentration and protein yield. In contrast, the lowest values for these traits were consistently recorded under 15 April sowing, while June and July sowings resulted in intermediate responses. Nitrogen fertilization exerted a strong positive effect on dry matter accumulation and forage quality. Increasing nitrogen rate from 167 to 309 kg N ha⁻¹ significantly increased dry forage yield, crude protein percentage, and protein yield in both seasons.

The highest nitrogen rate produced dry forage yields of 834.5 and 878.9 g m⁻² and the highest protein yields across seasons (Table 5). The interaction between sowing date and nitrogen rate further emphasized the importance of integrated management. The combination of 15 May sowing with 309 kg N ha⁻¹ resulted in the highest dry forage yield (1147.0 and 1183.7 g m⁻²) and protein yield (110.1 and 120.8 g m⁻²) in 2024 and 2025, respectively (Table 5).

3.3. Phenotypic Correlation Among Growth, Yield, and Quality Traits

Phenotypic correlation analysis revealed strong and consistent relationships among maize growth, yield, and quality traits in both seasons (

Table 6. Correlation between all studied traits below (2024), and above (2025) diagonal.

	Plant Height	Chlorophyll.	Fresh Leaves/Stems Ratio	Dry Leaves/Stems Ratio	Total Fresh Forage Yield	Dry Matter Percentage	Dry Forage Yield	Protein Percentage	Protein Yield
Plant height cm	-	0.93	0.82	−0.96	0.91	0.99	0.94	0.97	0.95
Chlorophyll.	0.96	-	0.90	−0.82	0.98	0.93	0.98	0.89	0.97
Fresh leaves/stems ratio	0.77	0.77	-	−0.74	0.85	0.80	0.85	0.71	0.81
Dry leaves/stems ratio	−0.94	−0.93	−0.74	-	−0.79	−0.96	−0.84	−0.93	−0.86
Total fresh forage yield	0.95	0.98	0.78	−0.90	-	0.92	0.99	0.88	0.98
Dry matter percentage	0.98	0.96	0.80	−0.92	0.97	-	0.95	0.96	0.96
Dry forage yield	0.95	0.97	0.80	−0.90	1.00	0.98	-	0.91	1.00
Protein percentage	0.98	0.92	0.69	−0.93	0.89	0.95	0.89	-	0.94
Protein yield	0.97	0.98	0.80	−0.93	0.99	0.99	0.99	0.93	-

). Plant height and chlorophyll content were positively and strongly correlated with total fresh forage yield, dry matter percentage, dry forage yield, protein percentage, and protein yield. Fresh leaf-to-stem ratio also showed positive correlations with most productivity traits, whereas dry leaf-to-stem ratio was negatively correlated with yield and quality parameters.

The strongest positive correlations were observed between total fresh forage yield and dry forage yield, as well as between dry forage yield and protein yield, indicating a close association between biomass accumulation and protein production. These relationships were stable across both growing seasons.

3.4. Stepwise Regression Analysis

Stepwise regression analysis identified dry forage yield (DFY) as the primary determinant of total fresh forage yield (FFY) in both seasons (

Table 7. Stepwise regression analysis for total fresh forage yield (FFY) kg m² via growth and components yield indices of mazing in 2024 and 2025 seasons.

Season	Model No.	Fitted Independent Traits	R2	Regression Equation
2024	Mod.1	DFY	0.996	Ŷ = 1.33 + 0.003 DFY
2025	Mod.1	DFY	0.988	Ŷ = 1.26 + 0.003 DFY
	Mod.2	DFY + DMP	0.998	Ŷ = 2.93 + 0.004 DFY − 0.11 DMP
	Mod.3	DFY + DMP + PH	0.999	Ŷ = 3.27 + 0.004 DFY − 0.185 DMP + 0.008 PH

PH: Plant height, DFY: Dry forage yield, DMP: Dry matter percentage.

). In the 2024 season, DFY alone explained 99.6% of the variation in FFY, indicating a dominant contribution of dry biomass accumulation to fresh yield formation. In the 2025 season, DFY initially explained 98.8% of FFY variation, and the inclusion of dry matter percentage further increased the coefficient of determination to 99.8%. The final regression model for 2025 included DFY, dry matter percentage, and plant height, resulting in an R² value of 0.999. Predicted values derived from the regression models closely matched observed fresh forage yields across all treatments (

Table 8. Actual and expected values of total fresh forage yield kg m² for all models of stepwise regression analyses forage yield growth and components yield indices of maize in 2024 and 2025 seasons.

Combination Treatment	2024		2025
	Actual Values	Expected Values	Actual Values	Expected Values
		Mod. 1		Mod. 1	Mod. 2	Mod. 3
1	2.90	2.92	3.13	3.06	3.22	3.20
2	3.17	3.17	3.27	3.24	3.35	3.33
3	3.34	3.37	3.37	3.40	3.46	3.46
4	4.28	4.19	4.30	4.22	4.35	4.37
5	4.42	4.42	4.44	4.45	4.55	4.52
6	4.72	4.77	4.75	4.81	4.92	4.87
7	3.38	3.40	3.45	3.53	3.54	3.52
8	3.55	3.55	3.65	3.68	3.72	3.73
9	3.75	3.76	3.74	3.82	3.84	3.83
10	3.38	3.36	3.39	3.46	3.48	3.44
11	3.42	3.40	3.44	3.51	3.53	3.52
12	3.46	3.44	3.49	3.56	3.58	3.56
r	-	1.00	-	0.99	1.00	1.00
t	-	0.35	-	−2.21	−10.42	−12.95

Note r = correlation, t = t-paired test.

), confirming the high predictive accuracy of the selected models in both seasons.

3.5. Factor Analysis of Forage Yield Components

Factor analysis results indicated a strong underlying structure among maize forage yield traits in both growing seasons. The Kaiser–Meyer–Olkin (KMO) values exceeded the acceptable threshold in both years (0.759 in 2024 and 0.715 in 2025), and Bartlett’s test of sphericity was highly significant (p < 0.001), confirming the suitability of the dataset for factor analysis (

Table 9. Kaiser–Meyer–Olkin (KMO) and Bartlett’s test for assessing the adequacy of the data for the factor analysis.

Type of Test	Indicators of Significance
KMO Measure of Sampling Adequacy		2024	2025
		0.759	0.715
Bartlett’s test of sphericity	Χ2	255.7	234.0
	d. f	36	36
	significance	<0.001	<0.001

). The first extracted factor exhibited high eigenvalues (8.33 in 2024 and 8.22 in 2025) and accounted for 92.57% and 91.35% of the total variance, respectively (

Table 10. Portion of the variance of observed traits of maize forage yield encompassed with the Extraction Method: Principal Axis Factoring.

Factor	Initial Eigenvalues						Extraction Sums of Squared Loadings
	Total		% of Variance		Cumulative %		Total		% of Variance		Cumulative %
	2024	2025	2024	2025	2024	2025	2024	2025	2024	2025	2024	2025
1	8.33	8.22	92.57	91.35	92.57	91.35	8.33	8.22	92.57	91.35	92.57	91.35
2	0.36	0.44	3.96	4.94	96.53	96.29
3	0.18	0.27	1.98	2.97	98.50	99.27
4	0.09	0.04	0.94	0.44	99.44	99.71
5	0.04	0.01	0.42	0.14	99.87	99.85
6	0.01	0.01	0.10	0.09	99.97	99.94
7	0.00	0.00	0.03	0.05	100.00	99.98
8	0.00	0.00	0.00	0.01	100.00	100.00
9	0.00	0.00	0.00	0.00	100.00	100.00

). The second factor contributed only a small proportion of additional variance, while the remaining factors had negligible explanatory power.

The cumulative variance explained by the first two factors exceeded 96% in both seasons. High communalities were observed for all evaluated traits, ranging from 0.70 to 0.99 (

Table 11. Common structure of yield components and their correlation with rotated factor axes.

Yield Components	Common Structure of Yield Components				Correlations Between Yield Components and Rotated Factor
	2024	2025	2024	2025	2024	2025
	Initial		Extracted		Factor 1
Plant height	1.00	1.00	0.97	0.97	0.99	0.98
Chlorophyll	1.00	1.00	0.97	0.96	0.98	0.98
Fresh leaves/stems ratio	1.00	1.00	0.70	0.72	0.84	0.85
Dry leaves/stems ratio	1.00	1.00	0.90	0.84	−0.95	−0.92
Total fresh forage yield	1.00	1.00	0.96	0.93	0.98	0.96
Dry matter percentage	1.00	1.00	0.98	0.96	0.99	0.98
Dry forage yield	1.00	1.00	0.97	0.97	0.98	0.99
Protein percentage	1.00	1.00	0.90	0.91	0.95	0.95
Protein yield	1.00	1.00	0.99	0.97	0.99	0.98

), indicating that most of the variation in growth, yield, and quality parameters was effectively captured by the factor model. Plant height, chlorophyll content, fresh forage yield, dry matter percentage, dry forage yield, protein percentage, and protein yield exhibited strong positive loadings on the first factor, whereas dry leaf-to-stem ratio showed a strong negative loading. These results demonstrate that maize forage yield traits are highly interrelated and predominantly governed by a single dominant factor associated with overall plant productivity and biomass accumulation.

4. Discussion

Sowing date and nitrogen fertilization are among the most influential agronomic factors determining maize forage productivity, as they jointly regulate crop exposure to environmental conditions and nutrient availability during critical growth stages. The present study demonstrated that both factors, individually and in combination, exerted significant effects on maize growth, forage yield, and quality across two growing seasons under the semi-arid conditions of Upper Egypt. The superiority of the mid-May sowing date was evident across nearly all measured traits, including plant height, chlorophyll content, fresh and dry forage yield, and protein accumulation. This response can be attributed to more favorable thermal and radiation conditions during early vegetative development, which likely enhanced photosynthetic efficiency and prolonged the effective growth period. In contrast, early April sowing exposed plants to cooler temperatures that may have constrained early growth and nutrient uptake, while late sowing in June and July shortened the growing period and increased exposure to heat stress, thereby limiting biomass accumulation. Similar responses to sowing date have been widely reported in maize forage systems under Mediterranean and semi-arid environments [22,23,24,25].

Nitrogen fertilization significantly improved maize vegetative growth, biomass production, and forage quality in both seasons. Increasing nitrogen rates enhanced plant height and chlorophyll content, reflecting improved leaf area development and photosynthetic capacity [26,27]. These responses are consistent with the central role of nitrogen in chlorophyll synthesis, protein formation, and carbon assimilation [28,29]. The observed increases in dry matter production and crude protein content with higher nitrogen supply further confirm that nitrogen availability directly influences both forage quantity and nutritive value.

However, although the highest nitrogen rate (309 kg N ha⁻¹) consistently produced maximum forage yield and protein content, this response should be interpreted cautiously. High nitrogen inputs may reduce nitrogen-use efficiency and increase the risk of environmental losses, particularly under irrigated conditions. Therefore, while elevated nitrogen rates may be agronomically effective for maximizing short-term forage yield, their long-term sustainability should be evaluated in future studies that incorporate nitrogen-use efficiency, soil nitrogen dynamics, and environmental indicators. The interaction between sowing date and nitrogen rate highlights the importance of synchronizing nutrient supply with favorable environmental conditions. The strongest nitrogen responses were observed when maize was sown in mid-May, indicating that nitrogen uptake and utilization were optimized under suitable temperature and radiation regimes. Under suboptimal sowing dates, particularly early April, the yield response to nitrogen was reduced, suggesting limitations in nitrogen absorption or assimilation during early growth. This interaction supports previous findings that nitrogen-use efficiency is highly dependent on climatic conditions and crop phenology [30,31].

Dry matter accumulation and protein yield followed trends similar to those observed for fresh forage yield, reinforcing the importance of proper sowing time and nitrogen management in forage maize production. The higher dry matter percentage and protein yield recorded under mid-May sowing reflect improved assimilate partitioning and nitrogen assimilation during vegetative growth. These findings are consistent with earlier reports indicating that optimal sowing dates enhance biomass quality by improving the balance between structural and metabolic tissues [24,25,32,33].

Correlation analysis revealed strong positive associations among plant height, chlorophyll content, forage yield, and protein yield, indicating that vegetative vigor and photosynthetic efficiency are key drivers of forage productivity, consistent with previous trait association studies in maize genotypes [34]. Conversely, the negative correlations observed between dry leaf-to-stem ratio and most yield traits suggest that increased stem proportion and tissue maturity may reduce forage quality and digestibility. These relationships confirm that traits related to canopy development and physiological activity are closely linked to both forage quantity and nutritive value. Stepwise regression analysis identified dry forage yield as the primary determinant of total fresh forage yield in both seasons, reflecting the close relationship between biomass accumulation and water content in forage crops. The inclusion of dry matter percentage and plant height in the 2025 models further indicates that morphological traits and tissue composition can influence fresh biomass production under varying environmental conditions. Although the high coefficients of determination reflect strong statistical relationships, these models primarily confirm structural dependencies among yield components rather than revealing independent biological drivers.

Factor analysis further demonstrated that maize forage yield traits are governed by a dominant underlying factor explaining more than 91% of total variance across seasons. This dominant factor likely represents overall plant productivity, integrating growth, photosynthetic capacity, biomass accumulation, and protein formation. The stability of this factor structure across years indicates a strong and consistent physiological linkage among yield-related traits, as previously reported in multivariate studies of maize forage and grain yield [35,36,37,38,39,40]. The negative loading of dry leaf-to-stem ratio on the primary factor suggests that increased tissue lignification and maturity may negatively affect forage performance.

Overall, the results indicate that maize forage productivity under semi-arid conditions is driven by a tightly integrated set of growth and physiological traits that respond collectively to planting date and nitrogen availability. Optimizing sowing time allows crops to exploit favorable climatic conditions, while adequate nitrogen supply supports vegetative growth and protein accumulation. These findings align with broader concepts of sustainable productivity improvement in tropical maize systems [41] and with evidence that variability in forage yield and quality traits is influenced by both genetic and management factors [42]. The combined optimization of these factors is therefore essential for achieving high forage yield and quality, while future research should focus on balancing productivity with nitrogen-use efficiency and environmental sustainability.

5. Conclusions

This study demonstrated that both sowing date and nitrogen fertilization significantly influence maize forage growth, yield, and quality under the semi-arid conditions of Upper Egypt. Among the evaluated treatments, sowing in mid-May consistently provided the most favorable environmental conditions for maize development, resulting in superior vegetative growth, biomass accumulation, and forage quality compared with earlier or later planting dates. Nitrogen fertilization markedly enhanced forage productivity and crude protein content, with increasing nitrogen rates leading to progressive improvements in fresh and dry forage yield as well as protein yield. The highest nitrogen rate produced maximum yields; however, these results should be interpreted with consideration of potential limitations related to nitrogen-use efficiency and long-term environmental sustainability. The interaction between sowing date and nitrogen rate was a key determinant of forage performance, as the strongest yield responses to nitrogen occurred when maize was planted at the optimal sowing date. This finding highlights the importance of synchronizing nutrient availability with favorable climatic conditions to maximize nitrogen uptake, utilization, and overall crop productivity. Correlation, regression, and factor analyses consistently indicated that vegetative growth traits, particularly dry forage yield, plant height, and chlorophyll content, are strongly associated with fresh forage yield and protein accumulation. A single dominant factor explained more than 91% of the variation in forage yield traits across both growing seasons, reflecting the integrated nature of growth, physiological activity, and biomass production in maize forage systems. Overall, the results confirm that optimizing sowing time in combination with appropriate nitrogen management is essential for maximizing maize forage yield and quality in semi-arid environments. These findings provide a practical basis for improving forage maize production in Upper Egypt, while future research should focus on refining nitrogen management strategies to enhance nitrogen-use efficiency and reduce potential environmental impacts.