Introduction
Cowpea (Vigna unguiculata L.) is particularly important due to its versatility in human nutrition, animal feed, and its critical role in nutrient cycling within farming systems, owing to its ability to biologically fix atmospheric nitrogen. Due to climate change, areas experiencing water scarcity are expected to increase, as precipitation becomes less frequent and its spatiotemporal patterns shift. Consequently, reduced access to water resources in agriculture is inevitable. One strategy for managing water resource use in agricultural systems is to schedule irrigation based on supplying the specific water volume required by the plant at each irrigation event. Given the varying results from different studies, there is a need for precise location-specific investigation into cowpea's response to water deficit stress. However, research has shown that applying fertilizer and adjusting of planting dates can improve crop grain yield even under water-deficient conditions, thereby mitigating the effects of drought stress on crops. This study aimed to determine the response of cowpea yield and water productivity to reduced irrigation while evaluating the potential of nutrient sources and planting dates to ameliorate the effects of water deficit.
 
Materials and Methods
The experiment was conducted using a split-split plot design based on a randomized complete block design with three replications. The treatments included three planting dates (June 15, July 5, and July 25), three irrigation regimes (100%, 80%, and 40% of the crop’s water requirement), and four fertilizer treatments (control or no fertilizer application, application of chemical fertilizer including 100 kg.ha^-1 urea + 100 kg.ha^-1 triple superphosphate + 100 kg.ha^-1 potassium sulfate, animal manure at 30 tons.ha^-1, and compost at 30 tons.ha^-1). Planting dates were assigned to the main plots, irrigation regimes to the subplots, and fertilizer treatments to the sub-subplots. Cowpea was manually sown at the specified planting dates (according to the treatments) at a density of 20 plants per square meter. At harvest, morphological trait (plant height), grain yield and related traits (number of pods per plant, number of seeds per pod, number of seeds per plant, 1000-seed weight, grain yield, biological yield, and harvest index) were measured. Seed quality traits, including seed protein content (percentage) and protein yield, were also measured. Water use efficiency for grain production was calculated using the amount of water consumed and grain yield. Data were analyzed using analysis of variance (ANOVA) based on the split-split plot design and the underlying randomized complete block design with MSTATC software. Analysis of variance of the data was performed based on the split-split plot experiment and the underlying randomized. complete block design using MSTATC software. Mean comparisons were performed using Tukey’s test.
 
Results and Discussion
The highest plant height, 1000-seed weight, number of seeds per plant, and seed yield of cowpea were obtained on the second planting date (July 5) under 100% water requirement supply and with chemical fertilizer application. However, this treatment showed no significant difference in the number of leaves per plant compared to the first planting date across all irrigation and fertilizer regimes (except for the no-fertilizer treatment under the 40% water requirement irrigation regime). The lowest plant height (61 cm) was recorded on the third planting date with 40% water requirement supply and no fertilizer application, which was not significantly different from other irrigation and fertilizer treatments at the same planting date. The difference between the highest (2845 kg.ha^-1) and the lowest (647 kg.ha^-1, recorded on July 25 with 40% water requirement supply and no fertilizer application) seed yields exceeded 400%. In other words, planting cowpea on July 25 with only 40% of its water requirement supplied and no fertilizer application results in a more than fourfold decrease in grain production. The absence of fertilizer application on July 25 resulted in cowpea seeds produced under the 40% water requirement irrigation regime being not only smaller (lower 1000-seed weight) but also having a lower protein percentage compared to other treatments. Under these conditions, seed weight decreased by more than 50%. Fertilizer application combined with reduced water consumption under the 40% water requirement irrigation regime led to the highest water use efficiency for grain production in cowpea at the second planting date (July 5). The lowest water use efficiency for grain production was observed on the third planting date under both 80% and 100% water requirement supply conditions, and fertilizer application could not improve water use efficiency at this planting date and under these irrigation regimes. Cowpea achieved the highest protein yield on the second planting date (July 5), which was not significantly different from the first planting date (June 15). The lowest protein yield was recorded on the third planting date (July 25).
 
Conclusions
Considering growth- and seed-related traits (plant height, number of leaves per plant, biological yield, grain yield, and harvest index) as well as seed quality (protein content and yield), planting on July 5th with 100% water supply and chemical fertilization is recommended as the optimal treatment for achieving the highest grain and protein yield in cowpea. However, in the cases of water scarcity, the water supply for cowpea can be reduced to 80% of its requirement. In this scenario, by applying chemical fertilizer, a satisfactory grain yield (approximately 2450 kg per hectare, with about a 14% decrease compared to supplying 100% of the water requirement) and a protein yield comparable to that obtained with 100% water supply and chemical fertilizer application can be obtained.