AbstractSimple linear and multiple regression analyses were used to evaluate the effect of solar radiation, potential evapotranspiration, and other weather indices on the nutrient composition (N and K) and yield of napiergrass (Pennisetum purpureum), in an attempt to better understand previously published results on the effect of harvest frequency, supplemental irrigation, and nitrogen fertilization on the yield of this forage. The inverse relationships between yields and nutrient composition (dilutional effects), previously reported as more significant for the frequently-harvested forage, were also evident in the present study. An inverse and more significant relation between nutrient composition and evaporative-demand indices was again observed for the frequently-harvested forage. Inasmuch as both the yield-nutrients and the nutrients-weather relations are inverse, positive relations should be expected between yields and weather. This was the case, and more significantly so for the frequently-harvested and irrigated forage yields than for the less frequently harvested forages as a function of evaporative-demand indices. The use of more than one weather index in multiple regression analysis appeared to improve significantly the prediction of irrigated forage yields, especially of those under the low-N regime. A harvest-frequency factor (Hf) contributed further toward the accuracy of the prediction of irrigated forage yields. This confirms the overall effect of harvest frequency on forage yields and the significant harvest frequency-irrigation interaction.
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