Response of Cowpea Dry Matter, Yield Attributes and Grain Yield to Graded Nitrogen Fertilizer Application Rates.

It is well established that the application of high nitrogen (N) fertilizer rates inhibits the development of root nodules in legumes but application of low-level ‘starter' N doses is sometimes recommended to improve nodulation and N2 fixation. Numerous research results have often suggest that N is the most limiting nutrients on farmers’ fields. We evaluated the response of two cowpea varieties (IT00K-1217 and Oloyin) to varying N rates (0, 10, 20, 30 & 40 kg ha^-1) applied as urea under supplementary irrigation over a 2-year growing seasons. Treatments were laid out in a split plot arrangement and fitted into randomized complete block design with four replicates. Results revealed that N fertilizer application exerts significant (p<0.05) effects on fodder production, the total dry matter yield, grain yield and the harvest index. Grain and total dry matter yields obtained over the 2-year planting seasons ranged from 688 to 2326 kg/ha and 3523 to 7904 kg/ha, respectively depending on N rate and cowpea variety. Mean grain yield across the 2 planting seasons was highest at 30 kg N/ha application rate for both cowpea varieties; and higher with IT00K-1217. The difference in the mean number of seeds per pod, pod length and haulm weight for the two cowpea varieties differed significantly (p<0.05). Significant N fertilizer x variety interaction effect observed on fodder production and harvest index during 2007/08 planting season indicated differential varietal responses to N fertilizer application, which may have practical implication for cowpea production. Grain yield showed strong positive correlation with fodder production, total dry matter yield and harvest index. Nitrogen use efficiency of the two cowpea varieties decreased with increase in N rate and generally higher with IT00K-1217 variety.

Key words: Cowpea production, Fertilizer N, N-use efficiency, grain yield, soil available N