Yield and water use efficiency of cowpea under water deficit/Produtividade e eficiencia do uso da agua do feijao-caupi sob deficiencia hidrica

The state of Para is the main regional producer of cowpea, but its yield is still low compared to other states of the Northern region such as Amazonas and Tocantins, due to the management adopted and the water regime during the cycle, since its cultivation is conducted on a rainfed basis. The objective of this study was to evaluate how water deficit imposed during reproductive stage interferes in the yield of cowpea and in its water use efficiency under the climatic conditions of Castanhal, Para, Brazil, for agricultural planning purposes. The experiment was carried out in Castanhal, northeastern region of the Para state during the dry season of 2014, 2015 and 2016. The experimental design was randomized blocks with six blocks and four treatments, corresponding to different irrigation depths in the reproductive stage, defined as 100, 50, 25 and 0% of the crop evapotranspiration. Water use efficiency (WUE) was determined by the ratio between total grain yield and total water used in each treatment. Maximum water availability led to an average increase in yield of 58% compared to the treatment without irrigation. Water depths below 260 mm limited yield to values lower than 1,000 kg [ha.sup.-1]. The cultivar adopted had WUE of 4.63 kg [ha.sup.-1] [mm.sup.-1], in response to the higher levels of water supply, but showed WUE of 4.31 kg [ha.sup.-1] [mm.sup.-1] under water depth of 50% of water demand. Key words: water deficit, water demand, irrigation management O estado do Para e o maior produtor regional de feijao caupi, mas a produtividade alcancada ainda fica abaixo de outros estados da regiao Norte como Amazonas e Tocantins, devido ao manejo adotado e ao regime hidrico durante a safra, uma vez que seu cultivo e realizado em regime de sequeiro. Objetivou-se neste trabalho avaliar como a deficiencia hidrica imposta durante a fase reprodutiva interfere na produtividade e na eficiencia do uso da agua pelo feijao-caupi nas condicoes climaticas de Castanhal, Para, para fins de planejamento agricola. O experimento foi realizado no municipio de Castanhal, regiao nordeste do estado do Para durante o periodo menos chuvoso dos anos 2014, 2015 e de 2016. Adotou-se o delineamento experimental em blocos ao acaso com 6 blocos e quatro tratamentos, correspondentes a diferentes laminas de irrigacao na fase reprodutiva, definidas como 100, 50, 25 e 0% da evapotranspiracao da cultura. A eficiencia do uso da agua (EUA) foi determinada pela razao entre a produtividade de graos