Cuphea nitrogen uptake and seed yield response to nitrogen fertilization
Cuphea (Cuphea viscosissima Jacq. x C. lanceolata W.T. Aiton, PSR23) is an oilseed crop that is a rich source of medium-chain fatty acids. Progress has been made on improving cuphea agronomically, but little is known about N fertility requirements for optimum cuphea production. The objective of this study was to determine the N necessary for maximizing seed yield and oil content. Experiment 1 was conducted at Casselton, ND, in 2005 and at Glyndon, MN, in 2005 and 2006 in which fertility treatments (soil + fertilizer N) were 44, 60, 80, 100, 150, and 200 kg N ha–1. Experiment 2 was conducted at Morris, MN, in 2005 and 2006, in which fertility treatments (soil + fertilizer N) were 51, 93, 140, and 185 kg N ha–1. As N fertility increased, plant tissue NO3–N increased, but as developmental stage advanced, plant NO3–N was diluted and decreased. According to the regression model, maximum total N uptake at harvest occurred at 139 kg N ha–1, which includes N from the fertilizer and from the soil. This could classify cuphea as a medium-N-requirement crop. Seed yield was enhanced with N fertility only at Morris, where maximum seed yield occurred at 185 kg N ha–1. However, seed yield increase obtained with added N fertilizer (134 kg N ha–1) was only 71 kg ha–1. Nitrogen fertilizer cost would be greater than the profit obtained with the incremental seed yield; thus N fertilizer application was not economical in this study.