NITROGEN-FERTILIZER AND CULTIVAR EFFECTS ON YIELD AND NITROGEN-USE EFFICIENCY OF GRAIN AMARANTH

Knowledge of fertility requirements or nitrogen-use efficiency of amaranth (Amaranthus spp.) cultivars is limited. The objectives of this research were to determine the response of grain amaranth to applied N, P, and K, and to determine the effect of cultivar and fertilizer N on nitrogen-use efficiency. Two field experiments were conducted. In the first, five rates of N, P, and K were applied in a central composite design with the cultivar Plainsman grown in five environments. In a second experiment, six amaranth cultivars, Amont, K266, K283, Plainsman, K432 and D136, were grown in three environments with five N rates (0, 45, 90, 135, and 180 kg N ha-1). There was no response to P and K applications when initial soil tests were above 68 kg P ha-1, and 172 kg K ha-1, respectively, but grain amaranth responded linearly to applied P at one location with initial soil P tests of 11 kg ha-1. Grain yield ranged from 794 to 1980 kg ha-1 and responded to N in most environments. Forage yield ranged from 6.1 to 16.6 Mg ha-1 and was increased at higher N rates. At all locations, lodging increased with application of N. Fertilizer N increased pre-flowering N accumulation but not post-flowering N accumulation. Nitrogen-use efficiency (NUE) ratio of grain yield to total soil N supply ranged from 3.48 to 7.91 kg kg-1 across cultivars and environments. Nitrogen-use efficiency decreased with increased soil N mainly because of decreasing N-uptake efficiency (ratio of total plant N to total soil N). Grain amaranth is relatively inefficient in N-use primarily because of its low harvest index (HI = 9-15%) and N harvest index (NHI = 12-26%). This suggests that selection for higher HI and NHI could be effective in improving grain yield and nitrogenuse efficiency of grain amaranth.