TIMING OF LIMITED IRRIGATION AND N-INJECTION FOR GRAIN-SORGHUM

When subsurface irrigation sources are lacking in humid and subhumid r egions, high yearly precipitation may allow for storage of surface wat er in farm ponds and lakes for irrigation. Irrigation at selected grow th stages may avoid critical stress for crops with some drought tolera nce, such as grain sorghum [Sorghum bicolor (L.) Moench]. Because grai n sorghum is responsive to N, injecting fertilizer N through the irrig ation system also may improve production. The objective of this study was to determine the effect of timing of limited-amount irrigation and N fertigation on grain sorghum yield; yield components; grain N conte nt; and N uptake at the 9-leaf, boot, and soft dough stages. The exper iment was conducted from 1984 to 1986 on a Parsons silt loam (fine, mi xed, thermic, Mollic Albaqualf). The experiment was designed as a 6 x 2 factorial plus two reference treatments. Six timings for irrigation were targeted at the 9-leaf (9L), boot (B), soft dough (SD), 9L-B, 9L- SD, and B-SD growth stages. N application systems were either 112 kg N ha-1 surface-banded preplant or 56 kg N ha-1 preplant and 56 kg N ha- 1 injected through the irrigation at a rate of 28 kg N ha-1 per 2.5 cm of irrigation. Two reference treatments included were one receiving N but no irrigation and one receiving neither N nor irrigation. In 1984 , irrigation generally increased grain sorghum yield by nearly 1 Mg ha -1. However, yield was not affected by selection of irrigation timing, N application method, or the interaction of the two factors. This was partly because early irrigations increased kernels/head, whereas late r irrigations increased kernel weight. Above average rainfall during t he growing season, especially just prior to the 9-leaf, boot, and soft dough growth stages, resulted in no irrigations in 1985. In 1986, yie ld was increased by early (9-leaf) irrigations as compared to soft dou gh irrigations. Early irrigations resulted in higher kernels/head; how ever, rainfall after the soft dough irrigation may have masked any tre atment effect on kernel weight. As in 1984, N application method did n ot affect grain sorghum yields, even though yield was reduced to less than 3 Mg ha-1 with no N nor irrigation. In both 1984 and 1986, N upta ke at succeeding growth stages appeared to respond to irrigations made at previous growth stages. Injecting half of the fertilizer N through the irrigation system did not affect N uptake compared to applying al l N preplant. The lack of response to fertigation may be related to th e low leaching potential of the soil used in this study.