RESUMO
Three sorghum x Sudangrass hybrids were planted in twelve 0.2-ha plots to test the effect of date of harvest and hybrid on plant maturity, DM yield, chemical composition, and in situ DM and fiber disappearance. Sweet Sunny Sue (a non-brown midrib (BMR) hybrid; nonBMR), NutriPlus BMR (a BMR hybrid; NP-BMR), and Dry Stalk BMR (a BMR hybrid; DS-BMR) were planted on 26 June 2003 at 22.4 kg of seed/ha. Beginning 34 d after planting, plant height and phenological growth stage were assessed weekly in 10 random, 0.5-m(2) quadrats per plot. Plants were clipped to 2.5 cm in height and analyzed for CP, NDF, and ADF using near-infrared spectroscopy. Composite samples harvested from each plot on d 34, 48, and 63 were incubated in the rumen of 3 steers to determine the in situ disappearance of DM and NDF in a 3 x 3 Latin square. Forage yield was greater (P < or =0.02) for nonBMR than NP-BMR on d 41 and 55 and tended (P = 0.08) to be greater on d 48. The DS-BMR hybrid produced more (P = 0.04) forage DM than the NP-BMR on d 48. When DM yield was regressed on growth stage at harvest, BMR hybrids were predicted to produce 265 kg/ha more DM (P < 0.01) than nonBMR, at the late-boot stage. At all harvest dates, NDF concentrations were less (P < or =0.02) for BMR than nonBMR. The DS-BMR had greater (P < or =0.02) NDF concentrations than NP-BMR on d 41, 48, 55, and 63. Detergent fiber concentrations were predicted to be greater (P < 0.01) in nonBMR than BMR when regressed on growth stage at harvest, but the magnitude of the differences in fiber concentration diminished with growth stage. The A fractions of DM and NDF were greater (P < 0.01) and the C fraction was less (P < 0.01) for BMR hybrids than nonBMR. The B fraction of DM was not affected (P = 0.15) by hybrid type. The B fraction of NDF was not different (P = 0.28) on d 34 but was greater (P < 0.01) on d 48 and 63 for BMR than nonBMR. Effective degradability of NDF and DM was greater (P < 0.02) for BMR than nonBMR on all harvest dates. The A fraction of DM was less for DS-BMR than NP-BMR (P < 0.01), but the B and C fractions of NDF and DM did not differ (P > 0.13) between BMR hybrids. This research indicates that forage chemical composition and ruminal in situ disappearance are improved in the BMR sorghum x Sudangrass hybrids tested compared with the nonBMR. Yield reductions are commonly reported for BMR hybrids, but predicted DM yields in the current study were not reduced if harvested at a similar phenological growth stage.
