CANOPY STRUCTURE AND NUTRITIVE-VALUE OF LIMPOGRASS PASTURES DURING MIDSUMMER TO EARLY AUTUMN

Cattle (Bos spp.) grazing warm-season, perennial grasses in the lower Southeast USA often gain at a slower rate during mid-summer to early autumn than they do before or after this season. This has been observed with 'Floralta' limpograss [Hemarthria altissima (Poir.) Stapf et C.E. Hubb.], and speculation is that low average daily gains (ADG) may result from changes in canopy structure or herbage nutritive value. During four consecutive 21-d periods beginning in July 1987 and 1988, a study was conducted on Pomona sands (sandy, siliceous, hyperthermic Ultic Haplaquods) to detect periods of low ADG on rotationally grazed limpograss, and to determine if changes in canopy structure or herbage nutritive value occurred concurrently. Canopies were stratified into two layers of equal depth during each period to characterize herbage bulk density, plant part composition, herbage crude protein (CP), and in vitro digestible organic matter (IVDOM). Additionally, the potential of limited protein supplementation [none, 0.63 kg d-1 (LO) of a 21% CP corn (Zea mays L.)-urea mixture, or 0.73 kg d-1 (HI) of a 50% CP corn-urea mixture] to prevent the decline in ADG of steers grazing limpograss was evaluated. Daily gains of steers receiving the 50 and 21% CP supplement treatments (0.59 and 0.53 kg, respectively) were greater than those of nonsupplemented steers (0.29 kg) during the 84-d trial. Regardless of supplement treatment, steer ADG was lowest during the second period in 1987 (30 July-20 August) and third period in 1988 (25 August-15 September). Upper (UL) compared with lower canopy layer (LL) had three to six times greater leaf blade to stem plus sheath ratio, 30 to 50% as great a bulk density, 5 to 10% greater IVDOM, and 25 to 100% greater CP. Canopy factors associated with low gain were not identified conclusively, but during periods of low steer ADG, LL stem plus sheath mass was greater than for other periods, whole canopy CP concentration was lower or tended to be lower, and pre-graze canopies were tall (57-60 cm), possibly indicating greater physiological maturity of herbage.