Utilising temperate and tropical pastures and compensatory growth during winter to maximise growth in cattle

Abstract

The effect of combinations of temperate and tropical pastures, and of interactions with compensatory growth, on liveweight gain in steers was investigated over the winter-spring period in the subtropics. Sixteen Brahman crossbred steers (294 ± 4.4 kg; ± s.e.) were either not supplemented (low growth rate; L) or supplemented with silage and with a mixed sorghum-barley grain-based ration (2 kg/hd/d) (high growth rate; H) in autumn. Growth rates over ca. 100 d (hereafter autumn) for L and H groups were –0.21 and 0.71 kg/d, and liveweights at the end of autumn were 270 ± 5.4 and 365 ± 8.2 kg, respectively. Steers were re-allocated within original groups so that half grazed ryegrass (Lolium multiflorum) cv. Aristocrat alone (LR, HR) and half grazed ryegrass (3 d/week) and carryover pangola grass (Digitaria eriantha) (4 d/week) (LRP, HRP). Liveweight gains over the next 88 d (winter-spring) were 1.17 ± 0.056, 1.05 ± 0.055, 0.86 ± 0.057 and 0.67 ± 0.037 kg/d, and final liveweights were 373, 362, 441 and 423 kg for LR, LRP, HR and HRP, respectively. Growth rates over winter–spring were higher (P < 0.05) for steers grazing ryegrass alone than for the combined grazing treatment, and were higher (P – 0.05) for unsupplemented steers than for those supplemented in autumn, but there was no interaction between winter–spring and autumn treatments. Over the winter-spring grazing period, herbage mass on offer averaged 2725 and 4280 kg/ha dry matter (DM) for the ryegrass and pangola grass pastures, respectively. Average green leaf percentages were 43 and 22%, and within this component crude protein (CP) concentration was 24–29% and 11–14% and in vitro DM digestibility (IVDMD) was 75–78% and 56–62%, respectively. The concentrations of ammonia-nitrogen in rumen fluid from steers grazing ryegrass or pangola ranged from 93–126 mg/L, with no treatment effects.
The results indicate that carryover tropical pasture can be used in practical feed plans to augment the limited supply of temperate pasture, thereby reducing costs with only small reductions in animal performance. They also indicate that severe restrictions in growth rate during the autumn period will result in compensatory growth later on, but may jeopardise attainment of market specifications for heavy carcases at a young age.