The partial replacement of silage with straw, with or without barley or soybean meal, in rations for winter calving beef cows

Abstract

The effects of partially replacing grass-legume silage with barley straw, without or with either supplemental energy or protein, were assessed in 72 lactating beef cows. Diets were fed from calving in January-February until turnout to pasture in late May. Performance of cows and their calves was measured over this period, and weight and body condition were measured at weaning in September. Apparent digestibility and N balance of these diets was also assessed in growing steers. Replacing half the silage with barley straw reduced diet digestibility (P < 0.05), intake (P < 0.05), body weight (BW) gain (P < 0.001) and body condition (P < 0.001) of cows but did not affect BW gain or weaning weights of calves. Reproductive performance was also unaffected, although overall production efficiency (kg weaned calf kg-1 cow BW) was impaired (P < 0.10). When the silage-straw diet was supplemented with barley, metabolizable energy (ME) intake was increased to the level supplied by the all-silage diet. However, there was a tendency for cows to partition more ME to calf growth and less to body reserves as indicated by body condition score (P < 0.05). This may have been related to better N utilization, since N losses in urine were lower for steers fed the barley-supplemented mixture than those fed silage (P < 0.01). When the silage-straw was supplemented with soybean meal (SBM) to increase crude protein (CP) to equal the all-silage diet, there were no benefits for the cow or calf, because SBM reduced intake relative to silage fed alone (P < 0.001). Thus, although supplementing with SBM improved N balance, relative to silage (P < 0.01), there was no response in calf performance, probably because of insufficient ME intake in the cow. Although silage appears not to be well balanced for the nutrient requirements of lactating beef cows, attempts to optimise nutrient supply did not markedly improve growth performance of the calf or reproductive performance of the cow. Key words: Beef cow, cow-calf, silage, straw, gains, weaning weight, reproduction