The relationship between live-weight and herbage intake in grazing cattle

Abstract

Six dry cows, six in-calf heifers and six yearling calves grazed in groups of six, arranged either in single age groups or mixed age groups; the arrangements alternated, and each was repeated three times in a continuous grazing sequence with six-day periods. All the animals grazed together for a preliminary period of seven days. Under each arrangement the cattle grazed on a mature sward for two periods, and on an aftermath for one period. Access to fresh and grazed herbage was restricted by electric fences which were moved daily, but the amount of fresh herbage offered was well in excess of appetite at all times. Records of live-weight, herbage intake, grazing behaviour and social dominance ranking were obtained for all individuals. There was a close relationship between live-weight (LW) and herbage organic matter intake (OMI) in each grazing period, and the regression equations for the six periods were very similar. The best-fit exponent of LW for the pooled results was 0·61±0·038; this was significantly different from 0·73 and 1·0. The exponent increased to 0·71±0·045 when the cow data were excluded. The inclusion of weight change as a third variable was only of value in the mature sward. The calves ate significantly more, and the cows tended to eat more, when grazing in mixed age groups. These differences could not be explained by differences in the time spent grazing. There were high simple correlations between LW and OMI, between social dominance rank and OMI, and between LW and rank, particularly in the mixed age groups. When variations in LW were excluded, dominance rank had little influence on OMI in the mixed age groups, but the independent effects of LW and social rank on OMI were of approximately equal importance in the single age groups.