The relationship between the digestibility of a sward and the herbage consumption of grazing calves

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Abstract
SUMMARY: Nitro-chalk was applied to S.23 ryegrass swards, at approximately monthly intervals, at two contrasting levels in 1961 and three levels in 1962. Steer calves, 3–6 months old, grazed in rotation round a series of four paddocks on each treatment. Paddocks were trimmed and fertilizer applied after each grazing. The herbage intakes of the calves were determined on three occasions in 1961 and four occasions in 1962. Faecal output was estimated by chromic oxide dilution. In vitro digestibility determinations were carried out on samples of herbage clipped from the swards.There was a close linear relationship between the digestibility of the herbage and the amount eaten, over the full range of digestibility encountered (68–82%). Regression equations calculated within seasons and fertilizer levels, and within years, did not differ significantly. The regressions of faecal output on herbage digestibility were not significantly different from zero.The observed relationship between herbage digestibility and herbage intake may reflect (a) the greater sensitivity of young ruminants than adult stock to changes in the digestibility of the diet, and (b) the reduced importance of the chemostatic control of food intake, except at very high levels of digestibility, in young rapidly growing animals.