The usefulness of grazing tests for studying the ability of sheep and cattle to exploit reproductive patches of pastures

Abstract

Reproductive patches are a poor-quality but highly available feed resource of pastures which are partly lost because animals preferentially graze vegetative regrowths. This study was made to assess the usefulness of short-duration measurement periods to determine the ability of individual animals or species to graze on reproductive patches associated with vegetative patches. The grazing choices of heifers and ewes on cocksfoot plots half vegetative and half reproductive in area, with vegetative swards at 7 (T7) or 11 (T11) or 18 cm (T18) were recorded over a whole day or during 30-min tests. Preferences were defined as the proportion of grazing time that was spent feeding on each sward type. Both sets of measurements showed that the heifers consistently grazed reproductive patches longer than the ewes. The heifers' preference for vegetative patches increased linearly with sward height from 41 to 77% (T7-T18) in tests and from 27 to 79% over a day. The ewes' preference for vegetative patches was constant between T11 and T18 in both test and whole day recordings (99 and 92%, respectively), and only slightly decreased, to 89 and 75% for 7-cm high swards. In contrast the results on grazing preference within each species (heifers and ewes) showed a difference in the rank of individual animals between the two sets of measurements. The limitations of estimating preference from results obtained during short grazing periods are discussed.