Foraging behaviour of beef heifers and ewes in natural grasslands with distinct proportions of tussocks

Abstract: In natural grasslands, plant species preferred by grazing animals are intermixed with less preferred species. We hypothesise that increasing proportion of Eragrostis plana Nees tussocks, a non-preferred diet item in natural grassland, will constrain the accessibility of the preferred diet item for grazing animals, and sheep would be less restricted than cattle. The experimental paddocks contained approximately 0, 25, 50 and 75% E. plana, in vegetative stage. Inter-tussock areas were proportionally maintained at a non-limiting forage allowance. Records were made every 1-min of the stratum (tussock, inter-tussock or intra-tussock) grazed by each of four beef heifers or four ewes. Short-term intake rate was calculated for each 45-min grazing session by weighing the animals before and after grazing, and correcting for insensible weight losses (evaporative and gaseous losses). Beef heifers reduced grazing time on the inter-tussock areas by 0.60% per each 1% increment in tussock cover, whilst ewes exhibited a smaller reduction of 0.36% (P <0.05). Virtually all of beef heifers’ reduction in inter-tussock grazing time was replaced by tussock grazing. However, the ewes were more selective and attempted to find preferred food in the intra-tussock stratum. Ewes adjusted their foraging strategies and maintained a constant short-term intake rate regardless of percentage of tussock cover. Beef heifers exhibited the highest short-term intake rate with 34% tussock cover. Beef heifers bite mass decreased when tussock cover increased over 44%, whereas no trend was detected for ewes. Tussocks may act as a vertical and/or horizontal barrier interfering with the process of bite formation and affecting bite mass of beef heifers. Conversely, ewes were able to adapt their grazing behaviour with increasing tussock cover, so that no effects on the measures taken in this study were observed. This response is consistent with the finer resolution of grazing selectivity by ewes and with the allometry of short-term intake rate. [Copyright &y& Elsevier]