Spray-topping annual grass pasture with glyphosate to delay loss of feeding value during summer. II. Herbage intake, digestibility, and diet selection in penned sheep

The treatment of annual ryegrass-silver grass pasture with the herbicide gl yphosate at seed head emergence delayed the loss of soluble carbohydrates a nd improved herbage digestibility. The present study investigated whether t his improvement in nutritive value had any influence on the diet preference , intake, and performance of housed sheep. Since the effects of glyphosate on digestibility were most marked in stem tissue, preference was examined n ot only in relation to sprayed v. unsprayed herbage, but also in terms of p reference for individual plant parts. In the preference testing, sheep showed a marked preference for sprayed rat her than unsprayed herbage. They also consumed more of it, such that their digestible dry matter intake from sprayed herbage was nearly twice that fro m unsprayed herbage. When sheep were offered different plant fractions from the control herbage, they selected against stem material and preferred lea f or seed head. By contrast, when offered fractions from the sprayed materi al, they preferred stem to leaf, but showed no strong preference for stem o ver seed head. Preference was related more to the soluble carbohydrate cont ent of a plant part than its digestibility, and only when soluble carbohydr ate contents were similar was there preference for the more digestible mate rial. The consumption of sprayed herbage, compared with control herbage, led to h igher concentrations of rumen volatile fatty acids in the period 4 h after feeding. Moreover, the feeding of these herbages over a 6-week period resul ted in significant reductions in pre-feeding plasma urea and beta-hydroxybu tyrate concentrations. When control herbage was fed with supplements of fru ctose, urea, or fructose+urea, in an attempt to mimic the possible differen ces in nutritive value between control and sprayed herbage, the responses i n rumen and blood metabolites were less marked than when sprayed herbage wa s fed. As a consequence of the higher nutritive value of the sprayed herbage and t he higher intakes by the animals fed this material, sheep fed sprayed herba ge were able to maintain weight over the feeding period, whereas those offe red the control herbage lost weight at 130-150 g/day.