Spray-topping annual grass pasture with glyphosate to delay loss of feeding value during summer. III. Quantitative basis of the alkane-based procedures for estimating diet selection and herbage intake by grazing sheep

Previous papers in this series have demonstrated that, when annual pastures were sprayed at seed head emergence with low rates of the herbicide glypho sate, the nutritive value of the sprayed herbage was improved. Housed sheep preferred sprayed herbage to unsprayed herbage and, within sprayed herbage , appeared to have a preference for stem material. The housed sheep ate mor e of the sprayed material and their weight gains were improved. If such res ponses occurred in grazing animals, they could result in substantial improv ements in animal liveweights or wool production. The present paper describes the alkane-based procedures used to investigate diet selection and herbage intake in grazing animals, with the emphasis pl aced on methodological and statistical issues. The alkane concentrations in the plant fractions (leaf, leaf sheath, stem, seed head) of sprayed and un sprayed herbage are reported. Similarly, the alkane concentrations in extru sa samples collected by oesophageally fistulated sheep given access to the grazed areas, and in the faeces of sheep grazing the areas, are reported. P atterns of alkane concentrations in extrusa and faeces were similar and ind icated that, in the short term, oesophageally fistulated animals consumed a diet of similar composition to that selected by the animals resident on th e plots. Using these data for alkane concentrations, multivariate statistic al analyses are presented which provide an objective basis for selecting th e alkanes to be used in the estimation of diet selection. The results of the first of 4 grazing periods are presented, as an example of the application of the alkane-based procedures for estimating herbage in take and, in particular, diet composition. Use of these techniques indicate d that, when given access to sprayed senescent pasture, sheep consumed more digestible dry matter and selected more of it from the stem fraction than was the case with unsprayed herbage. These results are similar to those fou nd with housed animals and demonstrate that patterns of herbage and faecal alkane concentrations can be used to estimate diet composition of grazing a nimals in terms of the plant parts on offer.