The digestion of perennial ryegrass (Lolium perenne cv. Melle) and white clover (Trifolium repens cv. Blanca) by grazing cattle

Abstract

1. Pure swards of perennial ryegrass (Loliwn perenne cv. Melle) as a primary growth (May), trimmed primary growth (early June) and regrowth (late June), and white clover (Trifolium repens cv. Blanca) as a mature primary growth (July) and vegetative regrowth (August), were grazed by twelve Friesian steers (mean body-weights throughout experiment 152–231 kg) at daily allocation rates of forage which provided dry matter (DM) intakes ranging from 28 to 36 g/kg body-weight). 2. Total nitrogen contents of the three ryegrasses declined with season (37–20 g/kg DM), but in vitro organic matter (OM) digestibilities were relatively constant (0.76–0.79). The clovers had higher N (average 366 g/kg DM) and lower fibre and water-soluble carbohydrate contents than the grasses, and in vitro OM digestibilities of 0.70 and 076 respectively. 3. Duodenal digesta samples were obtained, using a portable sampling apparatus from the animals grazing the pasture, and estimates of the flow of nutrients into the small intestine were derived using two indigestible markers which were continuously infused into the rumen using a portable infusion apparatus. Forage intakes were calculated from estimates of faecal output of indigestible OM and the predicted in vivo OM digestibilities of the forages consumed. Coefficients of variation for OM flow to the small intestine and OM intake were 11.8 and 10.9 % respectively. 4. The apparent digestion of OM in the rumen ranged from 722 to 741 g/kg digestible OM intake and from 681 to 711 g/kg digestible OM intake for the grass and clover diets respectively. Substantial losses of ingested N before the small intestine were measured on all diets except the regrowth ryegrass. Losses were 0.30 and 040 g/g N intake on the primary growth ryegrass and the regrowth clover respectively; N contents were 37 and 39 g/kg DM respectively. 5. Comparison of the values obtained at pasture with that obtained when similar diets were offered to housed cattle (Beever et al. 1985) indicated that combined relations relating duodenal OM and non-ammonia-N (NAN) flows to OM and N intakes respectively could be established for each diet. For two diets (primary-growth grass and regrowth clover) the relations were curvilinear; for the remaining diets, the derived relations were linear. 6. The effects of forage species, stage of maturity and variations in the stem: leaf ratio in the grasses, on nutrient supply in grazing animals are discussed, and a combined relation for all grazed and harvested forages examined in this and in a previous study reported by Beever et al. (1985) is established. The relation Y = 1.430 - 0.0169X (r072, residual sd 0.140). where Y is NAN flow/N intake (g/g) and X is N content in the crop (gfkg OM), indicates that net losses of N across the rumen will occur on diets which contain more than 25.5 g N/kg OM. 7. Possible limitations in the techniques available to measure nutrient digestion and supply in grazing ruminants are discussed.