Provision of certain carbohydrate-based supplements to pasture-fed sheep, as well as time of harvesting of the pasture, influences pH, ammonia concentration and microbial protein synthesis in the rumen

Abstract

A series of experiments was conducted to test the hypothesis that ‘synchronising’ the availability of carbohydrate and nitrogen (N) in the rumen of sheep, either by the feeding of carbohydrate-based supplements in relation to pasture intake or by provision of pasture with a higher ratio of these 2 nutrients, would benefit microbial protein synthesis (MPS). The response variables were rumen pH and ammonia (NH3), and urine allantoin was measured as an indicator of MPS. The animals used were adult castrate male sheep fitted with a rumen cannula. In experiment 1, rumen NH3 and pH were found to be at a maximum and minimum, respectively, 1–2 h after feeding kikuyu (Pennisetum clandestinum) grass and 3–4 h after feeding biennial ryegrass (Lolium multiflorum) with rumen N degradability (at 18 h in sacco) being 55 and 75%, respectively. In experiment 2, the infusion of sucrose at 4 h post-ingestion of ryegrass pasture (the anticipated time of peak rumen NH3 concentration) lowered peak NH3 concentration by 40 mg/dL (from 187 to 147 mg/dL) and raised urine allantoin excretion from 6.0 to 9.0 mmol/sheep.day compared with infusion of sucrose at –1, +1 or +7 h. When sucrose was infused, 1 h after being fed kikuyu (synchronous) at the expected time of peak rumen NH3 concentration, the NH3 peak (from 152 to 110 mg/dL) and rumen pH were reduced and urinary allantoin excretion raised (from 6.6 to 7.6 mmol/sheep.day) compared with sheep given sucrose 7 h after ingestion of kikuyu grass (asynchronous). In experiment 3, the feeding of fine-rolled barley grain at, or 2 h before, feeding kikuyu grass, to ‘synchronise’ the availability of rumen-fermentable carbohydrates (RFC) in the grain with the availability of N released from the grass, substantially reduced rumen NH3 and pH, and increased urine allantoin levels (from 10.1 to 11.8 mmol/sheep.day) compared with feeding barley at 4 or 6 h before feeding grass. In experiment 4, sheep were fed kikuyu or ryegrass cut in the early morning (AM) or late afternoon (PM), with the nutrient content varying only in water-soluble carbohydrates (64 v. 114 g/kg DM for ryegrass and 41 v. 80 g/kg DM for kikuyu, respectively) and percentage of dry matter. Sheep fed PM-ryegrass had a significantly lower rumen pH compared with sheep fed AM-ryegrass but there was no difference in rumen NH3 concentrations. Urinary allantoin secretion was substantially higher in the PM group than the AM group (8.7 v. 3.9 mmol/sheep.day). When sheep were fed PM-kikuyu the pH fell to lower values than for the AM-kikuyu (6.4 v. 6.8) and there was again no effect on rumen NH3 levels. Urine allantoin levels were higher (P = 0.006) (6.2 v. 5.6 mmol/ sheep.day) in PM-fed sheep. The results of these studies support the hypothesis that there are benefits in synchronising the availability of RFC with N in the rumen on MPS but this is not always associated with significant changes in rumen pH and NH3. The implications of these results are discussed in relation to the situation in which synchronisation of RFC and N may potentially benefit dairy cattle.