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.