Growth and rumen digestion characteristics of steers grazing autumn grass supplemented with concentrates based on different carbohydrate sources

The objective of this study was to determine the effects on animal performa nce and rumen digestion characteristics of supplementing cattle grazing aut umn grass offered at two levels of allowance, with concentrates formulated from different carbohydrate sources. Seventy-two steers (494 (s.e. 38.4) kg ) were assigned to one of six treatments which were arranged in a two grass allowances (5.5 or 11 kg grass dry matter (DM) per head per day) by three concentrate types (starch, starch + fibre or fibre-based diets) factorial d esign. The experiment commenced on 22 August and had a mean duration of 84 days. Concentrates were formulated to have similar metabolizable energy and crude protein and animals were offered 5 kg per head daily. Grass allowanc es were offered daily and concentrates were offered individually. Six rumen fistulated steers were used concurrently in a Latin square design to deter mine the effect of the diets on rumen digestion characteristics. Grass and concentrates degradabilities were measured using the nylon bag technique. T he concentrate DM was degraded at 0.189, 0.102 and 0.094 per h for the star ch, starch + fibre and fibre-based diets, respectively. Concentrate carbohy drate source and grass allowance did not affect (P > 0.05) degradation rate of grass DM or nitrogen in the rumen, rumen fluid pH, rumen fluid concentr ation of volatile fatty acids, total lactic acid or ammonia concentration o r plasma concentration of glucose, urea or beta-hydroxybutyrate. Animals of fered the low grass allowance had larger (P < 0.05) diurnal variation in th e rumen fluid acetate: propionate ratio than those offered the high grass a llowance. Increasing grass allowance increased (P < 0.01) carcass gain wher eas concentrate type had no effect. There was no interaction between concen trate carbohydrate source and grass allowance on any of the production vari ables measured. It is concluded that the rumen environment in cattle grazin g autumn grass buffered the effects of concentrate carbohydrate degradabili ty such that similar productivity was achieved with each concentrate.