Turnips with chemically-treated straw for beef production 2. Effect of turnips on the degradability of straw in the rumen

Abstract

ABSTRACTThe nylon bag technique was used to determine the effects of offering to steers an allowance of turnips in a diet based on straw on the degradation of dry matter and acid-detergent fibre of the straw. Rumen-cannulated steers were offered basal diets of (a) sodium hydroxide (NaOH)-treated strawad libitum, (b) NaOH-treated straw plus turnips (50 g dry matter per kg W0·73) offered once daily or (c) turnipsad libitum. Samples of straw, treated with NaOH, anhydrous ammonia (NH3,) or untreated straw were incubated in nylon bags in the rumen along with each of the basal diets. Additionally molasses (66 g dry matter per kg W0·75) was infused into the rumen of the steers offered the NaOH-treated straw. Dry matter and fibre losses of straw from nylon bags were in the order NaOH-treated > NH3,-treated > untreated; there was no interaction (P> 0·05) between method of straw treatment and effect of basal diet on 40-h or 72-h dry matter or fibre degradability. Supplementation of the basal straw diet with either turnips or molasses depressed (P< 0·01) degradability of both dry matter and fibre of the incubated straw at 40 and 72 h, molasses infusion producing a greater depression (P< 0·05) than offering turnips. There was a fall in rumen fluid pH shortly after steers consumed their turnip allowance or molasses was infused into the rumen, minimum pH values were 6·09 ± 0·241 and 5·75 ± 0·205 respectively. The pH of rumen fluid and degradability of dry matter and fibre of barley straw were depressed by the presence of carbohydrates in turnips and molasses. A reduction in the activity of cellulolytic bacteria may have been the cause of the depression in straw degradability.