Influence of energy source and dietary protein degradability on the voluntary intake and digestibility of barley straw by pregnant beef cows

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Abstract
Three experiments, designed as three simultaneous 4 × 4 Latin squares (21-day feeding periods), were conducted using pregnant beef cows. Diets were based on barley straw given ad libitum together with 1·7 kg dry matter (DM) of either molassed sugar-beet pulp (MSBP) (experiment 1), unmolassed sugarbeet pulp (USBP) (experiment 2), or rolled barley (BARLEY) (experiment 3). In the respective experiments, MSBP, USBP and BARLEY were given either alone, to provide about 200 g crude protein (CP) per day (CONTROL) or were supplemented with urea (U), untreated soya-bean meal (SBM) or formaldehyde-treated soya-bean meal (FT-SBM) to provide a further 200 g CP per day. The contrasting dietary treatments in the three experiments provided intakes of rumen-degradable protein (RDP) ranging from 16 to 388 g/day.All protein supplements improved overall straw DM intakes, the improvement being significant for U only (P < 0·01). A linear relationship between the daily intake of RDP (g) and straw DM (kg) was obtained. The regression equation was:DM intake = 5·03 + 0·0035 × RDP (R2 = 0·703; residual s.d. 0·561; d.f. 35; P < 0·001).All protein supplements also improved the apparent digestibility of straw organic matter (OM) (P < 0·05) and the metabolizable energy (ME) obtained from straw (P < 0·01). Blood urea was increased due to supplementation with urea (P < 0·001), SBM (P < 0·01) and FT-SBM (P < 0·05). Supplementation with BARLEY appeared to improve straw DM intake over MSBP and USBP but this was not reflected in improved ME intake from straw.

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