Influence of carbohydrate or protein supplementation on intake, behaviour and digestion in dairy cows strip-grazing low-nitrogen fertilized perennial ryegrass

The effect of concentrate composition on intake, grazing behaviour and ruminal fermentation was studied on dairy cows strip-grazing on perennial ryegrass swards receiving a low level of nitrogen (N) fertilization. Four treatments were compared in spring 1994 on eight cows according to a 4 x 4 Latin square design: 1) control with no supplement (NO), 2) rapidly fermentable wheat concentrate (WHE), 3) slowly fermentable maize and soya-bean hulls concentrate (MSH), and 4) protected soya-bean meal (SBM). The supplementation level was 2.5 kg organic matter (OM).d(-1). Individual herbage intake was calculated by estimating faecal output (using chromic oxide) and herbage digestibility (faecal N and ADF as indicators). Herbage intake was 13.7, 13.4, 13.8 and 15.8 kg OM.d(-1) for NO, WHE, MSH and SBM, respectively. The intake level for unsupplemented cows was low and appears to have been mainly limited by a strong deficiency in ruminally degradable protein (ruminal ammonia: 0.8 mmol.L-1; blood urea: 1.2 mmol.L-1). WHE and MSH had no effect on the level of herbage intake, while total digestible OM intake was increased by 2.0 kg. Carbohydrate supplementation had no effect on grazing behaviour or on the intensity of ruminal fermentation. The SBM concentrate clearly increased herbage intake by way of improved ruminal digestion due to the input of N. Supplementation with SBM increased ruminal ammonia (+0.7 mmol.L-1), total volatile fatty acids (VFA) (+14 mmol.L-1), as well as blood urea (+2.5 mmol.L-1), estimated total neutral detergent fibre (NDF) digestibility (+0.05) and OM digestibility (OMD) of herbage (+0.02). A stimulatory effect of nutritional status, protein mainly, on intake cannot be excluded, as suggested by behavioural data. ((C) Elsevier / Inra).