Choice of grass or maize silages by lactating dairy cows: influence of supplementary protein, concentrate level and milk yield

Preference testing of two foods provides a means of assessing the relative value an animal places on the foods. Two experiments examined the relative choice of grass and maize silages by lactating dairy cows, and investigated whether protein supplementation, concentrate level or milk yield level of cow influenced this choice. In experiment 1, 16 cows including eight high (30 kg/day) and eight low (17 kg/day) yielding (milk level, ML) cows were offered a low (171 g/kg dry matter (DM)) or a high (300 g/kg DM) crude protein (CP) concentrate in eight 2 x 2 Latin squares. In experiment 2, four cows were offered a low (112 g/kg DM) or a high (334 g/kg DM) CP content of concentrate at a low (3.5 kg/day) or high (7 kg/day) concentrate level (CL) in a 4 x 4 Latin-square design.CP of concentrate did not influence the proportion of grass silage to maize silage eaten in either experiment. Also, neither ML of cows (experiment 1) nor CL (experiment 2) affected proportions of the two silages eaten. The mean proportions selected were 0.88:0.12 and 0.52:0.48 for grass and maize silages in experiments 1 and 2 respectively, which reflected differences between experiments in the digestibilities of the silages, and indicated that digestibility may be an important factor influencing the choice of silages. The CP, CL and ML treatments produced significant responses in silage and total DM intakes. Feeding behaviour studies showed the responses to CP were associated with differences in meal size, which is a function of duration of each meal and rate of intake. The number of meals taken per day was similar for all treatments and averaged 8.0 and 8.1 for experiments 1 and 2 respectively. It was concluded that whilst lactating cows modified their feeding behaviour in response to the diets available, there was no evidence they modified the proportions of the two silages selected in order to meet a target protein content in the total diet. It would appear therefore that other factors, of which digestibility may be important, have a higher priority in choice.