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1 Animal Biology Division, Scottish Agricultural College, The King's Buildings, Edinburgh EH9 3JG, United Kingdom
2 Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth SY23 3EB, United Kingdom
3 BOCM PAULS Ltd., 47 Key Street, Ipswich IP4 1BX, United Kingdom
Diet choice, expressed as grams of high protein feed selected per kilogram of intake, was measured for lactating cows with ad libitum access to two feeds. First, cows were given a choice between a low protein feed and a high protein feed for 9 wk while feeds were alternated between feeders. The proportion of feed chosen from a feeder depended on the feed it contained, which showed that cows selected for feed characteristics. Diet choice was then measured over six periods during which cows had access to either a low protein feed and a high protein feed or to these feeds plus added urea. Diet choice was lower when urea was added, suggesting a substantial effect of the content of ruminally degradable protein (RDP) in feeds on diet selection. All feeds had equal RDP contents in Experiment 3 when early and late lactating cows had access first to two feeds with a low and high yield of metabolizable protein and then to two feeds with a low and medium yield of metabolizable protein. Diet choice did not differ from what was considered to be random and was not affected by milk yield or stage of lactation. No evidence suggests that cows selected for metabolizable protein when both feeds contained adequate RDP.
Key Words: cows • lactation • diet selection • protein
Submitted on January 28, 1998
Accepted on June 15, 1998
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