DIET CHOICE BY DAIRY-COWS - 2 - SELECTION FOR METABOLIZABLE PROTEIN OR FOR RUMINALLY DEGRADABLE PROTEIN

Diet choice, expressed as grams of high protein feed selected per kilo gram of intake, was measured for lactating cows with ad libitum access to two feeds. First, cows were given a choice between a low protein f eed and a high protein feed for 9 wk while feeds were alternated betwe en feeders. The proportion of feed chosen fi om a feeder depended on t he feed it contained, which showed that cows selected for feed charact eristics. Diet choice was then measured over six periods during which cows had access to either a low protein feed and a high protein feed o r to these feeds plus added urea. Diet choice was lower when urea was added, suggesting a substantial effect of the content of ruminally deg radable protein (RDP) in feeds on,diet selection. All feeds had equal RDP contents in Experiment 3 when early and late lactating cows had ac cess first to two feeds with a low and high yield of metabolizable pro tein and then to two feeds with a low and medium yield of metabolizabl e 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 ev idence suggests that cows selected for metabolizable protein when both feeds contained adequate RDP.