Diet choice by dairy cows. 1. Selection of feed protein content during the first half of lactation.

Effects of weeks in milk and milk yield on diet choice were recorded during the first half of lactation. Low and high protein feeds were used, and both consisted of 30% concentrate and 70% grass silage (fresh matter basis). Both feeds contained similar amounts of energy, but the crude protein contents were 131 and 185 g/kg of dry matter (DM), respectively, for the low and high protein feeds. In a nutrient flow experiment with three cannulated lactating cows, the metabolizable protein yields of the low and high protein feeds were 75 and 114 g/kg of DM, respectively. Thirty-seven cows were divided into control groups for the low and high protein feeds, and a choice group had access to both the low and the high protein feeds. Intake of DM and milk yield by cows in the control group fed the high protein feed were higher than those by cows in the control group fed the low protein feed, but these measurements did not differ from those of cows in the choice group. Cows in the choice group consumed a mean of 683 g of high protein feed/kg of total intake, which differed from what would be considered random intake (500 g/kg total intake). Diet choice did not systematically change during the experiment and was not correlated with weeks in milk, milk yield, or milk protein output. We concluded that diet selection differed significantly from what would be considered random, which allowed cows in the choice group to perform well. However, diet choice did not reflect the estimated metabolizable protein requirements of the cows.