Diet Choice by Dairy Cows. 2. Selection for Metabolizable Protein or for Ruminally Degradable Protein?

Abstract

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.

Diet selection and animal state: an integrative framework

“…The idea that there is a 'requirement' for degradable protein is therefore not an unreasonable one. The results of a series of subsequent experiments were consistent with the idea that cows, given a choice between foods with different RDP contents, selected sufficient RDP and also avoided eating an excess (Tolkamp et al 1998b). These findings need to be reconciled with the results of other research in which ruminants seemed to be able to select a diet that reflected their protein needs according to their physiological state (Kyriazakis & Oldham, 1993;Cooper et al 1994;Kyriazakis et al 1994).…”

Effect of maize processing on diet selection in cows

“…However, despite the same chemical composition of diets, there are physical differences among processed maize grains that lead to degradation variability. These results seem to agree with the hypothesis of Tolkamp et al (1998) who, after concluding that animals did not actually select their diets based on feeding environment (feed bunk, feed position), proposed that animals could be choosing feed according to its ruminal degradable protein (RDP) content. However, they could not conclude which factor led the animals to make non-random choices.…”

“…Kyriazakis and Oldham (1993) sheep were able to select a diet that met their crude protein (CP) requirements and thereby avoid feeding excess protein. Lawson et al (2000) suggested that protein demand might guide lactating cows' diet selection, although Tolkamp et al (1998) observed that diet selection by lactating cows did not differ from a random choice among diets with different metabolizable protein content. Rumen environment may also influence voluntary choice.…”

Diet selection by growing lambs offered whole barley and a protein supplement, free choice: Effects on performance and digestion

“…It is possible that protein may have been selected as a buffer (Phy and Provenza, 1998) or as a source of degradable N as suggested by the low rumen ammonia-N concentrations recorded in Experiment 2. The involvement of degradable N supply in feed selection has been invoked by Tolkamp et al (1998) to explain the high protein intake observed in choice feeding trials with pregnant sheep (Cooper et al, 1994) and lactating cows (Tolkamp et al, 1998;Lawson et al, 2000), although James et al (2001) failed to show that food selection by sheep is regulated on the basis of rumen degradable N supply.…”