When diets of concentrates and grass silage (70:30) were offered to cows to compare barley (B) and molassed sugar beet feed (SBF) as the major component of the concentrate, calorimelric studies revealed enhanced energy partition towards milk at the expense of tissue repletion on B, during lactation weeks 11-21, whilst heat production was unaffected (Beever et al., 1989). In contrast when Sutton el al. (1988) fed the same diets to early lactation cows SBF increased milk fat and energy yields particularly during weeks 4-10, but less so during weeks 11-14. One possible explanation for this difference in response may relate to the different stages of lactation examined. To investigate this further, energy partition was examined in cows between weeks 6 and 14 of lactation given barley or molassed sugar beet feed based concentrates with approximately equal proportions of grass silage.