Phenotype x nutritional environment interactions in forage intake and efficiency of Angus cows grazing fescue-legume or fescue pastures.

Abstract

Forage intake, milk production, calf growth and efficiencies of milk production and calf growth were estimated for 198 lactations of 91 mature Angus cows grazing fescue-legume or fescue pastures. Cows calved January through March and calves were weaned in October in each of the 5 yr of the experiment. Cows grazing fescue-legume consumed 1.7 kg/d more (P less than .01) dry matter (DM) that was 4.6 percentage units more (P less than .01) digestible than cows grazing fescue. Cows grazing fescue-legume produced .4 kg/d more (P less than .01) milk and weaned 23.8 kg more (P less than .01) calf than cows grazing fescue. This increased production required an additional intake/cow-calf unit of 2.3 kg of digestible DM for each additional kilogram of calf weaned. Cows grazing fescue-legume were less efficient in calf production at least partially because they gained 33.3 kg more (P less than .01) weight and 1.82 mm more (P less than .01) fat cover at the 12th rib during lactation than cows grazing fescue. Cows producing relatively large amounts of milk consumed more digestible DM, weaned heavier calves, and were more efficient producers of milk and weaned calf regardless of pasture type grazed. The influence of milk production on these variables was more dramatic for cows grazing fescue than for those grazing fescue-legume. Cow type favoring maximum conversion of digestible DM intake by cow and calf to calf gain was different for the two types of pasture. For cows grazing fescue, those having either small amounts of fatness or having small structural sizes, regardless of milk production, were the most efficient. Structural size was not related to efficiency for cows grazing fescue-legume. Fatness had little influence on efficiency of low milking cows. The relationship of structural dimension and efficiency for cows grazing fescue was due to interrelationships of structural dimension, change in cow weight and fatness during lactation, and calf weaning weight: cow weight ratio. Relationships between efficiency and fatness could not be attributed to these variables.