Energy and Amino Acid Requirements for Growth, Pregnancy, and Lactation
Growth requirements of dairy cattle are based on body weight gain needed to meet target growth rates and body weights, which are based on expected mature size for breeding herd replacements. The content of protein in the weight gain during growth of dairy heifers varies from over 20% in early growth to less than 13% as an animal approaches maturity. Prediction of amino acid requirements will not be accurate without accurate predictions of body gain and milk composition.
Pregnancy requirements are predicted from uterine and conceptus demand with varying expected birth weights and day of gestation. These become critical in accuracy of ration formulation during the last 60 days of pregnancy.
The milk from dairy cows ranges from 2.5 to 3.5% protein. Lactation requirements are computed for the amount and composition of milk. Energy and protein stored in body tissue (body reserves) are used to meet requirements when nutrient intake is inadequate. The empty body (full body minus the feed and water in the rumen and intestinal tract) of dairy cows averages about 16% protein. Body reserves must be taken into account when evaluating ability to meet requirements, especially under environmental stress, feed shortage or early lactation conditions. Visual appraisal is used to assign a body condition score, which in turn is used to predict body fat and energy reserves. The cycle of reserve depletion and replenishment during lactation and the dry period is reflected by predicted condition score change.