Utilization of energy for maintenance and production in dairy cows given protected tallow during early lactation

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Abstract
Summary: During the first 13 weeks of lactation, four groups of seven cows were given hay and a basal concentrate mix containing 0, 1·7, 3·3 or 5·0 kg/day of a protected lipid supplement (320 g long-chain fatty acid/kg D.M.). The proportion of hay to total concentrate was maintained at 25:75 and cows were fed to appetite.The total amount of energy digested corrected for endogenous loss (DEc) was partitioned into crude protein energy (CPEc), fatty-acid energy (FAEc), ‘carbohydrate’ (non-protein non-fatty acid) energy (CEC) and non-fatty-acid energy (NFAEc = CPEc + CEc).Partial efficiency of milk production and live-weight gain was curvilinearly related to FAEc, maximum efficiency occurring at about 30 MJ/day of FAEc.Net energy utilized was significantly related to DE0c for both of the periods considered, Weeks 2–6 and weeks 7–13. In addition, a curvilinear effect of the ratio of fatty acid energy to non-fatty-acid energy was established. At fixed intakes of digestibl energy, net energy utilized was maximal for ratios of FAEc/NFAEc of 0·14 and 0·18 for weeks 2–6 and 7–13 respectively.Milk yield was significantly related to DEc for weeks 1–6 and 7–13; milk energy yield was significantly related to DEc for weeks 7–13. The multiple regression relationships between milk yield or milk energy yield and CPEc, FAEc and CEc gave a negative effect of crude protein, possibly indicating that the proportion of total energy supplied as crude protein (18–26% of DEc) was above the optimum requirement.At fixed intakes of digestible energy, outputs of milk and milk energy were maximal at ratios of FAEc/NFAEc of 0·16 and 0·15 for weeks 1–6 and 7–13 respectively. The inclusion of a curvilinear effect of total energy (DEc2) gave some indication that utilization of energy for milk production for weeks 7–13 diminished at the highest digestible energy intakes whilst energy utilized for live-weight gain increased.