High genetics of little merit in energy study
ENERGY metabolism studies using high yielding cows have produced no evidence that genetically superior animals use metabolisable energy more efficiently for milk production.
In a limited trial at the Centre for Dairy Research, University of Reading, four Holstein cows with previous milk yields of over 10,000kg and an average post calving weight of 605kg were offered a TMR comprising maize silage, grass silage, dried lucerne and a range of concentrates. These included Sodagrain, molassed beet feed, maize, soya, wheatfeed and Megalac.
Full energy and nitrogen balances were obtained at five week intervals up to week 20 of lactation.
Average dry matter intake was 24.4kg a head a day with an initial mean milk yield of 51.7kg a day. Individual cows ate over 28kg of DM a day yielding more than 60kg milk a day.
While high yielders ate considerable amounts of feed, the ME intake did not exceed 300 MJ a day and there was a significant compromise in digestive efficiency resulting in a mean dietary ME content of 10.9 MJ/kg DM.
In early lactation the energy shortfall resulted in a considerable mobilisation of body energy with a subsequent compromise in milk composition. While ME intake increased as lactation progressed there was relatively little change in milk energy output. By lactation week 20, cows were still in negative energy balance despite showing a positive gain in weight.
In the first 10 weeks of lactation both milk protein and fat contents were low, but while milk protein only increased gradually there was a substantial improvement in milk fat as lactation progressed. Gross energy intakes were increased by the tenth and fifteenth week stages of lactation while energy digestibility was largely unchanged and low in relation to the type of diet offered.
After allowing for maintenance ME requirements, total energy available for milk production and including mobilised body tissue, was estimated to be used with 58% efficiency.
High genetic merit dairy cows use metabolisable energy for milk production no more efficiently than their lower merit contemporaries.