2 April 1999

Study shows half extra energy converts to milk

WHEN a cow is given a unit of energy, and after her needs for milk and maintenance are met, she uses almost half to produce more milk and the same amount to increase body tissue.

Thats the initial conclusion from one of a series of studies on energy partitioning in dairy cows at the Agricultural Research Institute of Northern Ireland, Hillsborough, reported at BSAS.

Rosemary Agnew said producers often ask whether feeding a cow extra energy will produce more milk, or increase body weight, and whether it will be economic.

"This study showed that if you could feed an extra 2kg above an animals requirement, 1kg would be used to produce milk and 1kg to produce body tissue. For each MJ of extra energy eaten, half is used for milk production." Whether this is economic depends on how cheaply you can provide the extra feed relative to milk price.

To complete the study, four early lactation cows averaging 65 days in milk and yielding 29kg were offered a series of diets providing them with between 120 and 240MJ of metabolisable energy.

Richard Kirkland of Hillsborough said a further study shows the cow partitions extra energy to milk yield and body tissue in the same proportions at other stages of lactation.

This was studied by reducing the energy supply to cows at 150 and 400 days calved, yielding 40 and 20 litres, respectively, he explained. Higher yielding cows were dried off in two quarters to take out the effect of milk yield.

"We found days in milk had no effect on nutrient partitioning," he said. Cows continued to use 0.44 of each MJ supplied for milk and an equal amount for body tissue, with the rest being lost through heat production and other routes.

Other factors which may influence how extra energy is used are genetic merit and milk yield, said Dr Agnew.

"Its more likely that high yielding, high merit cows will partition more of the extra energy to milk and less to body condition."

Further studies should provide enough data to allow the development of a computer model to predict milk yield responses from extra feeding.

NUTRIENTPARTITIONING

&#8226 Half extra feed used for milk.

&#8226 Other half for body tissue.

&#8226 A little lost as heat.