In theory, it’s true that cows use energy more efficiently to gain condition while lactating, as noted by former DairyNZ staff members John Roche and Kevin Macdonald in 2011. However, other variables can influence the actual body condition that is gained.
For example, maintenance energy costs and the energy costs of grazing and walking are substantially greater when cows are lactating than if they were dry. Not only that, lactating cows tend to partition most of the extra energy to milk production and not to body condition.
Although there’s evidence that lactating dairy cows require 25 percent fewer megajoules of metabolisable energy (MJ ME) to gain a body condition score (BCS) unit than dry cows, they also require:
- between 10 and 20 percent more energy for maintenance than dry cows
- approximately 2.5 MJ ME for every kilometre (km) walked, which is approximately 10 MJ/day if the average paddock is 1km from the shed and if the cow is milked twice a day.
These additional energy requirements negate much, if not all, of the difference in energetic efficiency for BCS gain.
Another factor offsetting body condition gain in the lactating cow is that they partition only a small proportion of energy eaten towards BCS gain.
Genetic selection priorities over several decades have resulted in a cow that willingly mobilises condition in early lactation to support milk production, and only reluctantly partitions energy to condition score gain in preference to milk production in mid and late lactation.
Rates of BCS gain
Due to all the factors mentioned above, BCS gain per unit of feed eaten is much greater in dry cows. For example, feeding a lactating cow 3kg DM concentrate per day for 100 days during mid or late lactation would increase BCS by approximately 0.12 BCS units.
In comparison, feeding 3kg DM concentrate per day to a dry pregnant cow (for 60 to 100 days pre-calving) would increase BCS by one unit.
Cows are more efficient at gaining condition while they’re lactating than when they’re dry.
Busted - yes and no
Lactating cows do use energy more efficiently for body condition gain, but dry cows partition more of the energy eaten to BCS gain.
Find out more about BCS at dairynz.co.nz/bcs
This article was originally published in Inside Dairy April 2019