The myth: Cows are more efficient at gaining condition while lactating than when dry.
Busted: Yes... and no!
Although cows use energy more efficiently to gain condition while lactating, their maintenance energy costs and the energy cost of grazing and walking are substantially greater than if they were dry. Furthermore, cows don't gain much condition while lactating, irrespective of diet.
Feeding 300kg of grain will increase a dry cow by 2 BCS. If milking, BCS gain is 0.12.
Are cows more energetically efficient at gaining condition while milking?
Although there is evidence that lactating dairy cows require 25 percent less energy to gain a body condition score unit than dry cows, they also require:
- Between 10 and 20 percent more energy for maintenance than dry cows
- Approximately 2.5 MJ for every km walked (approximately 10 MJ/day if the average paddock is 1 km from the shed and the cow is milked twice a day).
These additional requirements negate much, if not all, of the difference in energetic efficiency for condition score gain. In addition, lactating cows partition only a small proportion of available nutrients towards condition score gain.
Genetic selection priorities over several decades have resulted in a cow that willingly mobilises condition in early lactation and only reluctantly partitions energy to condition score gain in preference to milk production in mid and late lactation.
Figure 1 below highlights the effect of supplementing a high starch concentrate to lactating cows. Milking cows partition very little consumed energy to BCS gain - 600 kg concentrate eaten resulted in a 0.3 unit increase in BCS. In comparison, 3 kg/d concentrate to a dry pregnant cow two to three months precalving would increase BCS by 1 unit in 40 to 50 days.
Figure 1: The effect of concentrate on condition score gain. Every kg of concentrate eaten per day resulted in an increase in condition score of 0.04 BCS units in 100 days. In other words, if a cow was fed 3 kg concentrates each day during mid or late lactation, she would be 0.12 BCS units fatter after 100 days.
By John Roche, DairyNZ Principal Scientist, Animal Science and Kevin MacDonald, DairyNZ Senior Scientist, Farm Systems
This article was originally published in Inside Dairy March 2011.