More milk means reduced fertility and body condition score
Take Home Message: Selecting for milk yield decreases fertility and body condition score.
Selecting for increased milk production leads to a deterioration in genetic merit for fertility and condition score. That was the unequivocal conclusion of a study carried out by geneticists at SAC on the college’s Langhill herd, which was under maximum selection for milk production between 1980 to 2005.
“Several authors have reported on the changes that have occurred in dairy cow productivity and fertility at national or regional level,” SAC’s Geoff Pollott told delegates at this year’s British Society of Animal Science annual conference.
“The general trend is for an improvement in milk production at the expense of fertility. But such studies do not account for the effects of farm, management, feeding changes and health status on the measured traits. So that’s something that we set out to do.”
Selection is the main tool used by animal breeders to improve the performance of their stock. Despite large gains being made at industry level in the major dairy traits, very few reports exist for the results of experiments on persistent selection for dairy traits in a single herd, particularly with reference to the correlated effects on other dairy traits.
Dr Pollott’s team’s work was based on the intensively recorded Langhill herd of cows, originally maintained at the University of Edinburgh’s Langhill Farm until 2002, and then transferred to SAC’s Crichton Royal Farm.
The 200-cow herd was equally divided into a selection line and a control line – only results from selection line animals are reported in this paper. Bulls were chosen from all AI bulls available at the time of mating, which had the highest fat plus protein PTAs, had semen available and were not related to the cows back to grandparent level.
The data used in this study were the PTAs for all traits produced as part of the national genetic evaluations carried out by EGENES on behalf of DairyCo.
“We found that selecting sires on the basis of their fat+protein PTA resulted in an average annual PTA increase of 2.66kg per lactation,” said Dr Pollott.
“Milk production increased by 35kg a year and test-day yield at about day 110 also increased. All other traits deteriorated with cows having poorer fertility with days to first service and calving interval increasing by 0.4 days and 0.7 days per year respectively. Cows are also becoming thinner.”
Summary (pdf) Presentation (pdf)
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