Dairy Pipeline: May 2005
Extension Dairy Scientist,
Genetics and Management
(540) 231-4762 email: firstname.lastname@example.org
PTA's for Daughter pregnancy rates (DPR) measure genetic differences between bulls in fertility of their daughters. DPR predicts genetic improvement (or deterioration) in pregnancy rate for future daughters of a bull compared to a bull that is expected to produce no change (PTA DPR = 0). Better pregnancy rates reduce semen usage and can be an important part of efforts to reduce days open. Producers wanting more fertile cows should select the bulls with higher DPR ratings. Change will be slow, as genetic differences between bulls range from lows of -3 or -4 % to highs of similar magnitude. Some of the critics of DPR say that genetics don't control fertility enough to justify selection. I will plead guilty to ignoring fertility in the past for just that reason, though not having genetic evaluations for fertility made the decision for me. Heritability of DPR is low - only about 4% - so producers should not expect dramatic improvement in fertility from selection. However, genetic control of fertility is real. Pregnancy rates in Holsteins declined by about 9% over the past 40 years, and 5 to 6 of those percentage points were due to genetic change. During those years, fertility was not part of selection programs. We selected to improve production and type. The decline in fertility was a correlated response. If fertility can be hurt by correlated response to selection, it can certainly be improved by selective use of higher fertility bulls. DPR needs to be included with all the other traits of economic importance in dairy cattle breeding, however, rather than being singled out as "the" trait in a breeding program. The reason is that one of the more effective ways to improve fertility is to select for lower milk yield, which would likely cause more pain than gain. The Net Merit index includes DPR with 7% of the total emphasis on all traits. That compares with 55% emphasis for milk, fat, and protein. Selection on Net Merit will improve fertility by 1% across an entire population of cows over a ten year period. The magnitude of change isn't impressive, but the real point is that fertility improves at the same time that production, productive life, somatic cell score, functional type traits and calving ease are substantially improved. That's a much better situation than the old approach where improved milk and type came at the cost of less fertile cows.