IOWA Cattleman
Official publication of the Iowa Cattlemen’s Association Vol. 38, Num. 7, October 2011
Cooling Off, summer breeding rates may be down
Grant Dewell, DVM, Veterinary Diagnostic and Production Animal Medication, Iowa State University
Fall is finally here and the temperature has cooled off. However, cows may still be recovering from the heat of summer.
In July an estimated 4000 feedlot cattle died from excessive temperatures. Most feedlot cattle experienced reduced feed intake during the heat event and will have reduced weight gain.
Typically, cows on pasture are able to mitigate their heat load and are not at risk of death loss.However, the high temperatures experience in July in Iowa may negatively impact the reproductive performance of cows and bulls. Cattle producers with a July through mid-August breeding season should be especially mindful of their pregnancy rates this fall.
Reproductive review
Follicles in ovaries can be damaged due to excessive body temperatures. These damaged follicles will be sub-fertile when they are ovulated, sometimes even well into the fall. Even if cows were ovulating fertile follicles in July, cattle may not express enough estrus behavior because of the extreme temperatures. If the cow did get bred the embryo is at risk of embryonic death from heat stress until the fourth day after breeding.
Bulls can also have heat related issues. During high temperatures, bulls will seek shade and will be less active in seeking receptive females. If the bull cannot adequately control testicular temperatures then sperm production will be reduced and abnormal sperm will increase. Decreased motility of sperm cells is evident within two weeks of a heat event and may not return to normal for eight weeks.
The combination of reduced cow and bull fertility in July may result in poor pregnancy performance. Cows should be pregnancy tested this fall to determine pregnancy status, Keeping open cows through the winter with the high cost of feed this year will be very expensive. Cull cow prices are still above average but testing early and selling before seasonal price reductions is advisable.
If your breeding season begins in July you may want to reconsider your breeding and calving strategy. High temperatures in July can affect fertility and if cows do conceive they will calve in April which often wet and muddy for calves.
As you look ahead, consider delaying breeding season into August and September for May and June calving when pasture is green and drier. Enjoy the fall weather while it lasts.