I am no stranger to the effects of summer heat stress in cattle. Back when I was working on my master’s thesis at the University of Manitoba (the effect of molybdenum on copper status in beef cattle nutrition, 1983), I had 35 Shorthorn/Angus crossbred yearlings on full feed. During that particular summer, after July 1 southern Manitoba cooked for nine weeks in a row under temperatures often exceeding 35 C. By the end of August, most of my cattle lost about 30 to 50 pounds each.
Breeding cattle can suffer from heat stress in the summer, too. It is well accepted by most local beef producers that air temperatures will often exceed 30 C during most of our summers. Add in the days of high humidity, not a whisper of breeze and the expected onslaught of horn and face files, it might be a wonder that cows can get rebred at all. Fortunately, there are a couple of things that we can do in order to reduce heat stress in breeding cattle.
Heat-stressed cows are less likely to show signs of estrus or heat due to decreased amounts of blood-circulating reproductive hormones.
Even if a heat-stressed cow is bred by the bull and is luckily enough to conceive, there might be more bad news to come. That’s because these cows suffer from a high chance of early embryonic deaths at two different times of pregnancy during very hot weather; zero to seven days (loss of developing embryo), and from day 25-45 (early fetal loss). Cows that experience fetal death much later on (for similar reasons) can cycle sometimes after the breeding season is over, when the bulls are pulled.
That’s assuming the bulls are active enough to breed the cow herd in the first place. Excessive heat may build up inside their testes, which can cause temporary sterility. If it is an extended period of heat stress they may not fully recover for about 60 days. In one particular case, a producer showed me a 20-month old bull that failed to breed a pot of 18 replacement heifers during days of 35 C weather and only to later successfully breed them in a fall-calving herd.
Lots of water
As the photography that I took at a ranch last summer shows, beef producers should provide lots of water to all cattle in the summer. Lactating beef cows and bulls usually drink about 40-50 litres of water per head per day in comfortable weather and dramatically increase water consumption to 80-90 litres per head per day during heat-stressed conditions. Some cattle field studies show that water must be cool, because water temperatures over 25 C can actually add to heat-stress in cattle. The water surface area should be large enough for a number of cattle to drink at the same time, and the water flow within the waterers and tanks replenished, quickly.
Aside from plenty of water, beef producers can also alleviate some heat stress in cattle by controlling exploding fly populations that parallel hot summer weather. Three main methods of fly control are:
- Ear tags Impregnated with insecticide, one ear tag per season per cow, which is good up to a few months of control (face and horn flies).
- Back rubbers — the standard recommendation is to set them up to force cattle to pass by them. It’s a good idea to check them on a weekly basis to see if they need recharging.
- Pour-on insecticide — 30- to 60-day control (for horn flies). I would buy a commercial brand, because some generic types have had difficulties in proper absorption rates.
I cannot stress these points enough — lots of clean, cool water and good fly control in order to help ease heat stress in the cow herd during breeding season. I realize that some cattle have some trees on their pastures, so they can get out from under the sun or some cattle are quickly move through rotational fields to graze more digestible grass (less internal heat generated). However, I have walked in many beautiful parts of the Prairies, where there isn’t a tree in sight.