Getting cows bred is only part of the story. Keeping them bred, especially through the summer months, also takes attention to detail.
In normal situations where the bull is fertile and covers cows at the right time, fertilization rates approach 100%. So, if a normally expected single-service conception rate is 60-80%, the difference comes from embryonic or fetal loss. Most of this loss occurs in the first few days of development and those cows or heifers come back in heat and get another opportunity to get settled 19-24 days later. Missing one cycle means that cow's calf will be younger and lighter at weaning. But, losing a pregnancy after they pass over as heat cycle compounds the economic loss.
For beef cows and heifers, relatively little pregnancy loss occurs beyond 60 days after breeding in normal situations. However, heat stress can increase pregnancy loss well into the second and even third trimesters. To compound the problem, spring calves are often weaned and the cows worked at an extremely hot (and often dry) time of the year. Fall calving cows and heifers tend to calve earlier than the calculated due date when long periods of heat and drought are experience through the third trimester.
Some of the original research on heat stress and pregnancy rates was done at Oklahoma State University. Those researchers found that when cattle were bred at a relatively cool temperature but then exposed to moderate and severely hot temperatures, their pregnancy rates were decreased by as much as 50%. Furthermore, they found that the surviving fetuses were smaller in heat stressed cows and were more prone to loss later in pregnancy.
It is extremely important to remember that cattle have an upper critical temperature that is 20 degrees lower than humans. That means that when we feel uncomfortable due to heat, cattle can be extremely stressed. Humidity adds to the problem by increasing the heat index and limiting the ability of cattle to dissipate heat.
Adding other stressors to heat stress compounds these problems. For instance, fescue toxicosis limits a cow's ability to dissipate heat because it reduces blood flow to the skin. If cows and heifers are exposed to diseases they are not properly vaccinated against, their immune system can be overwhelmed. Heat stress decreases grazing and feed intake while drought can limit forage availability. In recent years, droughts and the resulting dust – along with malnutrition - have increased the rate of respiratory issues leading to more late pregnancy loss. In extreme cases of pasture loss, cattle might eat weeds and leaves they would normally leave alone; leading to pregnancy loss or even mortality of the cow itself.
Also remember that heat stress will increase water consumption. Increasing the ambient temperature from 70°F to 90°F can double water intake. So, it is important not to neglect the availability of a clean, fresh and cool water supply. Pay special attention to pond water quality as the level goes down in summer. Increased water intake leads to increased urine output that depletes mineral stores. So, along with the water, make sure a balanced mineral supplement is continually available. Most cattlemen know that shade is important but just having a few trees in the pasture might not be adequate. Make sure that the entire herd in a single pasture can find shade areas without congregating in one small area and compromising air flow.
Working cattle during the summer months is usually necessary but makes it difficult to manage heat stress. Try to target days with a forecasted temperature between 80 - 85°F and relative humidity between 30 - 40%. Begin working as early in the morning as possible and try to be done by 10:00 AM. If there is no other alternative to working cattle during extended periods of extreme heat, consider scheduling the work between the hours of midnight and 8:00 AM. Working cattle in the evening can still be stressful because they have not had a chance to dissipate heat and reduce their core body temperature after the sun has gone down.
Other ways to combat the negative effects of heat stress on reproductive efficiency include changing breeding and calving season or incorporating heat tolerant genetics. However, total pregnancy loss is not the only reproductive concern during heat stress. The efficiency of the placenta can be reduced and lead to decreased calf vigor. So, altering genetics or the calving season should be considered in relation to how that will impact the value of your eventual end product (whether it is weaning weight, yearling weight, or beef) and overall profitability.