– Dr. Les Anderson, Beef Extension Specialist, University of Kentucky
One of the most challenging aspects of spring calving is trying to determine when to calve to maximize reproductive rate. Reproductive efficiency in a cow herd is most accurately measured by the term “percent calf crop weaned” which is calculated by dividing the number of calves weaned by the number of cows that were in the cow herd when the breeding season began the previous year. The two factors that affect the ability of a cow to wean a calf is pregnancy rate and calf death loss.
Most spring-calving herds begin calving sometime in February or March and end sometime in May or June. Calving in February and March can be challenging because both of these months are typically wet and/or cold. Wet/cold environments result in higher calf death loss; calf death losses average 5-7% for most spring calving herds. One method to reduce calf death loss is to calve when the weather is more accommodating. For example, death loss is much lower (1-2%) for cows that calve in the fall (September and October). One might think that calving in April and May could be a better option; the weather is certainly warmer and calf death loss will likely be lower. To calve in April and May, the breeding season would be start June 23rd and would last through the month of August. Unfortunately, breeding cattle during this time results in lower pregnancy rates and would put most beef cattle producers out of business.
Data from the University of Kentucky Research Center at Princeton demonstrate the impact of breeding season on reproductive rate. In this trial, cows were exposed to a 45-day natural service breeding season. The breeding seasons were early (4/21-6/5), typical (5/21-7/6), or late (6/19-8/4). Pregnancy rates declined dramatically in cows that were bred later in the summer. Pregnancy rates were 89% for cows bred early, 78% for cows bred during the typical time, and only 59% for cows bred to calve later (April/May). Therefore, in Kentucky, cows that are bred to calve later in the spring will likely have lower calf death loss but considerably fewer of the cows will actually get pregnant. Why is pregnancy rate so low for cows in July and August?
The main factor that reduces pregnancy rates in our state, and others in the fescue belt, is heat stress. Heat stress occurs when the body temperature is elevated for more than two degrees above normal for more than 48 consecutive hours. Heat stress reduces pregnancy rates by increasing embryonic mortality. Developing embryos/pregnancies can be lost at two different periods of pregnancy; before Day 7 (loss of the developing embryo) and from Day 25-45 (early fetal loss). Cows that experience embryonic loss in the first week of pregnancy are repeat-breeders; they come back into heat 20-21 days after service. Cows that experience fetal loss from Day 25-45 are normally those cows that conceived early in the breeding season (end of May) but were exposed to extreme heat stress 25-45 days later. Data from trials at the University of Kentucky illustrate that fetal death loss ranges from 5-25% depending upon the level of heat stress. Cows that experience fetal death loss are typically open at the end of the breeding season.
The heat stress problems in our state are the result of consumption of endophyte-infected fescue. Endophyte is a fungus that grows in fescue and it produces chemical compounds that reduce the ability of a cow to dissipate heat. These chemicals redirect blood flow in an animal’s body such that the blood supply pools in the interior regions of the body. Normally in the summer an animal’s blood supply flows more to the exterior of the body so that it can be cooled. The redirection of the blood flow reduces the ability of an animal to cool itself during the night and results in tremendous heat stress on the body and lower pregnancy rates.
How can we reduce the impact of heat stress? The first logical approach would be to limit the access of your cows to endophyte-infected fescue during the heat stress months (mid-June thru August). Grazing options include warm season grasses, endophyte-free fescue, predominately legume pastures, and/or sorgum sudan grass. Cows could graze endophyte-infected pastures until late-May to mid-June while the summer grazing pastures grow. Cows could then be turned out on the “summer pastures” until the end of the breeding season. If non-endophyte pastures are not feasible, then diluting the fescue with legumes and/or other feedstuffs will help reduce the impact of the heat stress. One supplemental feed that appears to reduce the effects of heat stress is fat. Research at the University of Kentucky has demonstrated that feeding cows high fat diets while grazing highly infected endophyte fescue during the breeding season can help reduce heat stress and improve pregnancy rates. In these trials, cows were fed either a commercial fat supplement free choice or whole soybeans (3 lbs/hd/day) during the breeding season (6/5-8/15). Fat supplementation increased hair shedding, reduced cow body temperature, and improve pregnancy rates from 56% to 78%.
The decision of when to concentrate your calving in the spring is tough. Life is easier if your cows could calve later but fewer of them will calve. With little doubt, calving earlier will increase pregnancy rates but will also likely increase calf death loss. Economically, 5-7% death loss is more financially sound than only 60-70% pregnancy rates. Use of alternative summer grazing systems to reduce the effects of endophyte-infected fescue is a logical but sometimes difficult solution. Feeding cows fat supplements will help but perhaps the best solution is to completely change your breeding and calving season. Cows that calve in the fall have lower calf death loss, higher pregnancy rates, and shorter calving seasons than cows that calve in the spring.