– Dr. Les Anderson, Beef Extension Specialist, University of Kentucky
Jeff, Darrh, and I were chatting the other day and, amazingly, we all agreed on something! Over our many miles of travel this winter/spring, we have seen more ribs on cows that any of us can remember. The wet, cold winter and poor hay quality has really stressed cows and if we don’t watch out, it will impact rebreeding.
A successful breeding season begins with nutritional management decisions made prior to calving but most spring-calving herds are past that now. “Ribs” are best maintained over the winter during the two trimesters of pregnancy. Visible ribs are one component of body condition score. Body condition score (BCS) is a numerical estimation of the amount of fat on the cow’s body. It ranges from 1-9; with 1 being emaciated and 9 extremely obese. A change in a single BCS (i.e. a 4 to a 5) is usually associated with about a 75 pound change in body weight. Evaluation of BCS prior to calving and from calving to breeding is important to ensure reproductive success.
Rebreeding performance of cows is greatly influenced by BCS at calving. Cows that are thin (BCS < 5; visible ribs) at calving take longer to resume estrous cycles and therefore are delayed in their ability to rebreed. As precalving BCS decreases, the number of days from one calving to the next (calving interval) increases in beef cows. Females with a precalving BCS <5 tend to have production cycles greater than 1 year. For example, cows with a precalving BCS of 3 would be expected to have a calving interval of approximately 400 days, while a cow with a precalving BCS of 6 would have a calving interval of approximately 360 days. Thin cows are anestrous for a longer period of time and are therefore more likely to be open at the end of the breeding season. They may also result in lighter calves to sell the next year because the calves from these thin cows will be born later in the calving season.
Management of BCS after calving also impacts rebreeding efficiency. Maintenance requirements for energy and protein increase 25-30% for most beef cows after calving. Producers need to plan their supplementation to match or exceed this increased nutrient requirement. Rebreeding efficiency is enhanced in cows that calved thin if their energy intake is increased. Although the best management plan is to calve cows in a BCS of 5+, increasing the energy to cows that are thin at calving can boost reproductive performance. If you see ribs, increase the energy intake of your cows even if they are on pasture. They need to gain weight or rebreeding will likely be hindered.
Thin cows (on an increasing energy intake plan!), young cows, and late-calving cows have one characteristic in common that will greatly impact their reproductive success; anestrus. After each calving, cows undergo a period of time when they do not come into estrus. This anestrous period can be as short as 17 days but can also last as long as 150 days depending upon a number of factors. Typically, mature cows in good BCS will be anestrus for 45-90 days (avg about 60 days) while first-calf heifers will be in anestrus for 75-120 days.
Let’s consider the impact of anestrus and calving date for a herd that calves from March 1 until May 10. Bull turnout is May 20 and the length of anestrus for mature cows (BCS 5+) is 60 days, for thin (BCS <5) mature cows is about 80 days, and for young cows is 90 days. A mature cow with no ribs showing (BCS 5+) that calves on March 1 will begin to cycle on May 1 and is highly likely to conceive early. However, the mature cow that calves on April 20 won’t cycle until June 20 and her opportunity to conceive early is less likely. Mature cows with “ribs” showing and first-calf heifers that calve on April 20 won’t begin to cycle until July 20 and will have limited opportunities to conceive.
Cattlemen can reduce the anestrous period by fenceline exposure to a mature bull or by treating the cows with progesterone for 7 days prior to bull turnout. Sources of progesterone include the feed additive melengestrol acetate (MGA) or an EAZI-Breed CIDR insert (Zoetis Animal Health). Both sources induce estrus in anestrous cows and exposure of anestrous cows to progesterone for 7 days before bull exposure and increase pregnancy rate. Pregnancy rates increase in these females because inducing estrus will increase the number of opportunities these cows have to conceive in the breeding season.
Normally, I love “ribs” but not showing in my cows. If ribs are visible then plans need to be made to reduce the anestrous period. Energy intake must increase and estrus must be stimulated to give these cows a chance to conceive and give you a chance at a profit.