*Dr. Les Anderson is an extension professor with the University of Kentucky. To expedite answers to questions concerning this column, please direct inquiries to Feedstuffs, Bottom Line of Nutrition, 7900 International Dr., Suite 650, Bloomington, Minn. 55425, or email email@example.com.
A SUCCESSFUL breeding season actually begins with management decisions that are made at calving.
Cattle producers can affect rebreeding efficiency by focusing on body condition score (BCS), early assistance during calving difficulty, scheduling a breeding soundness exam for the herd sires, planning a herd reproductive health program and developing a plan to regulate estrus in first-calf heifers and late-calving cows.
Reproductive management begins with evaluation and management of BCS, which is a numerical estimation of the amount of fat on the cow's body. The BSC scale typically ranges from one to nine, with a score of one being emaciated and a score of nine being extremely obese.
A single change in BCS (e.g., from four to five) is usually associated with about a 75 lb. change in bodyweight. 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 less than five) at calving take longer to resume estrous cycles and, therefore, are delayed in their ability to rebreed (Richards et al., 1986). Research has clearly demonstrated that as precalving BCS decreases, the number of days from one calving to the next — the calving interval — increases in beef cows (Kunkle et al., 1994).
Females with a precalving BCS of less than five tend to have production cycles greater than one year. For example, cows with a precalving BCS of three would be expected to have a calving interval of approximately 400 days, while a cow with a precalving BCS of six would have a calving interval of approximately 360 days.
South Dakota State University research illustrates the influence of precalving BCS on the percentage of cows that initiated estrous cycles after calving (Paterson, 1993). This experiment demonstrated that the percentage of thin cows that were cycling in the first month of the breeding season (June) was considerably lower than for cows that were in more moderate body condition.
During the second month of the breeding season in this study, 55% of the cows with a BCS of four still had not initiated estrous cycles, while more than 90% of the cows in more moderate condition had begun to cycle.
Thin cows need a longer breeding season, which often results in more open cows or younger, 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 affects rebreeding efficiency. Maintenance requirements for energy and protein increase 25-30% for most beef cows after calving. Ranchers need to plan their supplementation to match or exceed this increased nutrient requirement. Rebreeding efficiency is enhanced in cows that were thin at calving if their energy intake is increased (Rutter and Randle, 1984).
Although the best management plan is to calve cows with a BCS of greater than five, increasing the energy to cows that are thin at calving can boost reproductive performance.
Dystocia (calving problems) can severely delay the onset of estrus after calving. Research shows that for every hour a female is in stage 2 active labor, there is a four-day delay in the resumption of estrous cycles after calving. Early intervention helps; 16% more cows conceived when cows were assisted within 90 minutes of the start of calving. The best method is to reduce the incidence of dystocia via selection, but early calving assistance will increase the opportunity for cows to rebreed.
One often overlooked management tool that can improve reproductive performance is breeding soundness exams in bulls. Ranchers need to think of breeding soundness exams as breeding season insurance.
These exams are a low-cost method of ensuring that herd bulls are not infertile. Bulls should be examined for breeding soundness about 30 days before they are turned out.
I have worked in reproductive management for nearly 20 years, and it amazes me how many cattle producers still do not vaccinate their cow herd against reproductive diseases. Several diseases are associated with reduced reproduction, including leptospirosis, bovine viral diarrhea, vibrio, trichomoniasis, etc.
The main problem is that most loss due to reproductive disease is subtle, and ranchers don't notice the loss unless they have a massive failure. Most cattle producers are not aware of losses due to aborted pregnancies. Ranchers need to work with their local veterinarian to develop an annual vaccination plan to enhance reproductive success.
Last, ranchers need to develop a plan to enhance the rebreeding potential of their first-calf heifers and late-calving cows. Young cows and late-calving cows have one characteristic in common that will greatly affect their reproductive success: anestrus.
After each calving, cows undergo a period of time when they do not come into estrus. This anestrus 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 (average of about 60 days), while first-calf heifers will be in anestrus for 75-120 days. Research has shown that only 64% of mature cows have initiated estrous cycles about 70 days after calving, while only 50% of first-calf heifers have initiated estrous cycles at nearly 90 days after calving.
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 is 60 days for mature cows and 90 days for young cows. A mature cow 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 very limited.
A first-calf heifer that calves on April 20 won't begin to cycle until July 20 and will have limited opportunities to conceive.
Cattle producers can reduce the anestrous period by providing fence-line exposure to a mature bull (Zalesky et al., 1984) or by treating the cows with progesterone for seven days prior to bull exposure (Lucy et al., 2001). Sources of progesterone include the feed additive melengestrol acetate or an EAZI-Breed CIDR insert (Zoetis Inc.).
Both sources have been shown to induce estrus in anestrous cows, and exposure of anestrous cows to progesterone for seven days before bull exposure will not reduce fertility.
Pregnancy rates will actually be increased in these females because inducing estrus will increase the number of opportunities these cows have to conceive in the breeding season.
The Bottom Line
Managing for reproductive success actually begins at calving. Cows need to calve with a minimum BCS of five and with little assistance. Effective planning for reproductive health and for limiting the impact of anestrus will ensure that ranchers see the results they want at the end of the breeding season.
Kunkle, W.E., R.S. Sand and D.O. Rae. 1994. In: Factors Affecting Calf Crop, First Edition. CRC Press Inc. p. 167.
Lucy, M.C., H.J. Billings, W.R. Butler, L.R. Ehnis, M.J. Fields, D.J. Kesler, J.E. Kinder, R.C. Mattos, R.E. Short, W.W. Thatcher, R.P. Wettemann, J.V. Yelich and H.D. Hafs. 2001. Efficacy of an intravaginal progesterone insert and an injection of PGF2-alpha for synchronizing estrus and shortening the interval to pregnancy in postpartum beef cows, peripubertal heifers and dairy heifers. J. Anim. Sci. 79:982-995.
Paterson, J.A. 1993. Effects of body condition on reproductive performance. In: 1992-1993 Regional Beef Meetings, Management for Efficient Reproduction. University of Missouri Press. p. 32.
Richards, M.W., J.C. Spitzer and M.B. Warner. 1986. Effect of varying levels of postpartum nutrition and body condition at calving on subsequent reproductive performance in beef cattle. J. Anim. Sci. 62:300.
Rutter, L.M., and R.D. Randel. 1984. Postpartum nutrient intake and body condition: Effect on pituitary function and onset of estrus in beef cattle. J. Anim. Sci. 58:265.
Zalesky, D.D., M.L. Day, M. Garcia-Winder, K. Imakawa, R.J. Kittok, M.J. D'Occhio and J.E. Kinder. 1984. Influence of exposure to bulls on resumption of estrous cycles following parturition in beef cows. J. Anim. Sci. 59:1135-1139.