"Critical Point" Feeding of Rumensin® in the Beef-Cow Operation

Elvin E. Thomas, Ph.D.

Elanco Animal Health

Greenfield, Indiana

Profitability of the beef-cow operation depends substantially upon achieving high pregnancy rates while at the same time maintaining feed costs as low as possible. Research has shown that body condition of cows at calving and breeding plays a major role in length of the postpartum interval and conception rate. Rumensin® has long been used to increase performance and control coccidiosis in feedlot animals which has led to its increased use in grazing situations for cow-calf and stocker operations. With increased awareness of the importance of having sufficient body condition on cows coming into the calving season, realization of the importance and use of Rumensin as a way of improving utilizable energy from the feeding program has increased markedly. This paper will focus on the role Rumensin can play regarding attainment of puberty, management of body condition, and the economics of feeding Rumensin during critical points in the production cycle. Rumensin is the only ionophore approved for use in reproducing beef cows.

Rumensin is a feed additive that alters ruminal fermentation such that the propionate to acetate ratio is increased thereby increasing the amount of energy available to ruminant animals for growth and other productive purposes. The effect is seen with both forage- and grain-based diets.

The effect of dietary energy upon reproductive performance of beef cows has been extensively documented. Adequate energy consumption is needed if a short postpartum interval and high conception rate is to be achieved. In addition, it has been shown that replacement heifers reach puberty earlier when fed higher energy diets compared to those fed lower energy diets, particularly when diets consist largely of low quality forages. Factors identified as influencing the onset of puberty in replacement heifers include body size, prepubertal growth rate, growth hormone, insulin, leutenizing hormone and propionic acid (4,6,7,8). Rumensin has been shown to increase growth hormone and leutenizing hormone secretion (4,5,7,10). In addition, Rumensin increases propionic acid production in the rumen. A 10-trial summary (1) of the effect of feeding Rumensin to beef heifers showed an average increase in weight gains of 0.14 lb per day. Other data have shown that Rumensin-fed heifers reached puberty and first estrus from 10 to 21 days sooner than non-supplemented controls (2,4,5,6,10). This is of interest to cattlemen who traditionally calve their heifers for the first time at two years of age.

Body condition of cows refers to the "degree of fatness or condition" and is commonly scored on a scale of 1 to 9 in which a score of 1 is for extremely thin and emaciated cows and 9 is for cows that are extremely fat (3,9). Most cows have a body condition score (BCS) ranging from 3 to 7 depending upon nutritional status and parasite/disease conditions. BCS at calving affects postpartum interval, services per conception, conception rate, calving interval and weaning weight of calves (3,9). Generally, BCS's of 4 at breeding have resulted in conception rates of approximately 60 percent which is significantly lower than conception rates of approximately 78 percent and 91 percent in cows with BCS's of 5 and 6, respectively (3). It is readily apparent that minimum BCS's of 5 be achieved before calving and that they be maintained until breeding if good conception rates are to be achieved.

Body condition scores of cows should be utilized extensively in establishing the management scheme for the cow herd. Such factors as stocking rate, forage species, forage quality, supplemental feeding strategies, parasites, etc., must all be considered. Proper forage management can pay big dividends by reducing the amount of supplemental feed required to achieve desired BCS's in the herd. This is true regardless of whether grazed forage is available to the herd or harvested forage stored for later feeding is utilized. Calculations involving expected energy content from forages grazed or harvested as high quality compared to low quality, show that costs of supplemental feed can be reduced by approximately 50%.

In an effort to maintain an average BCS of 5 in a cow herd, it is advisable to separate the herd into groups based on their current body condition. Providing supplemental feed to thin cows can be justified economically if they are maintained separate from cows with BCS of 5 or greater. Approximately 0.20 lb body weight gain can be realized from feeding each 1.0 lb of total digestible nutrients (TDN) above that needed for maintenance and pregnancy (3). In beef cows, approximately 75 lb of weight must be gained (above pregnancy needs) to realize an improvement in BCS of 1.0. Thus, a mature cow with a BCS of 4.0 would need to gain 75 lb to improve her BCS to 5.0. That could be accomplished in approximately 60 days by feeding 8.0 lb of grain mixture daily in conjunction with a forage program capable of supporting maintenance of body weight and pregnancy requirements.

Research has shown that Rumensin provides additional energy available to the animal through manipulation of rumen fermentation (8). From growth trials, it may be calculated that the additional energy derived from feeding 200 mg Rumensin daily to growing heifers is equivalent to a minimum of 1.0 lb of corn grain. The price differential between 1.0 lb corn grain compared to 200 mg of Rumensin shows that a significant savings may be realized by inclusion of Rumensin into the feeding program of the cow herd.

In an effort to manage BCS in the cow herd better, there are "critical periods" during which feeding of supplemental energy may be essential. Because Rumensin increases the energy available from a given ration, it follows that Rumensin should be added to the supplemental feed as a very economical source of energy. First, it is important to observe BCS of cows before they enter the last 1/3 of gestation. Thin cows (BCS below 5.0) should be sorted into another group and feed additional energy along with Rumensin. The goal is to have all cows calve with a body condition score of 5.0 or higher. Supplemental energy and Rumensin may need to be fed through the postpartum interval and into the breeding season in an effort to maintain a BCS of 5.0 during breeding. Following breeding, Rumensin may continue to be fed or may be removed depending upon the availability and quality of forage. Good management of BCS results in improved conception rates and heavier calves at weaning.

Utilization of Rumensin in the replacement heifer program is of even greater importance than in the mature cow herd because of the energy requirements for body growth in addition to pregnancy and body condition needs. It is recommended that Rumensin be fed to heifers from birth through calving and then continued until the heifer is confirmed pregnant with her first calf. Rumensin may be recommended until she gives birth to her second calf and is again confirmed to be pregnant. From that point forward, they would be managed as described for mature beef cows.

Literature Cited

Elanco Animal Health. 1978. Rumensin Technical Manual for Pasture and Range Cattle. Elanco Animal Health, Indianapolis, IN. U.S.A.

1. Granger, A.L., W.E. Wyatt, F.G. Hembry, W.M. Craig and D.L. Thompson, Jr.

2. Effects of breed and wintering diet on heifer postweaning growth and development. 1990 J. Anim. Sci. 68:304.

3. Kunkle, W., and R. Sand. 1990. Good body condition improves rebreeding. Florida Cattleman. July. Pg. 46-48.

4. McCartor, M.M., R.D. Randel and L.H. Carroll. 1979. Dietary alteration of ruminal fermentation on efficiency of growth and onset of puberty in Brangus heifers. J. Anim. Sci. 48:488.

5. Meinert, R.A., C.M.J. Yang, A.J. Heinrichs, and G.A. Varga. 1992. Effect of monensin on growth, reproductive performance, and estimated body composition in Holstein heifers. J. Anim. Sci. 75:257.

6. Moseley, W.M., M.M. McCartor and R.D. Randel. 1977. Effects of monensin on growth and reproductive performance of beef heifers. J. Anim. Sci. 45:961.

7. Randel, R.D., and R.C. Rhodes III. 1980. The effect of dietary monensin on the luteinizing hormone response of prepuberal heifers given a multiple gonadotropin-releasing hormone challenge. J. Anim. Sci. 51:925.

8. Randel, R.D., L.M. Rutter and R.C. Rhodes, III. 1982. Effect of monensin on the estrogen-induced LH surge in prepuberal heifers. J. Anim. Sci., 54:806.

9. 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.

10. Sprott, L.R., T.B. Goehring, J.R. Beverly and L.R. Corah. 1988. Effects of ionophores on cow herd production: A review. J. Anim. Sci. 66: 1340.

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