Elvin E. Thomas, Ph.D.
Elanco Animal Health
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
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
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
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.
Elanco Animal Health. 1978. Rumensin Technical Manual
for Pasture and Range Cattle. Elanco Animal Health, Indianapolis, IN. U.S.A.
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3. Kunkle, W., and R. Sand. 1990. Good body condition improves rebreeding. Florida Cattleman. July. Pg. 46-48.
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