Ionophores for Replacement Heifers

August 9, 2009 07:00 PM

By Glenn Selk

The fall breeding season for fall-born replacement heifers will be here before you know it.  Late summer and fall supplementation of these yearling heifers is often necessary to achieve the growth needed for a successful breeding season.

In an effort to insure more replacement heifers are bred to calve early in their first calving season, ranchers should consider using a supplement containing an ionophore in the growing diet of the heifers. "Ionophore" is the generalized name for the feed additives monensin and lasalocid. Both are presently approved for use with growing programs for replacement heifers. 

Research (Moseley et al., 1977) has shown that ionophores, such as monensin and lasalocid, will hasten onset of puberty in heifers (Scroll down to view Table 1).Age to puberty decreased and percent cyclicity at the beginning of the breeding season increased in monensin fed heifers, resulting in increased pregnancy rate during a short breeding season. Similar data is available for lasalocid.   

These heifers were fed the label-recommended dosage of monensin for approximately 100 days before the start of the breeding season.  This should mean that a higher percentage of heifers would be cycling at, or before the start of the breeding season. 

Most stocker cattle research has indicated that the addition of a label-approved dosage of an ionophore will increase average daily gain by 0.1 to 0.2 lb./day. Over a 150 day growing period of a replacement heifer, this means an additional 15-30 lb. in average weight improvement of the heifers by breeding time. 

There is an additional benefit that has often been observed when replacement heifers are fed a diet with an ionophore. The likelihood of heifers becoming ill with coccidiosis is greatly reduced due to the presence of the ionophore. 

Heifers that have been fed an ionophore will not be eligible to be sold in most "natural” or "organic” marketing programs.

(Scroll down to see Table 1.)

Table 1
Effect of Ionophore (monensin)
on Puberty, Conception, and Pregnancy Rates in Yearling Heifers


No. of Heifers

% Cycling

% Conception
of cycling heifers

% pregnant to AI
of total heifers











(Moseley, et al. 1977. Journ. of Amim. Sci.  vol. 45:961)      


Glenn Selk is Extension beef cattle specialist at Oklahoma State University. You can read more from Selk at his Web site "Cow-Calf Corner."

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