This article originally appeared in the Jan. 2011 Ag News and Views newsletter.
The traditional recommendation for developing replacement heifers is to feed them to achieve 65 percent of their mature weight by the beginning of the breeding season. This recommendation was developed decades ago based upon some research that indicated that almost all heifers would attain puberty by 65 percent of mature weight. However, some recent research may call this long held recommendation into question.
Researchers from Nebraska (Martin et al., 2008. Journal of Animal Science 86, p. 451-459) conducted a very interesting study. These researchers fed crossbred heifers to achieve either 55 percent or 50 percent of their mature weight by breeding season. The heifers were exposed to fertile bulls at a ratio of one bull to 25 heifers. The heavier heifers were exposed to bulls for 45 days, while the lighter heifers were exposed for 60 days. Interestingly, there was no difference in conception rate between these two groups - the conception rate was a very acceptable 88 percent. These data would seem to indicate that a target breeding weight of 65 percent may need to be reconsidered in some situations.
Let's take a look at what a reduced target breeding weight can mean. If we assume that mature weight of an example herd of cows is 1,200 pounds, then our 65 percent target breeding weight would be 780 pounds. If we assume we weaned a 450-pound heifer calf at 210 days, then we need this heifer to gain 330 pounds in the next 200 days. That is 1.65 pounds per day. During the winter, an average daily gain (ADG) this high will likely require good quality pasture or hay and a significant amount of supplement. If we reduced our target to 50 percent, then the required ADG is only 0.75 pounds per day. This gain could likely be achieved by grazing dormant native range with minimal protein supplementation. This low input program would likely reduce expenses and labor requirements. It is also possible that a low input development system would challenge these heifers a little, and the least efficient and least adapted heifers would be eliminated from the herd.
There are a few issues to consider with developing heifers to lighter weights. Obviously, there is a point at which heifers that are too light in weight won't cycle and therefore won't get bred. Secondly, in the Nebraska research, calves born from heifers bred at 50 percent of their mature weight were, on average, seven days younger and 13 pounds lighter than calves born from the 55 percent group. However, the lighter heifers cost an average of $17 less to develop than the heavier heifers.
Every ranch has a unique set of resources and opportunities. Be sure to put a pencil to the decisions you are making and determine which production practices may pay off in your situation. We may not be ready to recommend that you reduce your heifer breeding weight target based upon this one study, but don't get caught in the trap of doing things the same way just because that is the way you've always done them.