It’s not too late to get heifers to target weight by breeding

Elaine Grings, South Dakota State University | 

Depending on your calving season, breeding may be only a few months away. Will your replacement heifers be ready?
There has been much talk over the years about the proper weight for beef heifers at first breeding, with estimates ranging from about 55 to 65 percent of mature body weight as the target goal.
To be able to hit that target, you need to know the average weight of your mature cows. On average, mature cow weight seems to have increased by about 12 percent over the last 25 years. Be sure you have a reasonable estimate of cow weights for your current herd.
Once you know your target breeding weight, you can plan for the average rate of gain needed for your heifers between weaning and breeding. But does it matter when heifers add weight between weaning and breeding?
Over the years, a number of studies have shown that as long as heifers reach their target weight, the timing of the gain may not be critical. Development strategies that take advantage of low cost feeds early in development, followed by feeding heifers to make faster rates of gain in the last 60 to 90 days before breeding have been successful.
Let’s take a look at examples:
A heifer calf weaned at 550 lbs needs to gain between 1.0 and 1.6 lbs per day from weaning to breeding, depending on your goal weight (55 to 65 percent of mature weight) and your mature cow weight. If your cow size is 1400 lbs, your heifers need to gain between 220 to 360 lbs between weaning and breeding.

• 1400 lbs mature weight at 5-years of age x 0.55 = 770 lbs at breeding – 550 lbs at weaning = 220 lbs gain
• 1400 lbs mature weight at 5-years of age x 0.65 = 910 lbs at breeding – 550 lbs at weaning = 360 lbs gain
• Assume 230 days between weaning and breeding:
  • 220 lbs gain/230 days = 1.0 lb/d
  • 360 lbs gain/230 days = 1.6 lb/d

These rates of gain can be continuous for the entire 230 day period or altered such that heifers gain at slower rates for the first 140 to 170 days, followed by more rapid gains in the last 2 to 3 months before breeding. Several research studies have reported that this delayed gain strategy can be used to help save heifer development costs without impacting reproductive performance.
At USDA-ARS Miles City, one group of heifers in an early spring calving herd were raised on pasture with supplemental hay for 130 days after weaning and then moved to drylots where they were fed a diet based on corn silage and barley for 91 days. Gain during the pasture phase averaged 0.6 lb/d and during the drylot phase heifers gained 2.5 lb/d (for an average of 1.4 lb/d).
These heifers were compared with another group that was fed a corn silage and hay based diet in the drylot from weaning to breeding and gained an average of 1.5 lb/d. The number of heifers cycling at the beginning of the breeding season and the proportion pregnant at fall pregnancy check were not different for the two groups.
In this study, feed costs were similar for the two groups of heifers, but other diets could be formulated that might save on feed costs.
When using a delayed gain strategy, it is important to have enough time on the high gain diet to ensure that heifers will reach the target weight. Researchers in Kansas who fed heifers low gain diets that resulted in gains of 0.33 lbs/d followed by 2.7 lbs/d for the last 50 days before breeding for an overall average gain of 1.1 lb/d observed a decreased number of heifers cycling at the beginning of the breeding season but pregnancy rates were similar to heifers that gained at constant rate of 1.2 lbs/d from weaning to breeding.
Because heifers that conceive earlier in the breeding season will have greater lifetime productivity, this difference in cyclicity at the beginning of the breeding season may be quite important. Aggressive estrous synchronization protocols may help to get heifers bred earlier in the breeding season and these programs may be combined effectively with delayed gain strategies.
It is important to be realistic about the rate of gain heifers can make during the last few months before breeding. Heifers that had been making slow rates of gain may be more efficient in their gains when initially shifted to the higher gain diets.
However, this effect will be short-lived and should not be counted on for the entire period of greater gain. The chance of adverse environmental conditions also needs to be taken into account. Excessive mud in a drylot could cause sufficient performance losses to either cause heifers to miss the target, or wipe out any cost savings that might occur earlier in the development period.
If your heifers haven’t gained as well as you’d like this winter, there is still time to get them ready for the breeding season. Take a good look and evaluate whether you should step up their gains during this last few months before breeding.