In order to maximize profit potential, it is important to have heifers calving at 2 years of age. The time period required for replacement heifers to pay for their development and maintenance is referred to as the payback period. On average, six calves are necessary for the female to pay for her development and maintenance expenses. That is, a 7-year-old female weaning her sixth calf at the breakeven point.
If the female misses producing a calf one year, the payback period extends to nine calves. Research shows heifers becoming pregnant early in their first breeding season, specifically the first 21 days, remain in the herd longer and produce more total calf-weaning weight over their lifetime in production.
How do we select and manage replacement heifers so that they are having fertile heats and are ready to conceive by 14-15 months of age? Genetics, photoperiod, level of nutrition and growth rate all influence when beef heifers will reach puberty. That being said, heifers that have reached 65 percent of their mature weight by this age should have reached puberty and be ready to breed.
Obviously, age should be taken into account—along with other selection criteria—when selecting replacements, with older heifers having an advantage. Heifer calves born earlier in the calving season are produced by cows that conceived earlier in the breeding season.
After heifers are selected, how do we arrive at the target weight they need to gain from weaning until their first breeding season? First, we need an accurate estimate of the average mature cow weight. By using the weights taken at weaning time on the 4- to 7-year-old cows and adjusting to a body condition score (BCS) of five, we can calculate the average mature weight of the cowherd. For example:
• Average weight of mature cowherd: 1,300 pounds
• Average weaning weight of selected heifer calves: 527 lbs. (weaning weights taken on Oct. 1).
• Beginning of breeding season planned for May 1, corresponding to a calving season starting approximately Feb. 8.
• Days from Oct. 1 to May 1: 212 days.
• Target mature weight: 1,300 lbs.
• Mature weight (1,300 lbs.) x 0.65 = 845 lbs. by May 1.
• Gain needed: 845 lbs. – 527 lbs. = 318 lbs.
• Average daily gain needed from weaning to breeding season: 318 lbs./212 lbs. = 1.5 lbs./day.
What is the best way to feed to reach that target weight? In a normal year, spring-born heifers weaned in October are old enough to make good use of wheat pasture available by late November and gain 1.5 lbs. per day (or better) to reach the targeted weight. This would typically be the most cost effective way to develop replacements.
The way this year is shaping up, the cool season grass pasture looks iffy. The good news? It is documented that heifers can rough it and be grown very slowly through the winter months and fed harder for the couple of months going into breeding season in order to reach target weight by breeding season.
This is the development method referred to as SLOW-FAST in Chapter 29 of the newest edition of the OSU Beef Cattle Manual. The SLOW-FAST feeding method for replacement heifers can also be a more cost effective means of reaching the target weight than feeding for a consistent daily gain over the entire feeding period.
Additional management advice
• Don’t use growth implants. While it may be tempting to get the improved gain, replacement heifers should not be implanted at weaning (or later), as it will decrease fertility and pregnancy rates.
• Use ionophores (monensin or lasalocid) when possible. They are proven to cost effectively improve growth with no downside to fertility.
• Target a BCS of six by the beginning of breeding season. It’s not only important that heifers reach target weight, but it’s also important they don’t get overly fat, as this can reduce fertility as well. — Mark Z. Johnson, Oklahoma State University Extension beef cattle breeding specialist
