Monitoring heifer development
By Sandy Johnson
K-State Extension Livestock Specialist
Rations delivered in a dry lot setting have been commonly used to assure sufficient gains for heifers to achieve puberty and conceive at 14 to 16 months of age. Historically, recommendations have been to have heifers reach 60 to 65 percent of their mature body weight prior to breeding. As weaning weights and growth rates of cattle have improved over time, that target has become relatively easy to achieve, especially in a feedlot setting. From time to time there were reports of heifers that were artificially inseminated in a feedlot setting that apparently stopped cycling after they went to pasture. Generally there were no data to sort out possible problems. Now we may have a better understanding of what happens to heifer gain when taking feedlot developed replacement heifers to pasture.
Let's consider the performance of one group of heifers receiving a sorghum silage based diet. Heifers averaged 522 pounds at weaning and gained 1.1 pounds per day for the first 30 days post weaning. The next weight was taken in February when heifers averaged 774 pounds and had gained 2.2 pounds per day. To reach 60 percent of a mature weight of 1,325 pounds by the end of April, heifers only needed to gain 20 pounds more. Three weeks prior to breeding, heifers weighed 865 pounds and averaged a body condition score of 7. From weaning to breeding, heifers gained 2 pounds per day whereas 1.4 pounds per day would have achieved a target weight of 60 percent. In more recent research, target weights for heifers as low as 53 percent have been successful, which would have required only 0.9 pounds per day of gain.
Yearling operations rough cattle through the winter and take advantage of compensatory gains when these calves are turned out on spring grass. While weights and condition of these heifers were not recorded after turnout, experience says the most likely outcome was a loss of condition on grass. Pregnancy rate to AI for these heifers was only 45 percent, lower than expected. Included on the list of possible reasons why AI pregnancy rate was not higher was the change in diet when the heifers went to pasture shortly after AI. Nutritional stress around the time of breeding has been shown to be detrimental. Heifers that were provided 85 percent of energy and protein requirements had reduced embryonic development on day 3 and 8 compared to those that received 100 percent of requirements.
We know that grazing is a learned behavior and when exposed to novel feedstuffs, young livestock will consume small amounts and increase consumption if no negative effects occur. Considerably more time and energy may be spent foraging when animals are introduced to novel foods. Since these heifers had been in the feedlot from weaning until turnout after AI, they may not have consumed enough forage in the early days after turnout, resulting in some embryonic loss.
A South Dakota study (Perry and co-workers) originally designed to compare early weaned heifers developed on range from weaning to breeding to heifers weaned at a traditional age and developed in the feedlot, showed a large difference in gain the first month heifers were on native range as yearlings. The first 30 days on spring pasture, heifers that had been developed on range over the winter gained 2 pounds per day while the feedlot developed heifers only gained 0.3 pounds per day. Average daily gain for the remainder of the grazing season was not different.
Two additional studies by Perry and coworkers were developed to look at heifer performance when turned out on spring grass. In one study, feedlot developed heifers were moved to pasture after an AI program and half received a supplement (5 pounds per head day DDGS) the first 30 days on pasture. Unsupplemented heifers lost 37 pounds and supplemented heifers gained 45 pounds when forage quantity was not limiting. Pregnancy rate to AI was higher in the supplemented group.
A study done in two replicates compared feedlot developed heifers that grazed for 30 days prior to AI to heifers that remained in the feedlot through AI. Immediately after AI, all heifers grazed the same pasture. Gain for 35 days post AI was higher or tended to be higher for heifers that grazed prior to AI compared to those that remained in the feedlot. Pregnancy rate to AI was numerically higher for heifers that had grazing experience prior to AI but more observations are needed to conclude this reproductive response could be expected consistently.
These studies indicate performance of yearling heifers the first month on grass may be improved by previous grazing experience or supplementation. Avoiding nutritional stress during the breeding season may reduce embryonic loss and increase the number of early pregnancies. Over-conditioned heifers are expensive to produce and are not positioned to take advantage of spring grass. Weighing heifers at the beginning of the development phase is the first step in achieving targeted gains. Monitoring gains at least twice before breeding should allow time for adjustments to be made.