Growing Beef Newsletter
January 2025, Volume 15, Issue 7
Post-breeding performance of heifers developed at two different rates, part 2
Sarah Phelps, animal breeding and genetics graduate student, Iowa State University
(This article follows the December 2024 article titled Developing heifer body composition traits at two different rates.)
The profitability of cow-calf operations is largely influenced by female reproductive success. Heifer development practices should prioritize promoting early conception and improving overall pregnancy rates to achieve maximum production efficiency. The Iowa Beef Industry Council funded a research project with Iowa State University to characterize the body composition of developing heifers to increase the understanding of heifer development and pregnancy.
It was detailed in the first article that purebred Angus heifers from the Iowa State University McNay Memorial Research and Demonstration farm were split into two groups: a lower average daily gain (restricted; developed to 55% of mature body weight at time of breeding) and a moderate average daily gain group (non-restricted; developed to 65% of mature body weight at time of breeding). On average, the non-restricted group had more backfat, larger ribeye areas, more intramuscular fat, a higher percentage of mature reproductive tracts, and a higher response rate to estrus synchronization than heifers developed in the restricted group at breeding.
After artificial insemination (AI) breeding (Day 231), heifers were managed as one group on pasture. Bulls were turned out with heifers 10 days post insemination for a 45-day breeding season. Initial pregnancy determination was conducted 30 days post-insemination by ultrasound imaging. A final pregnancy diagnosis was performed 30 days following bull removal. Carcass and body weight measurements were collected during the final pregnancy diagnosis 30 days following bull removal to monitor changes in body composition due to the management of heifers on pasture.
The average group measurements from weaning (Day 0) until final pregnancy diagnosis (Day 338) are shown in Figure 1. On Day 178, it became apparent heifers were being overdeveloped relative to their target weight, and both rations were altered, explaining the decrease in developmental rate between Day 178 and Day 231. The result of managing both groups on pasture is shown on Day 338. Both groups experienced a reduction in ribeye area and backfat thickness, with a more pronounced decrease for the non-restricted group. The restricted group continued to gain weight on pasture, while the non-restricted group lost weight. Interestingly, intramuscular fat increased for both groups while on pasture.
Figure 1: Development of mean body weight (Weight) and carcass ultrasound measurements for ribeye area (REA), backfat thickness (FAT), and intramuscular fat (IMF) from weaning (Day 0) to final pregnancy diagnosis (Day 338) for heifers fed diets differing in energy concentrations (restricted vs non-restricted).
The average change in dietary group body composition measurements from transitioning heifers to pasture management are shown in Figure 2, utilizing boxplots to represent this change. The black horizontal line represents the median, or middle, value of that group, while the blue box represents the spread in measurements. Values below 0 indicate the heifers lost condition during this time frame. In Figure 1, we can see that on average, heifers lost REA but gained IMF during this period, regardless of group, and heifers in the non-restricted group lost more FAT and RUMP compared to the restricted group. This illustration allows us to see the more drastic reduction in REA, FAT, and RUMP for the non-restricted group compared to the restricted group when transitioning to pasture management.
Figure 2: The amount of change (Δ) in heifer ultrasound carcass measurements for ribeye area (REA), intramuscular fat (IMF), backfat thickness (FAT), and rump fat thickness (RUMP) from artificial insemination (AI) to final pregnancy determination, split by dietary group (restricted vs non-restricted). Restricted are heifers developed to 55% of mature body weight at time of breeder while non-restricted heifers were developed to 65% of mature body weight.
The pregnancy rates for both groups are shown in Table 1 based on the number of heifers exposed. Although there is a numerical difference between groups for AI pregnancies, there was no significant statistical difference. Two AI pregnancies were lost in the non-restricted group, and four were lost in the restricted group. The final pregnancy percentage was 1.85% higher in the restricted compared to the non-restricted.
Table 1: Pregnancy rates for heifers on different energy concentration diets (restricted vs non-restricted) bred by artificial insemination (AI) and natural service (NS).
These results illustrate the effect of development practices on heifer pregnancy. Developing heifers to a lower percentage of mature weight of 50 to 55% at breeding could result in more culling before breeding and lower AI conception rates compared to heifers developed to larger body weights. Heifers developed to larger body weights (60 to 65% of mature weight) at the time of breeding had a better response to estrus synchronization and AI conception, but when transitioned from the dry lot to pasture, lost more weight and condition compared to lighter contemporaries.
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