Cow-Calf Commentary for Iowa Cattleman Magazine
By Sarah Phelps, ISU graduate student
January 2025
Iowa Beef Industry Council Funded Research Project: Developing heifer body composition traits at two different rates
Part 1
Female reproductive performance drives the profitability of cow-calf operations. Heifer reproductive success is particularly important since a calf has not yet been produced to recover a proportion of the female’s input costs. Therefore, understanding the mechanisms that affect heifer pregnancy can improve heifer development practices and increase profitability for cow-calf operations. Traditional practices use body condition scores (BCS) and body weight to indicate puberty onset but may not accurately reflect body composition. The Iowa Beef Industry Council funded a research project with Iowa State University to characterize body composition of developing heifers to increase the understanding of heifer development and potentially identify indicators for heifer pregnancy.
The objectives of this study were to:
- Understand the development of body composition traits in pre-breeding heifers
- Evaluate muscle and fat deposition from two different management styles
- Determine the association between body composition traits and heifer pregnancy
Purebred Angus heifers from the ISU McNay Memorial Research and Demonstration farm were divided into two groups at weaning based on age and weight: lower average daily gain (restricted; n=61) and moderate average daily gain (non-restricted; n=63). The restricted group of heifers was developed to 55% of mature body weight at breeding, and the non-restricted group was developed to 65%. Weight, BCS, and carcass ultrasound measurements for ribeye area, backfat, and intramuscular fat were collected at six time points between weaning (October 2023) and final pregnancy diagnosis (September 2024). Ultrasound measurements were quantified by The CUP Lab in Ames, IA.
The average measurements of each group over time during pre-breeding development is shown in Figure 1. On average, heifers in the restricted group were lighter at time of breeding with smaller ribeye area, less backfat, and lower intramuscular fat percentage compared to the non-restricted heifers.
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 breeding (Day 231) for heifers fed diets differing in energy concentrations (restricted vs non-restricted).
Before breeding, pelvic area measurements and reproductive tract scores (RTS) were taken. Table 1 displays the average RTS, pelvic area, and pelvic ratio measurements between groups. On average, the non-restricted group had more developed reproductive tracts with larger pelvic areas than the restricted group at the time of breeding. Eleven heifers were culled due to RTS and pelvic measurements, with seven in the restricted group and four in the non-restricted group.
Table 1: Reproductive measurements for reproductive tract scores (RTS), pelvic area, and pelvic ratio at time of breeding in heifers.
Heifers were synchronized using a 7-day CO-Synch + CIDR AI protocol the last week of May. An Estrotect heat detection patch was placed on the tailhead of heifers to monitor estrus response. Heifers were considered “responded” if any portion of the patch was abraded. Table 2 displays the estrous response measured during artificial insemination (AI) breeding. All heifers in the non-restricted group displayed estrous, while only 75% of the heifers in the restricted group did.
Table 2: Estrous response rates to synchronization using the 7-day CO-Synch + CIDR AI protocol within dietary groups of heifers (restricted vs non-restricted).
Results indicate that heifers developed to 65% of mature body weight have more backfat, larger ribeye areas, more intramuscular fat, higher percentage of mature reproductive tracts, and higher estrous response rates to synchronization compared to heifers developed to a lower percentage of mature weight. More heifers in the restricted group were culled prior to breeding than in the non-restricted group. Previous research has demonstrated that heifers could be developed to 55% percentage of their mature body weight without a significant loss in pregnancy rate but this study illustrates the potential of culling heifers prior to breeding due to development and decreased response to estrus synchronization for AI. The pregnancy results from this study will be presented in a second article published next month.
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