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Weaning Weight Comparison of UT Angus
Calves Supports Validity of Milk EPDs

Bobby Simpson and Jackie Martin, Knoxville Experiment Station
David Kirkpatrick, Extension Animal Science,
The University of Tennessee, Institute of Agriculture

Introduction
The importance and use of expected progeny differences (EPDs) in beef cattle selection is often a topic of interest and discussion among beef cattle producers. An EPD for a particular trait is simply an estimate of how progeny of one individual would be expected to differ from progeny of another individual of the same breed for that particular trait. Most breed associations monitor EPDs for birth weight, weaning weight, yearling weight, scrotal circumference, and several carcass characteristics.

Another important trait for which EPDs are monitored is that of milk production. The milk EPD for a given sire pertains to the expected difference in weaning weight of calves weaned by daughters of that sire. Thus, producers breeding a group of cows with hopes of producing replacement heifers would ideally breed those cows to a sire with a moderate to high EPD for milk, assuming that pasture or feed resources are available to support moderate to high milking females.


Procedures
A recent analysis of data from The University of Tennessee, Knoxville Experiment Station purebred Angus herd demonstrates the importance of EPDs for milk production. Only first-calf Angus heifers and their first calf were analyzed in the study. All heifers calved first as 2-year-olds. Heifers (n = 115) from three consecutive calving seasons (1996, 1997, and 1998) were used in the analysis. Heifers were fed and managed as a single group during the time at and around calving. All heifers and calves were managed similarly from calving to weaning. The first-calf heifers were retrospectively divided into three groups, based on the milk EPD of their sire. The three groups were as follows:

High (n = 28) - Heifers sired by bulls with milk EPD of 22 or greater;
Medium (n = 54) - Heifers sired by bulls with milk EPD of 14 to 21;
 Low (n = 33) - Heifers sired by bulls with milk EPD of 13 or less

Adjusted 205-day weights (Adj205wt) of the heifers' first calf were compared among the three groups. Adj205wt comparisons were made after statistically taking into account the weaning weight EPD of each calf's sire. A regression analysis was conducted to further evaluate the relationship between milk EPD of the dam's sire with Adj205wt of the dam's calf. The calving interval between the first and second calves of the heifers was also compared among the three groups.


Results
The Adj205wt (lb) of calves from heifers in the High (649) and Medium (626) groups were significantly higher than calves from heifers in the Low (587) group (Figure 1). The pattern of average Adj205wt exactly mirrored the mi lk EPD of the first-calf heifersí sires (i.e., first-calf heifers sired by bulls with higher milk EPDs weaned calves with higher Adj205wt). In addition, regression analysis revealed that for every 1 unit increase in milk EPD of the heifersí sire, there w as a 4.7 pound increase in the Adj205wt of the heifersí first calf. These observations add credibility to the reliability of milk EPDs, at least in the Angus breed.

More interestingly, however, even though heifers in the High and Medium groups produced calves with higher weaning weights, assumedly as a result of higher milk production, the calving interval between their first and second calves did not differ stati stically among any of the three groups. The calving interval was 366, 372, and 362 days for the High, Medium, and Low heifers, respectively. These calving intervals were very close to our goal of 365 days. This indicates that with adequate nutrition an d management, the higher-producing first-calf heifers were able to produce more milk for their calves without having a prolonged interval between their first and second calves.


Implications
Overall, the first-calf heifers used in this analysis possessed a high level of genetic potential for milk production. This is largely a result of a progressive artificial insemination program that has been implemented over the past 15 years at The University of Tennessee. The average EPD for milk production for current sires in the Angus breed is 13 (American Angus Association Sire Evaluation Report, Fall 1999), compared to milk EPD averages of 23 (High), 18 (Medium), and 10 (Low) for the sires of t he first-calf heifers used in this analysis.

Results from the Adj205wt comparison of calves produced by first-calf heifers in the High, Medium, and Low groups were as expected. These results should provide confidence, at least to Angus breeders, that EPDs can be used to improve certain production traits. Each breed has its own set of EPDs and must be evaluated independently.

The finding that first-calf heifers with higher milk-producing ability had calving intervals comparable to heifers with lower milk-producing ability demonstrates that there are beef females with the capability to produce at high levels without compromi sing reproductive performance. However, it should be pointed out that all heifers included in this analysis were on a high plane of nutrition (free-choice corn silage, hay and a complete mineral mixture) during the calving and breeding seasons. We would expect, with great certainty, that heifers with high milk-producing potential whose nutritional requirements are not met would achieve much less desirable calving intervals.