ONA REPORT

published in

THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL


August 2014

Selecting Heifers as Replacements for the Cow Herd


For questions or comments regarding this publication contact: Dr. Pillip Lancaster, University of Florida, IFAS


As we pregnancy check the cow herd and approach weaning this year, it is again time to think about developing replacements for the cow herd. Developing replacement heifers is one of the most costly aspects of the cow-calf enterprise, especially to achieve high pregnancy rates in heifers developed to calve at 24 months of age. In order for heifers to have a high chance of rebreeding as first-calf cows, they must conceive early in their first breeding season, which requires attainment of puberty at 13-14 months of age to be able to conceive at 15 months of age. Good nutrition is an important part of developing replacement heifers, but there are some criteria we can use when selecting replacement heifers that can increase reproductive success. Some criteria to consider are age, body weight, potential mature size, reproductive tract score, and pelvic area.  These criteria should be used in a 2 stage selection process where age, body weight, and potential mature size are used to evaluate heifers before or at weaning, then reproductive tract score and pelvic area are used to evaluate heifers at 12-13 months of age before the start of the breeding season.

Age is an important factor affecting whether a heifer will achieve puberty prior to the breeding season that is sometimes overlooked. Recent research at the UF IFAS Range Cattle Research and Education Center (REC) indicates that heifers will attain puberty at a minimum age even when fed to heavier body weight prior to that age. Thus, feeding heifers to achieve a target body weight quicker can reduce age at puberty, but only so much. When looking at the replacement heifers at the UF IFAS Range Cattle REC over the last 3 years, the percentage of pregnant heifers decreases as heifers get younger. However, to use age as a selection criterion requires keeping some sort of birth date records, which can be difficult on large expansive ranches. It is not practical to tag and record birth date of all calves on large ranches, but it may be beneficial to designate some herds for selecting replacement heifers where birth date is recorded.

Body weight at weaning is also an important criterion for selecting heifers. Heifers should be fed to achieve a target body weight of 60-65% of mature body weight before the breeding season to have the best chance of becoming pregnant. Thus, heavier heifers will more quickly achieve the target body weight to attain puberty. However, relying on body weight alone to select replacement heifers is not a good practice, because heifers with larger mature size may be inadvertently selected, which can increase the feed requirements of the cow herd. Younger heifers that are heavy for their age may be larger frame, and have larger mature size. Thus, potential mature size should be evaluated to avoid increasing mature size of the cow herd. Frame score can be estimated using age in days and hip height in inches (Ht) with the following formula: Frame score = -11.7086 + (0.4723 x Ht) – (0.0239 x Age) + (0.0000146 x Age2) + (0.0000759 x Ht x Age). Additionally, tables relating age and hip height with frame score for heifers 5-21 months of age can be found by searching the internet. Then, by using frame score we can estimate mature body weight: Mature body weight (lb) = (frame score x 75) + 800. Thus, an estimate of mature body size can be used for selecting replacement heifers just by using age and hip height. Calculating frame score will help identify those heifers that are likely to have larger mature body weight allowing them to be removed from the group of potential replacements, while evaluating age and weaning weight can allow identification of heifers likely to achieve the target body weight before the breeding season.

Reproductive tract score is a subjective estimate of sexual maturity based on ovarian development and palpable size and tone of the reproductive tract. Heifers are given a score of 1 (infantile) to 5 (cycling) at 12-13 months of age. Previous research indicates that heifers with tract scores of 4 and 5 have pregnancy rates 20-30% greater than heifers with tract scores of 1 and 2. Additionally, heifers with tract scores of 4 and 5 conceive earlier in the breeding season, and have 20-30% greater pregnancy rates as first-calf cows during their second breeding season. Thus, it is recommended that heifers with tract scores less than 3 be culled before the breeding season. Reproductive tract score is moderately heritable (h2 = 0.32) indicating that selecting heifers with high tract scores will increase reproductive performance of heifers in the future. A good example of putting this into practice is the Missouri Show-Me-Select heifer program, where in 1998 when the program started 45% of heifers had tract scores of 4 and 5 compared with 100% of heifers having tract scores of 4 and 5 in 2009.

Pelvic area is the product of height and width of the inside of the pelvis, and has a strong relationship with calving difficulty in heifers. Although, calf birth weight is the most important factor impacting calving difficulty in heifers. Therefore, by using expected calf birth weight and pelvic area we can identify heifers with small pelvic area that would be expected to have calving difficulty (Table 1). For example, if we know the expected calf birth weight based on the bull we will use is 70 lb, then we can calculate a threshold pelvic area for 12-13 month old heifers weighing 700 lb to use as culling criteria (70 x 2.2 from Table 1 = 154 cm2 pelvic area). Therefore, heifers with pelvic area less than 150 cm2 should be culled. Pelvic area is highly heritable (h2 = 0.50) such that selecting for larger pelvic area can result in less calving difficulty in replacement heifers in the future. A good example of using pelvic area is demonstrated in the Missouri Show-Me-Select heifer program where 11 years after starting the program 9.5% of Show-Me-Select heifers required calving assistance compared with 25% national average in 2009. In this program, heifers not meeting minimum pelvic area were not bred.

Table 1. Pelvic area/calf birth weight ratios for various heifer weights and ages to estimate deliverable calf birth weight.

 

Age, mo.

Weight, lb.

8-9

12-13

18-19

22-23

500

1.7

2.0

-

-

600

1.8

2.1

-

-

700

1.9

2.2

2.6

-

800

-

2.3

2.7

3.1

900

-

2.4

2.8

3.2

1000

-

2.5

2.9

3.3

1100

-

-

-

3.4

In conclusion, using these criteria, age, body weight, mature size, reproductive tract score, and pelvic area, together to select replacement heifers can improve reproductive success of heifer development programs. This can result in reduced cost for developing heifers, improved lifetime productivity of heifers, and less calving difficulty in heifers.


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