Growing Pains


How To Get More Heifer Calves

by Mel DeJarnette, director of technical research, Select Sires Inc.


I thought that title might get your attention! Seems like everyone is interested in how to get more heifers from A.I. these days. Herd expansion is one reason for a high heifer demand and a second reason is often less than desirable conception rates.

Because dairy herds typically run a 20 to 30% annual cull rate, 40 to 60% of the cows must achieve a 12 to 12� month calving interval simply to maintain herd size. Higher fertility levels must be achieved if additional heifers are to be made available for expansion.

Unfortunately, herd expansion and conception rates often appear to be antagonistic. As herd size increases, herd fertility tends to decrease resulting in fewer calves born each year and thus making maintenance of herd size a difficult task, much less having any extra heifers. The reasons for this reduced fertility are numerous but are usually associated with increased stress on cows due to high production, overcrowding, inadequate nutrition, or the difficulties associated with heat detection and proper insemination timing in large herds.

Only Proven Method

Whatever the reason, everyone seems to be interested in a simple and easy short cut to get more heifer calves. As far as I know, there is only one time tested and proven method to get more heifer calves, and that is: get more calves. For every two calves born, one will usually be a heifer. In other words, focus on improving conception rates in your herd and the heifer needs will take care of themselves.

In the previous four "Growing Pains" articles I discussed procedures that will facilitate high fertilization rates in your herd. In general, this involves good heat detection and proper timing of insemination with high quality, fertile semen. However, getting the egg fertilized is just the first step. In order for maximal pregnancy rates to be achieved, cows and their reproductive systems must be in sufficient physiological and metabolic states to support and nurture further development of fragile embryos.

Body Condition Scoring

Body Condition Scoring (BCS) is undoubtedly one of the greatest indicators available of fertility potential for a given herd or for a given cow. Body condition scoring is a visual assessment of the overall fleshiness of the animal. Body condition is a direct indication of the amount of fat stored by the animal and thus, of the nutrition program, in general. The degree of body condition also reflects the potential energy reserves that will be available for utilization during periods of suboptimal nutrition (i.e. post partum negative energy balance).

The following table displays an example of the numerous controlled research trials, indicating body condition and the magnitude of loss in body condition after calving. It shows the tremendous impact BCS has on several important parameters of reproductive performance. Despite this overwhelming evidence, routine use of BCS in dairy management programs appears to be the exception and not the rule.

Body Condition Score Loss After Calving
  <.5 0.5-1.0 >1.0
Days to 1st ovulation 27 31 42
Days to 1st estrus 48 41 62
Days to 1st service 68 67 79
1st service conception rate % 65 53 17
(Adapted from Butler and Smith, 1989. J Dairy Sci.)

The single most critical time for a cow to be in optimum body condition is at calving. Energy needs during the first 30 to 40 days after calving are so great that it is basically impossible for cows to consume enough feed to satisfy the demand. Therefore, all cows will loose weight and body condition after calving. However, proper feeding and monitoring of BCS will ensure that weight loss is minimal and will induce cows to cycle back with fertile heats sooner after calving. Ideally cows should not loose more than one BCS postpartum before they start regaining weight.

All cows and heifers should calve with a BCS of about 3.5 + .25. Therefore, the most important time for you to score the body condition of your cows is at about 200 days in milk. This allows you adequate time to adjust rations so that BCS can be gradually adjusted (up or down). Dramatic changes in BCS during the last month or so of gestation are often associated with calving problems or post-partum metabolic disorders.

Use the scoring guide on page 9 to monitor BCS and the nutritional status of your herd. Remember that BCS is not only a reflection of your nutrition program, but also of the fertility potential of your herd. Proper body condition is essential to ensure that your cows are capable of maintaining embryonic development once fertilization occurs. Work with your nutritionist, your veterinarian and your Select Sires sales representative to establish a regular routine for body condition scoring your cows and coordinating needed ration and/or grouping changes.

Recent Research Related To Gender Preselection

Over the years, numerous methods have been suggested to alter the sex ratio of offspring. One of the most common is based on the notion that male sperm swim faster than female, but they die sooner. Thus, if you inseminate early, the male sperm die resulting in more heifer sperm fertilizing the eggs. To my knowledge, there is no conclusive, repeatable data to support the idea that male and female sperm vary in either swimming speed or longevity.

A recent article published in Animal Reproduction Science presented some interesting data to support the timing theory. Using a device which detects changes in vaginal/cervical mucous around the time of estrus, these researchers varied the time of insemination and supposedly skewed the sex ratio of resulting offspring with 90% accuracy. However, the data set presented was extremely small (13 to 14 animals/treatment). Until these results are duplicated in other laboratories and/or on a much larger scale, I�d consider this technology for altering sex ratio as strictly experimental.

An abstract presented at the 1998 Animal and Dairy Science meeting from Dr. Ray Nebel�s lab at Virginia Tech, compared the gender ratios for 822 calvings as affected by time from first mount to insemination. First mount was identified by the computerized "Heat Watch" system and time to A.I. and gender ratios were retrospectively analyzed. Dr. Nebel's data (figure 1) would not support the theory that timing of A.I. has any affect on gender ratios.




Dr. George Siedel and researchers at Colorado State University also presented some encouraging data at this meeting on their progress with sexed semen.




Using very low doses of sorted, non-frozen semen and deep horn insemination in virgin heifers, they achieved approximately 40% pregnancy rates with 90% of resulting offspring being of the desired sex. However, just as an example of what small numbers in a data set can do, the frozen control semen in this project that had not been sorted for sex resulted in 80% heifer calves.

Sexed semen technology has a way to go before commercial application will be realized. Among other problems, the current estimated cost of this technology at about $100 per straw. Additionally, because the sorting procedure damages many sperm, it is yet to be determined if acceptable fertility can be maintained with sperm that are frozen and thawed after the sorting procedures.

Only time will tell if technology will provide cost effective methods for altering the sex ratios of A.I. offspring in the future. At present, however, there is one and only one method that is available to every dairy and beef producer that has stood the test of time to get more heifers; get more calves!

Body Condition Scoring

Score

  1. Individual short ribs have limited flesh covering, and the ends are sharp to touch. Vertebrae of the chine, loin and rump regions are distinct, and hooks and pin bones are sharp. Severe depressions between hooks and pin bones are present.


  2. Individual short ribs are apparent but not prominent and are sharp to the touch but with more flesh than 1. Vertebrae of chine, loin and rump are not highly visible but can be felt. Area below the tailhead and between pins is depressed but more fleshed than 1.


  3. Short ribs can be felt but not easily seen. Vertebrae of the chine, loin and rump are rounded and smooth. Areas between the pin bones and around the tailhead are smooth, with no signs of fat deposits.


  4. Short ribs felt only through firm pressure. The ridge of the backbone in the chine region is rounded and smooth, and the loin and rump regions appear flat. Hooks are rounded, and the area around the tailhead and pin bones is rounded, with evidence of fat deposits.


  5. Bone structure of the backbone, short ribs, hooks and pin bones is not apparent. Extreme fat deposits. Tailhead is buried in fatty tissue.


  6. (Adapted from Wildman et al., 1982)