Getting heifers to their first breeding in the right body condition and on time can establish their “reproductive momentum” for a lifetime. An article in the January 2013 Bovine Veterinarian explained why creating this momentum in the herd is important.
Creating reproductive momentum so that all cows and heifers breed, calve and re-breed in a timely manner every year starts with the first-calf heifer. On any given operation there may be heifers ready to breed for the first time, and heifers ready to calve for the first time, whether they are home-raised or purchased replacements. Both groups need some attention.
Start with the end in mind
On a herd level, mature cows tend to calve near the same date each year if nutrition and bull fertility are adequate. To keep a beef herd rolling along reproductively year after year, it’s important to strategically plan the heifer’s first breeding.
The goal of at least 80% of the females cycling before the start of the breeding season is a minimum for all heifers, first-calf heifers, and mature cows. The best available estimate is that about two-thirds (65%) of matings between a fertile bull and fertile cow will result in a pregnancy that can be detected at typical preg-check times, says Bob Larson, DVM, PhD, Dipl. ACVPM, Dipl. ACT, Kansas State University.
Successful pregnancy is less likely to occur on the first cycle after calving (or on the first cycle after puberty), therefore, cows that are cycling before the start of the breeding season are more likely to become pregnant at their first exposure to bulls than even cows that resume cycling during the first 21 days of the breeding season. ”Therefore, in order to have at least 60- 65% of cows calving in the first 21 days of the calving season, essentially all of the cows need to be cycling by the end of the first 21 days of the breeding season,” Larson says.
To make sure that the mature cows have a good opportunity to calve in the first 21 days of the calving season throughout their lives, for their first calf they need to calve at least 80 to 100 days ahead of the start of the mature cow breeding season, which places this calving ahead of the start of calving for the mature cows, explains Larson.
Larson’s ideal is for heifers to calve two cycles ahead of the mature cows because on average, first-calf heifers require about 40 additional days to resume fertile cycles compared to mature cows. However, it is important to consider individual herd situations.
“My recommendations vary from targeting heifers to be bred 42 days ahead of the cows to 30 days to two weeks,” Larson suggests. “But because my goal is to have all the heifers bred before the start of the mature cow breeding season, starting the heifer breeding season 42 days ahead of the cows allows two opportunities to become pregnant – three if utilizing estrous synchronization at the start of the heifer breeding season. If I shorten the time frame for heifers to become pregnant ahead of the mature cows (for example 14 or 30 days), I must use estrous synchronization to have one or possibly two opportunities for the heifers to become pregnant.”
Utilizing estrous synchronization is certainly a useful tool to allow heifers multiple opportunities to become pregnant in a short breeding season, but it is not an essential tool if the heifers have a breeding season longer than 40 days, Larson adds.
Research has indicated that herd productivity is improved when heifers conceive early in their first breeding season. To breed early they have to reach puberty prior to the start of the breeding season. The mean age at puberty for cohorts of North American beef heifers is typically 11.5 to 14 months of age with some variation around this estimate. “This means that at 11.5 to 14 months of age, half of heifers are not cycling,” Larson says. “We need at least 80% cycling at the start of breeding to have a successful heifer breeding season.”
Depending on the situation (available forage and feed as well as marketing options) many times Larson recommends that under-developed heifers are exposed to bulls starting on his optimum date for the herd and then only recommends keeping for replacements those heifers that become pregnant early in the breeding season.
“If the group of heifers is underdeveloped, it is very possible that few will become pregnant early enough to calve ahead of the mature cows,” he says. “Regardless of the number of heifers that meet this goal, I do not recommend keeping heifers that are bred to calve much later than the start of the mature cow calving season.”
Heifer pre-breeding exams
Making sure a group of heifers is physically mature and cycling at breeding time is critical to the program. Larson says getting an arm in these heifers about six weeks prior to breeding is important. “It is important to have nearly all the heifers cycling before the start of the breeding season, and the only way I can be sure the group is reproductively mature is to palpate their reproductive tract.” (See reproductive tract scoring sidebar)
When Larson conducts a pre-breeding soundness examination of heifers he evaluates their body weight, reproductive tract maturity and pelvic area. “If a heifer has any abnormalities or is extremely immature, I will cull her as a replacement candidate.” Heifers should be at 60-65% of mature cow body weight at this time. Many herds rarely if ever weigh mature cows, but frame score and sale weights of cull cows can be used to estimate mature cow weight.
At the time of pre-breeding exams if heifers are extremely immature because of young age or low weight gain, it may not be possible to adjust their diet to ensure that they reach puberty by the start of the breeding season. However, many situations with groups of replacement heifers that are moderately immature for their desired breeding date can be greatly improved by six weeks of increased weight gain.
Larson notes that the pre-breeding evaluation is an assessment of whether or not the group of replacement heifers is well-developed and ready for breeding more than an assessment of the breeding potential of individual heifers. “Therefore, I will make recommendations for the entire group rather than very many individual culling decisions.”
In many situations, if adequate numbers of replacement heifers are born early in the calving season and forage quality and availability is very good, many heifers in the replacement pool will reach puberty at an age that will allow them to be bred to calve ahead of the mature cows. However, if forage production or availability is limiting for any substantial time from birth to puberty so that necessary weight gains are not met on forage alone (or forage with a small amount of supplementation), then few heifers in the replacement pool are likely to have reached puberty by the start of a breeding season targeted ahead of the mature herd.
“This variation in heifer performance based on year-to-year variation in forage production means that the reproductive performance will vary greatly from year-to-year if management is held constant,” Larson explains. “By evaluating heifers several weeks ahead of the breeding season, year-specific adjustments can be made in management to allow consistently good reproductive performance of replacement heifers.”
Exams six weeks ahead of breeding isn’t the only thing a veterinarian can do at this time, as it’s about the start of the calving season for the mature cows. “I’m on the farm anyway looking at these heifers, so I should check out the cows. This is also a good time to be looking at the bulls, especially the bulls that are going to go in with the heifers.”
First calving, second breeding
Most heifers in a replacement group should calve in a body condition score of 6 (on a 9-point scale) with the rest calving at a body condition score of 5. “Heifers that have a body condition score less than 5 are likely to have very long period of time before they resume fertile cycles,” Larson says.
On the flip side, heifers can be too fat at the time of calving and in some situations will deposit fat in the pelvic canal leading to greater risk of calving difficulty. In general, heifers greater than a body condition score of 6 are considered too fat. “However, if heifers are too fat going into calving and their first lactation, this is not the time to try to take weight off of them by restricting feed,” Larson cautions.
If herd calving records are available that will allow you to compare the date when heifers have their first calf to the date when they have their second calf, the producer and veterinarian can identify the average post-calving anestrous length for first-calf heifers that successfully rebreed, but will ignore those who fail to become pregnant for their second calf. Larson says this piece of information is helpful, but is probably underestimating the range in length of post-calving anestrous for the herd.
Most first-calf heifers will require 80 to 100 days to resume fertile cycles after their first calf. “But there is great individual and herd-to-herd variation; therefore, I am not confident that I can identify each individual first-calf heifer that will become pregnant for their second calf early in the breeding season simply by her calving date,” Larson says. “My recommendation is usually to give every heifer and cow an opportunity to become pregnant within the optimum breeding season for the herd and then have the discipline to cull any female that is open or bred to calve later than the predetermined herd goal.”
If the heifers are developing right, calve early, have plenty of time postpartum before the start of their next breeding season and are cycling early, they have three chances to get pregnant for their second calf – and the momentum just starts going Larson says.
Larson admits that some producers and veterinarians may not want to breed heifers that much earlier than the cows, but if heifers are bred at the same time cows are bred, following 283 days of gestation, the heifers will calve over a 45-65 day period. “In this scenario the first calf heifer post-partum anestrous period is covering up almost the entire breeding season,” he explains. “How many of these heifers that calved at the same time the cows calve will get pregnant? Very few. How can I solve that? I am probably leaving the bull out longer, so he’s getting some pregnant later. They are pregnant through the chute so I keep them, but now my first-calf heifers will calve very late for their second calf. So now I have momentum working against me.”
Getting involved in reproductive programs, especially with heifers, is a great opportunity for veterinarians. “If we don’t measure, interpret, look at and palpate these heifers, we are letting Mother Nature take her course and it’s a 50:50 chance if they are cycling when we need them to.
“Some years they will reach puberty when we want, some years they won’t,” Larson says. “If the veterinarian is involved every year, the heifers can do it every year because you make modifications if necessary. Without veterinary involvement, it works some years and some years it doesn’t. With us, producers can make it work every year.”
Sidebar: Two ways to tract score
Reproductive tract scoring can be a useful tool when evaluating heifers pre-breeding. Two scoring methods include the 5-point Colorado State University scale, or a 3-point scale that Bob Larson, DVM, PhD, Dipl. ACT, Dipl. ACVPM, prefers to use.
* Anderson KJ, LeFever DG, Brinks JS, Odde KG: The use of reproductive tract scoring in beef heifers.
Agri-Practice 12(4)19-26, 1991.
Larson’s 3-point scale for reproductive tract scoring is based on the following guidelines:
Tract Score 1: An immature reproductive tract (uterus and ovaries) or otherwise at high risk of not becoming pregnant during a limited breeding season with the targeted starting date (equivalent to a 1 or 2 on the CSU scale).
Tract Score 2: A tract (uterus and ovaries) that is consistent with a heifer that is not currently cycling, but is likely to reach puberty by the start of the breeding season with recommended management (equivalent to a 2 or 3 on the CSU scale).
Tract Score 3: Evidence of cycling by the presence of a palpable corpus luteum (CL) or mature reproductive tract (equivalent to a 4 or 5 on the CSU scale).
If the ranch is planning to use estrous synchronization and AI on the group of heifers, then nearly all should be a tract score 3 by the start of breeding, and at least 60% should be a tract score 3 when evaluated six weeks ahead of the breeding season, Larson says. “It may be wise to reconsider estrous synchronization and AI if the group has less than 50% of the heifers classified as a cycling (tract score 3) six weeks ahead of the desired start to the breeding season.”
Sidebar: Baby heifers – which do we keep ?
Making the decision on which heifers to keep back for replacements can start as early as that heifer’s birth. Bob Larson, DVM, PhD, Dipl. ABVP, Dipl. ACVPM, says the main criteria for whether or not heifer calves are potential replacements are their age and the presence of any undesirable traits that can be passed from their dam or sire.
“In most situations, I want to select heifers that are born in the first 40 days of the calving season as potential replacements to ensure that they are of sufficient age to reach puberty early enough to allow breeding ahead of the mature herd,” Larson says.
“If dams have any udder or behavioral problems that can be passed to their daughters, they should not be identified as potential replacements.”