(From: Proceedings) CVC IN KANSAS CITY PROCEEDINGS
Aug 01, 2011
By Brad White, DVM, MS
Objectives / key points
- The characteristics of females selected for retention into the breeding herd impacts long term herd performance and economic viability.
- Raising replacement heifers in an efficient manner is possible by paying attention to critical control points during each production phase.
- Major control point phases include: pre-weaning, weaning, pre-breeding, breeding, gestation, and calving.
- Veterinarians can provide a valuable service to clients by providing input at appropriate times to enable the economical and efficient production of replacement heifers.
Replacement heifers represent the future of the cow-calf operation. These females are the genetics behind the next generation of farm offspring. Efficient development is critical as raising a replacement heifer represents a significant asset that does not generate a return until the first calf is sold. Beef herd productivity is increased when a high percentage of the heifers are bred early in the season. Heifers have higher rates of calving difficulty when compared to mature cows. Yet, there are some management tools that can be used to identify and minimize risk factors for dystocia. Lifetime productivity is also an important consideration in planning the female development timeline. Heifers calving at 24 months of age produce an earlier return on investment and potentially have more calves than females who are older at calving.
Targets for the beef replacement heifer program are: increase / maintain a high conception rate early in the breeding season, minimize dystocia rate, promote good post-calving conception rates, and increase farm income through efficient lifetime productivity. To achieve these goals, we must utilize a plan that addresses rate limiting steps or critical control points during each production phase. From birth to calving, heifer production can be divided into six key phases: pre-weaning, weaning, pre-breeding, breeding, mid-gestation, and calving.
Development of a replacement heifer management program is a good opportunity for veterinarians to add value to client relationships. Many producers have failed to adopt proven techniques such as estrus synchronization, artificial insemination, and EPD-based selection. Creating and implementing a protocol for meeting farm targets for replacement females results in a mutually beneficial relationship between client and veterinarian. The client benefits through utilizing knowledge to improve production efficiency, while practitioners garner income for performing services and providing advice about best management procedures. The purpose of this presentation is to describe several specific points where veterinarians can assist producers develop quality heifers in an efficient manner.
Knowledge-based management of replacement heifers is based on good production records and the process is initiated with the birth of the animal. Individual identification allows not only performance, but pedigree tracking on the calf. Recording birthdates or at least birth week is useful because heifers born early in the calving season will have an age advantage relative to females born at the tail end of the season.
Weaning is the time to begin the immunization program and determine a nutritional plan based on animal performance. The goal with weaning vaccinations is to protect animals against common respiratory pathogens and also to begin building immunity against pregnancy wasting infectious agents. As a minimum, all replacement heifers should receive two rounds of modified-live viral (IBR-BVD-PI3-BRSV) vaccine at this time. Other specific antigens to be included should be determined by the veterinarian with direct consideration of herd biosecurity status and previous disease history. If BVD is suspected in the herd, all animals should be tested for persistent infection (PI) either prior to or at weaning. Heifers should also be treated with an appropriate anthelmintic as internal parasites can negatively impact weight gains from weaning to breeding.
Weaning weight can be used to determine a nutritional program necessary to meet animal target weight by the goal breeding date. The onset of puberty is heavily influenced by both age and weight. To become pubertal by yearling age (12-13 months), heifers must receive adequate nutrition to signal the body that the “luxury” of reproduction is available.
As a rule of thumb, target breeding weight can be calculated as 65% of the expected mature female weight. Thus, by subtracting the weaning weight from the target weight and dividing by the number of days between weaning and breeding, a goal for the rate of gain can be calculated. Using NRC guidelines and a knowledge of available feedstuffs, the ration can be formulated to reach the goals in an efficient manner. Achieving target weights prior to the onset of the breeding is key to achieving the goal of having a large number of heifers conceive early in the season.
A pre-breeding evaluation of heifer pubertal status is a critical control point to determine potential management interventions to ensure a successful outcome to the breeding program. The goal is to combine several objective criteria to determine the readiness of the group to breed. A reasonable goal is to have 80% of the replacement heifers cycling at the start of the breeding season. In order to allow time for management adjustments, pre-breeding examinations are typically performed 4-6 weeks prior to the target breeding date, with a goal of 65% pubertal animals at this time. If the pre-breeding herd target is not met, data from animals should be used to determine appropriate measures to correct the problem.
Individual animal weight, reproductive tract score, and pelvic area are measured to classify the animal’s pubertal status. The goal for animal weight is based on reaching 65% of mature weight by breeding. Reproductive tract scores were first described by CSU researchers and are a method to categorize heifer reproductive status by using ovarian structures and uterine size as palpated per rectum. A 5-point system can be used where a 1 is immature, 2 and 3 are non-cycling, and 4 and 5 have reached puberty (Table 1).
Pelvic area is often measured and used to decrease the risk of dystocia due to a disproportionately large calf compared to the heifer’s pelvic opening. The technique has been used for a long time and research has shown good correlations between yearling and calving measurements in this highly heritable trait. Pelvic area is also correlated with mature cow size and calf birth weight, thus selecting the maximum size is not desirable. The measurement can be used to set a minimum culling criteria (such as 150 cm-2) without ranking heifers based on this criteria.
The pre-breeding examination is also an appropriate time to booster vaccinations for prevention of reproductive losses. Viral vaccines and appropriate immunizations for vibriosis and leptospirosis may be given at this time.
Pre-breeding examinations should not be confined to the female component of the herd. Bulls scheduled to be used for natural service should have a breeding soundness examination performed prior to the onset of the breeding season.
Sire selection is an important decision impacting dystocia rates and offspring performance. Many breeds offer birth weight and calving ease (difficulty) expected progeny differences (EPDs), and veterinarians can offer a valuable service by helping clients select a bull appropriate for their operation. Birth weight and weaning weight are correlated – resulting in a common misconception that producers must choose between dystocia and reasonable weaning weights. Utilization of artificial insemination (AI) allows the farm manager to select a bull with the relatively rare combination of a low calving difficulty due to birth weight and a high growth performance potential.
The breeding season goal is to front-end load the calving season, or rather, get as many heifers bred at the onset of breeding as possible. One method to achieve this is through estrus synchronization. Many programs are available to synchronize heifers. Protocol selection should be based on farm goals and resource availability (financial, labor, facilities). Endogenous progesterone plays an important role in the onset of heifer puberty, and inclusion of a progestin product in the synchronization protocol may mimic this effect in heifers close to cycling.
Veterinarians should be in close contact with the farm manager during the breeding season to identify potential problems as soon as possible. In estrus synchronization and AI programs, benchmark goals should be set regarding synchronization response rates and timing of observed estrus. If these thresholds are not met, further investigation should be pursued immediately to determine intervention strategies.
Pregnancy diagnosis by rectal palpation is the next critical stage in the heifer development process. The veterinarian should palpate to identify open animals that can be culled, and also to provide an objective evaluation of the breeding program. The producer can select final replacements from this group of animals and sell excess stock to reduce further feeding and maintenance costs.
After palpation, the bulk of the pregnancy is relatively low maintenance for the heifers. Producers should be reminded that these animals are still growing and should be fed separately from the mature herd if possible. A ration should be formulated to grow the animals to 85% of mature weight at calving. Pre-calving nutritional status plays a critical role in dystocia, calf health, and post-calving conception rates. A farm visit two months prior to calving to visually evaluate heifer weights and body condition scores provides another potential intervention point. Heifers should be at a body score of 5.5 to 6.5 (on 1-9 scale) at calving for optimum performance.
Calving is the culmination of the replacement heifer program, but not the end of the intensive management. Employing the management tools described above (EPD selection, culling criteria) will reduce, but not eliminate, the frequency of calving difficulty. Heifers should be observed frequently and early intervention is indicated to reduce negative consequences of dystocia. The author prefers to use 30 minutes of non-progressive labor as a threshold to determine intervention in heifer calvings. Producers should be trained on normal stages of labor and instructed on when to call for assistance prior to the onset of calving to reduce potential problems.
The post-partum heifer has the highest nutritional requirements of any animal on the farm. The new dam is still growing, lactating, and preparing to breed back. If nutrients are limited, the first thing to suffer is conception rate. It is virtually impossible to supplement enough feed to place animals in a positive energy balance during this time period. Therefore, to prevent a “sophomore slump”, the critical nutritional control point is actually the nutritional status prior to calving.
Replacement heifer development is a critical component of many cow-calf operations. There are a variety of tools that greatly increase the efficiency and value of the bred heifer for the farm manager. Veterinarians can play a key role with clients by identifying and assisting with evaluation of critical control points in the heifer development program.