Case studies: Heifer development and reproductive failure (Proceedings)

Replacement Percentage

Because one goal of proper heifer development is to improve second parity pregnancy percentage, a beef producer may ask "what is the impact of higher pregnancy percentages during the second breeding season on costs and income?" Table 1 displays the effect of changing pregnancy percentage for first-calf heifers in 5-percentage point increments on the percent of the herd that must be replaced each year and the average age of the herd. In general, given the assumptions in the table, for every 5-percentage point improvement in first-calf heifer pregnancy percentage, the number of replacements needed for the herd decreases by about 1 percentage point and average cow age increases by .01 years.

Table 1. Effect of 2nd parity pregnancy percentage on herd replacement percentage and average cow age at the start of the breeding season.

Heifer Breeding Model: Critical Control Points in Heifer Development

A critical control point (CCP) can be defined as a point in the production process where a value or values can be measured that are a direct result of previous management and that impact the success of the remaining production process. A number of CCPs are possible in heifer development (Table 6). Not all of these points are important as control points for every farm. How well a farm can manage earlier or later CCPs in the production process impacts the necessity of measuring each of the following potential control points. The following CCPs are based on a production system where replacement heifers are moved into the replacement pool at weaning, estrous synchronization and artificial insemination (AI) are utilized for the first breeding opportunity at approximately 14 months of age, followed by exposure to bulls for the remainder of a 60-day breeding season, and use of natural service only for the second breeding season at approximately 27 months of age. The following potential heifer development CCPs start with the closest to the end of heifer development as I have defined it (bred for second calf) and work backwards to selection of heifers for the replacement pool.

Table 6. Calendar for Heifer Development Critical Control Points

I. Weight per day of age at weaning, sire EPDs for milk production, growth, and calving ease/birthweight, and structural soundness (feet, legs, genetic defects)

(fall for spring-calving herds)

Successful heifer development starts with selection of candidates for the replacement pool that are likely to reach puberty prior to the start of the breeding season, become pregnant early in the breeding season, have little calving difficulty, and re-breed early in the second breeding season. Because puberty is age and weight dependent, only heifers whose age and weight at weaning are compatible with being old enough and heavy enough prior to the start of breeding to reach puberty should be selected. Expected progeny differences for the sires and dams (when available) of individual heifers should be examined to find those heifers that are predicted to meet herd goals for mature size, growth rate, milking ability, and calving ease. Heifers with undesirable structural confirmation of feet and legs should not be included in the replacement pool, as well as heifers with a familiar history of genetic defects such as vaginal prolapse.

If subsequent CCPs have high failure rates, re-examining the selection criteria may be necessary to identify those traits most likely to be correlated with failure later in the heifer development system.

II. Vaccination protocol to enhance herd immunity to pregnancy-wasting diseases

For most beef herds, the potential list of diseases in a vaccination program would include: brucellosis, Infectious Bovine Rhinotracheitis (IBR), Bovine Viral Diarrhea (BVD), vibriosis (Campylobacteriosis), and leptospirosis. Other diseases for which vaccines are available include: Hemophilus somnus and trichomoniasis.

III. Body weight gain (ADG) during weaning to breeding period

(winter for spring-calving herds)

Because weight is a primary factor determining the onset of puberty, ensuring that the nutritional program is meeting average daily gain requirements for the period from weaning to breeding is critical for a successful heifer development program. If weight gain is not as projected, the energy content of the diet can be increased so that target weight will be met. In addition, the use of ionophores, progestogens, and anthelmintics will help ensure that heifers reach target weights and puberty prior to the start of the breeding season.

IV. Breeding Soundness of heifers: Reproductive tract score, pelvic area, yearling weight

(30-60 days prior to the start of the breeding season; early- to mid-spring for spring-calving herds)

Heifers should be 55-65% of mature weight, depending on breed, prior to the start of the breeding season to ensure that most animals in the group have already reached puberty. Palpation of the reproductive tract aids in determining the percentage of the group that has reached puberty and identifies individuals that have abnormal or infantile reproductive tracts. Heifers with pelvic areas that are abnormally small when considering age, weight, and maturity should be identified.

Sixty percent of heifers should already have reached puberty 30 to 60 days prior to the start of the breeding season. If the herd does not meet this criteria, the energy content of the diet can be increased so that weight gain is increased to the necessary level to meet target weight, the start of the breeding season can be delayed, or the breeding season can be abandoned with the heifers either being sold as feeder cattle, or being held over for a later breeding season.

V. First Breeding: Breeding Soundness of bulls

(late-spring for spring-calving herds)

All bulls used to breed heifers should pass a breeding soundness examination and their EPDs for birthweight and or calving ease should be consistent with the farm's goals.

VI. First Breeding: Estrous response to synchronization system

(Start of the breeding season; late spring for spring-calving herds)

If heifers have reached puberty and the synchronization system was applied appropriately, 70-80% or more of heifers should display estrus within the time window predicted by the synchronization system. If results do not meet this goal, the percentage of heifers that are puberal, the accuracy of estrous detection, and the success of administering the synchronization system should all be investigated.

If estrous response to synchronization is poor, alternate or additional synchronization systems can be implemented, the period of estrous detection and AI can be extended, or the date for the start of the natural breeding season can be altered.

VII. First Breeding: Pregnancy percentage to AI breeding

(40-60 days after AI breeding; mid-summer for spring-calving herds)

If heifers are synchronized and bred AI, bulls should be held out of the breeding pasture for 2 weeks following the last day of AI breeding so that AI pregnancy percentage can be accurately determined early in gestation via fetal aging by palpation. Sixty-five to seventy percent or more of the heifers identified in estrus and bred artificially should become pregnant to AI. Failure to meet farm goals could indicate inaccurate determination of estrus, poor semen delivery by the AI technician, poor semen quality, or poor condition of the females (stress, high environmental temperature, losing weight).

Identification of AI-sired pregnancies has value if more pregnant heifers are available than are needed as replacements and those heifers are either given preference to enter the herd, or are marketed at a higher value than natural service-sired pregnancies. By doing an early pregnancy determination, one not only knows the AI pregnancy percentage, but, because the bulls are still in the breeding pasture, they can be left longer than planned if the AI pregnancy percentage is less than expected to ensure that every heifer has several opportunities to become pregnant.

VIII. First Breeding: Body Condition Score / Body Weight at palpation for pregnancy determination

Heifers should be gaining about 1.3 to 1.5 lbs. per day in order to weigh 80 to 85% of mature weight at the time of calving as 2-year-olds. Failure to meet farm goals at this CCP indicate that forage quality and quantity do not meet the nutritional demands of growing, pregnant heifers in this herd. If BCS is lower than desired, the diet should be adjusted so that condition can be added prior to winter and late gestation when maintenance requirement increases. If low BCS and weight are not due to an unusually decreased forage quality or quantity compared to normal annual variation, either the nutritional strategy or the genetic potential for growth for the herd needs to be modified.

IX. First Breeding: Pregnancy percentage to first breeding season by 20-day periods

(40 days after breeding season; late summer/early fall for spring-calving herds)

Determining pregnancy percentage for the entire breeding season allows one to identify heifers that are not pregnant and to determine the best marketing plan for those animals. In addition, if more pregnant heifers are available than are needed as replacements, selection criteria can be used to market them to farms needing pregnant animals. A goal of 90-95% of heifers in the replacement pool being pregnant at the end of the breeding season is achievable if CCPs earlier in the heifer development process are used to remove heifers that do not meet the farm's standards. If CCPs were not measured or met earlier in the heifer development process, lower pregnancy percentages should be expected.

If pregnancy percentages do not meet farm goals even though earlier CCP criteria were met, bull fertility, nutrition during the breeding season, and the presence of pregnancy-wasting disease should all be investigated. If the number of pregnant heifer available will not supply the necessary replacements for the herd, additional replacements must be located and purchased.

X. First Calving: Body Condition Score / Body Weight at start of calving season (spring for spring-calving herds)

Heifers should have a BCS of 6 at the start of the calving season and should weigh 80-85% of mature weight after parturition. Failure to have adequate energy reserves will negatively affect lactation, weaning weight, and re-breeding percentage. Appropriate use of supplements after calving will be necessary to meet breeding season goals if this CCP is not met.

XI. First Calving: Calving distribution by one-week, two-week, or three-week intervals; Calving difficulty scores; pregnancy loss between pregnancy determination via palpation and calving(spring for spring-calving herds)

Data collected at calving is very valuable to determine whether earlier CCPs adequately monitored and guided the farm's heifer development program. Sixty-five percent or more of the calves should be born in the first 3 weeks of the calving season, with greater than 80% being born in the first 6 weeks. The prediction of AI pregnancy percentage should be compared to the percentage of calves born in the first 2 weeks of the calving season. In addition, the percentage of heifers that are confirmed to be pregnant but that fail to calve should not exceed 2%. Calving ease scores should reflect less than 15% of heifers having dystocia, with levels exceeding that goal causing one to examine both growth of the heifers and birth weight expected progeny differences (EPDs) of the bulls used.

Failure to have a high percentage of calves born when predicted by palpation will allow the palpator to re-calibrate his/her criteria for fetal aging, and to determine the stage of pregnancy where he/she is most accurate to improve future predictions of calving date. A high gestational loss of viable pregnancies should cause the herd's veterinarian to focus on biosecurity and vaccination protocols for diseases that cause pregnancy wastage. Excessive occurrence and severity of dystocia indicate that either heifers were underdeveloped, or more likely, calf birth weight was excessive due to genetic predisposition by either the dam or sire. Because each sire will affect many calves, accurate predictions of the sires' influence on birth weight by using EPDs is critical to avoiding excessive dystocia.

XII. Second Breeding: Breeding Soundness of bulls (BSE)(late spring for spring-calving herds)

Only bulls that pass a BSE should be considered for use in the breeding pasture. The need for breeding soundness examination of bulls is based on the fact that many prospective breeding bulls are infertile, subfertile, or unable to copulate. But, as we evaluate the use of BSE, one should recognize the limitations. It should be remembered that the BSE reflects an animal's breeding soundness only on the date tested. A BSE does not reflect the bull's soundness in the past, neither does it definitely define the bull's ability to cause conception in the future. The overall effect of BSE is to eliminate many infertile bulls and to improve the genetic base for fertility within the herd.

Breeding soundness examinations consist of a complete physical, scrotal measurement as an indication of testicular size, and a semen evaluation. Once the evaluator has collected all the available information, he/she determines if the bull is a satisfactory breeder on the day tested. The veterinarian may also give an indication of the severity of any abnormality and a prognosis for recovery and use as a breeder in the future.

XIII. Second Breeding: Body Condition Score / Body Weight prior to the start of the 2nd breeding season (late spring for spring-calving herds)

Because body condition at the start of the breeding season is a good predictor of breeding season success, it is important that the farm's goal be met for BCS (5 to 6 on a 9-point scale) and body weight (82-84% of mature weight). Adding body condition to a growing heifer that is lactating is very difficult, therefore meeting this CCP is largely dependent on having the heifers in adequate condition at the time of calving and then having either adequate quality and quantity of forage to meet her needs for maintenance, lactation, and growth, or a supplementation strategy that adds adequate calories to the forage base to meet her energy needs during early lactation.

If earlier CCPs for body weight and condition were met, but the herd failed to meet this CCP, a supplementation strategy or a change in forage quality and quantity for future groups of fist-calf heifers early in their first lactation is needed.

If body condition goals were not met at earlier CCPs and energy intake did not increase to the degree necessary to replace deficient condition and maintain lactation, BCS at this CCP are likely to be below the farm's standards.

XIV. Second Breeding: Body Condition Score / Body Weight at palpation for pregnancy determination

The adequacy of the nutrition/forage program on the farm for the late portion of the breeding season can probably be determined by BCS at the time of pregnancy determination if palpation is not done too late in gestation. BCS at this time is also a good indicator of whether the herd's genetic potential for growth and milk production matches forage quality and quantity. BCS at this time is not a good indicator of the nutritional program going into the previous breeding season. First-calf heifers in their first lactation should weigh at least 85% of their mature weight at this time and should be near a BCS of 5 (on a 9-point scale), with no individuals falling below a BCS 4.

If earlier CCPs for body condition were met, but the heifers do not meet farm goals at this measurement, one needs to determine if unusual weather caused forage production to be greatly decreased compared to normal variation, if milk production or growth rate causes energy requirements to exceed that supplied by forage within normal yearly variation, or if the farm should plan to supplement energy to 1st-calf heifers during lactation either routinely or if forage production falls below a given quantity and quality.

If BCS is lower than desired, calves should be weaned and the diet should be adjusted so that condition can be added prior to winter and late gestation when maintenance requirement increases.

XV. Second Breeding: Pregnancy percentage by 20-day periods

(40 days after breeding season; late summer/early fall for spring-calving herds)

The overall success of the heifer development system for a farm is determined at this point. The goal is to have a high percentage of heifers that became pregnant at 13-15 months of age and that were selected to enter the herd, to be pregnant for their second calves, and in addition, to have become pregnant early in the breeding season. A pregnancy percentage for first-calf heifers over 90-95% is an achievable goal for farms that have met previous CCPs.

If the farm's heifer development system was meeting the goals for all CCPs prior to this point, but did not meet the rebreeding goals of the farm, bull fertility and the nutritional program for 1st-calf heifers would be areas for further investigation.

Reasons that rebreeding percentage could be below farm goals in a system where earlier CCPs were not met could include heifers becoming pregnant late in the breeding season and consequently they that did not reach the farm's goal for days between calving and re-breeding, or 1st-calf heifers being under conditioned going into the breeding season, thereby reducing the number that are cycling at the start of the breeding season.