Chapter 40 Brad J. White Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University, Manhattan, Kansas, USA Reproductive efficiency is related to overall herd health, and both high reproductive success and low disease losses are paramount to beef cow-calf herd economic sustainability. A standard measurement of reproductive outcomes is pregnancy or calving percentage, which is often used to evaluate herd reproductive efficiency status. In addition to this standard performance assessment, the reproductive profile of the herd can also be used as a methodology to create a focused herd reproductive health program, assist in the diagnostic strategy for suboptimal reproductive efficiency, and to guide the design of intervention strategies. Utilizing a breeding season evaluation to assess the overall reproductive status of the herd is not a new concept,1 and the evaluation can be expanded to generate specific target areas to improve overall success. Managing a herd to optimize the reproductive profile serves as a tool to optimize herd health parameters, and comparison of the current herd profile with profiles of the same breeding group in previous years facilitates early diagnosis of potential problems. The objective of this chapter is to describe the cow-calf herd reproductive profile and how to use this combination of measurements to better customize a reproductive management program based on information generated through the profile. Weaned calves are the primary source of income in the cow-calf herd, and fixed costs associated with maintaining cows is an important expense category.2 Therefore high reproductive efficiency that results in many weaned calves per cows exposed for breeding is strongly associated with financial success of the cow-calf herd.3 One variable that encompasses both conception percentage and successful pregnancy maintenance is the number of calves born in the calving season relative to the number of cows exposed during the breeding season. Lower than expected reproductive success could be attributed to several potential causes and in order to narrow the differential diagnosis list and create focus for the preventive health program, a herd reproductive profile should be generated. The herd reproductive profile consists of three main components. The length of the calving season is influenced by a variety of factors including production goals, calving season, resource availability, and owner preferences. The goal is to produce one calf per cow per year, and herds typically sell the calves born within one season at a similar time point. The length of the calving season will influence the uniformity of the calf crop at the time of sale and longer calving seasons result in calves with more weaning weight variability due to the length of growing time from birth to weaning. Although a 60-day calving season is often recommended, producers with a slightly longer calving season can still maintain “front-end loaded” calving seasons each year if forage production and nutrition are adequate for short postpartum anestrous intervals. The effect of breeding season length on herd net returns is related to the estimated length of the postpartum interval and conception rate in the subsequent breeding season.4 Most (66%) producers report that their calving season is 4 months or less.5 The calving season length can be modified by either controlling the length of the breeding season (i.e., the time period that bulls are exposed to cows) or by utilizing gestational aging to select cows that will calve in the desired time period. When evaluating potential reproductive problems, an important aspect is to compare the length of the breeding season from the current to the previous year. Breeding season length, along with calving percentage, have been associated with overall profitability of cow-calf operations.3 If the breeding season length is the same and a disease causes reproductive losses, then the problem will be manifested as either a decreased pregnancy percentage at mid-gestation pregnancy evaluation or a higher than expected percentage of cows that were confirmed pregnant but which did not calve during the calving season. If the breeding season is unrestricted, the problem may best be evaluated by monitoring the change in the pattern of calving during the season (described below). The number of calves born in the calving season relative to the number of cows exposed to breeding is based on both successful conception and the maintenance of pregnancy to parturition. The target mid-gestation pregnancy percentage after a 65-day or longer breeding season is 95%, but some pregnancies may not be maintained until parturition due to normal reproductive loss.6 Pregnancy diagnosis by rectal palpation may be used to provide an evaluation of the previous breeding season and offers information to the herd manager relative to the success of the program. Pregnancy percentage may vary slightly from year to year and from herd to herd, but decreases below 85–90% should be considered problems that merit further investigation. A low percent pregnancy at a mid-gestation evaluation identifies that a herd problem is present, but does not provide insight into the root cause of the problem; therefore, the pregnancy (or calving) distribution should be evaluated. The distribution of calving within the herd can be combined with calving season length and the pregnancy rate to create the herd reproductive profile. The calving distribution is created by aggregating the number of calves (or percent of the herd) born in each 21-day period of the calving season. The time interval used in the distribution is based on the average period between estrus for cycling cows and the fact that a mating between a cycling cow and a fertile bull should result in a pregnancy that can be detected 50 days later 60–70% of the time.7 Therefore, if all the females are cycling at the start of the breeding season and they are exposed to an appropriate number of fertile bulls, about 65% of the herd should be bred in the first 21-day cycle. This pattern of calving is often designated as “front-end loaded” as the majority of the herd calves early in the breeding season. For example, if the herd has 100 cows, then after the first cycle, 65 cows would be bred leaving 35 open. In the second cycle, the 35 open cows continue to be exposed to fertile bulls and if 65% get bred that would result in 23 pregnancies, leaving 12 cows open. In the third cycle, 65% of the 12 cows (8 head) would be bred leaving four open and a 96% overall pregnancy percentage. This pattern would be recognized as an ideal distribution and is similar to ideal distributions reported elsewhere.8 The pregnancy percentage at the end of multiple bull exposures over the breeding season is predicated by the fact that the cows are cycling the entire period until bred and the bulls maintained fertility throughout the breeding season. Therefore, the overall pregnancy percentage at the end of the breeding period can be influenced by any factor that decreases bull fertility or results in females not cycling during the breeding season. All three aspects of the reproductive profile (calving season length, number of calves born per cow exposed, and expected or actual calving distribution) can be illustrated in a single figure created from herd data (Figure 40.1). The data necessary to create a reproductive profile is available on most cow-calf farms and profiles can be generated from data collected at the time of pregnancy diagnosis or the previous year’s calving or both. If cows have pregnancy status determined at a time when pregnancies can be categorized into 21-day gestational age groups, then the results of the pregnancy diagnosis can be used to generate the reproductive profile. If pregnancy diagnosis was not performed during this time period (or at all), the reproductive profile can still be generated by plotting the number of calves born by 21-day intervals. This does not require individual animal birth dates to be recorded by the producer, but can be done with a minimum of information such as the number of calves born weekly or even a cumulative count of the number of calves at 21-day increments throughout the calving season. One caveat with both pregnancy diagnosis and birth-date records is to insure that the appropriate number of cows exposed to the bull is included in the denominator when determining the percentage of the herd. As both pregnancy diagnosis and calving occur several months after the breeding season, simply counting the number of cows present at the time of the assessment may introduce bias by missing cows that dropped out of the herd or were culled due to not becoming pregnant during the breeding season. Often the number of cows exposed can be determined by asking the producer for sales records on cull cows and the number of cows that have died in the interim since the breeding season.
Beef Herd Record Analysis: Reproductive Profiling
Introduction
The herd reproductive profile
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