Mating Capacity of Bulls; Bull to Cow Ratio

– Dr. Mike Day, Ohio State University

General recommendations for bull:cow ratios: The textbook recommendation for bull:cow ratio is typically 1 bull:20-30 females, with mature bulls closer to the 1:30 ratio and yearling bulls at 1 bull:20 cows. In cows in which estrous synchronization is used, the recommendations are altered by necessity. One mature bull with 15 to 30 cows, confined to a small area, is a typical recommendation for a system where a majority of the cows are expected in estrus over a 5 day period. At least a 10 day rest between breeding periods is suggested with this type of breeding challenge.

As might be anticipated, there is general disagreement with these recommendations among cattlemen. Some feel these numbers are too low, while others feel that the recommendations are too high; especially for yearling bulls. An easy answer to the question of how many females a particular bull can service in a breeding season is not usually available.

Perhaps the most fool-proof way to determine the mating capacity of a bull, or in other words the appropriate bull:cow ratio, is from past experience with the sire. After 1-2 breeding seasons an observant manager would have a good idea of the potential of a given sire for mating capacity. Unfortunately, by the time a sire has demonstrated his potential, he has often outlived his usefulness from a genetic standpoint for a given operation.

What are the limits for the bull:cow ratio? Some information is available from research trials. Reports vary in regard to the limits. In some studies, performance for some bulls at ratios of 1:44 or 1:60 were equal to that achieved at ratios of 1:25 for other bulls. However, at these higher ratios, a few bulls failed and achieved unacceptable rates of conception. Thus, the problem still exists that general recommendations of bull:cow ratios for all situations do not exist. Rather, the mating capacity of an individual bull is more important to reproductive success than the bull:cow ratio. Two of the primary factors that affect mating are discussed here.

Determinants of mating capacity:

1. Seminal Quality; a) Semen quality and b). Semen quantity

2. Serving Capacity; a) Libido or sex-drive+willingness and eagerness and b) Mating ability = competence of fulfilling aspiration

Economic importance of mating capacity: The importance of the bull in reproductive efficiency is often overlooked by producers, educators and research scientists. It is clear that the capacity of the bull to perform is limited by the fertility of the cowherd. Thus, a majority of efforts have and are placed upon ensuring that reproductive potential of the females in the herd is optimized. In short, we have come to assume that the bull will do his job, if we make the correct management decisions regarding the cow herd.

While in a majority of cases we ‘get by’ with this type of approach, the economic implications of failure or reduced performance of bulls in a natural mating program can be disastrous. Even if the managerial and financial investments are made to ensure that the cow herd is reproductively competent, if reproductive failure occurs due to a defect in the bull, a tremendous loss in terms of reproductive efficiency is likely. Much of the work from previous years with a cowherd to ensure a short calving/breeding season which occurs at the appropriate time of the year to meet your production system, can be lost in a single experience with a bull of inadequate mating capacity. Unfortunately, a bull infertility problem is usually not detected until after the fact, or in the best cases, during the latter stages of the breeding season.

In the context of bull:cow ratios, the implications of our choices are relatively simple to identify, even if the costs can be quite high. Clearly, the more females that bull can effectively impregnate in a given season, the more optimal or efficient we are in the use of the capital invested in that sire. As one pushes the bull:cow ration up, however, the likelihood increases that the bulls mating capacity will be exceeded and fertility during the breeding season will be reduced. At the risk of sounding like an economist, there is a point of diminishing returns, in terms of reproductive efficiency, uniformity of calf crop, etc. if mating capacity of the bull is exceeded. Furthermore, the consequences of infertility of a given bull, when pastured with 60 cows, are much more serious than if exposed to only 25 females. Thus the risks increase as efficiency of bull utilization increases.

There is no fool-proof way to ensure that a new bull is going to perform adequately in a natural breeding system. However, there are some tools available, and management considerations that are important, to minimize the risk of reproductive failure. Use of the methods that are discussed here, along with careful observation of a new bull’s breeding competence should help minimize the threat that bull infertility poses to reproductive efficiency in cow-calf production.

Factors affecting seminal quality: ‘Seminal quality’ encompasses both the quality and quantity of semen produced by the bull. One definition of seminal quality for a given bull is the quality achieved as a result of the genetic composition of the bull, minus the detrimental effects of the environment in which the bull is reared and maintained. Thus, seminal quality is a function of both genetic and environmental factors.

Semen quality is usually defined as the number of abnormal sperm cells produced and the motility of the sperm cells, whereas, semen quantity refers to the number of sperm cells produced. The standard method of evaluating these factors is through a the Breeding Soundness Exam (BSE). Also, a multitude of factors, such as health, nutrition, disorders, etc. can affect seminal quality. I will only briefly highlight a few of these factors here to emphasize their importance in determining mating capacity of bulls.

1. Testicular Size – Scrotal Circumference: Scrotal circumference is controlled to a large extent by genetics of the animal. This trait is of moderate to high heritability and, as has been well documented, has an important influence on reproductive competence of the sire as well as his male progeny.

What are the relationships of scrotal circumference to seminal quality? Bulls with small testicles are more likely to have reduced sperm output and increased numbers of morphologically abnormal spermatozoa. In a survey of almost 2000 bulls of varying ages, only 23% of bulls with a scrotal circumference of 32 cm were satisfactory for seminal quality. Seminal quality increased up to a scrotal circumference of 38 cm, with no further improvement in bulls with larger testes. Thus, scrotal circumference can have major effects on seminal quality and, in turn, influence the mating capacity of an individual sire.

It is important to mention the influence of scrotal circumference of a sire on age at puberty for female progeny. For each 1 cm deviation in scrotal circumference among sires, it has been estimated that age at puberty in daughters is influenced by 3-4 days. Thus 2 bulls with scrotal circumferences that differ by 10 cm, would sire daughters with a 40 day difference in age at puberty. The benefits of assessing this trait, and including it in selection criteria for bulls are two-fold.

2. Other Genetic Factors: Other genetic factors that can influence seminal quality are testicular consistency, some recessive trait, and scrotal shape. Obviously age influences quality, with seminal quality shown to increase for about 4 months following the attainment of puberty in yearly bulls. Thus, careful consideration is necessary for evaluation of seminal quality of young bulls, and to the challenges we subject them to in terms of bull:cow ratio.

3. Environmental Factors: Bulls used for natural service are exposed to a wide range of environmental and management conditions. Typically, the factors in this category that effect seminal quality are most often associated with influences on hormonal mechanisms or on regulation of scrotal temperature. Only limited information is available regarding the effect of environment on the basic hormonal mechanisms that control seminal quality. More extensive information is available regarding the effects of various environmental and management factors on seminal quality. Often, these influences are directed through their impact on regulation of testicular temperature. The influence of elevated testicular temperature on sperm production and quality have been well-documented and often include reduced sperm production and decreased semen quality.

Optimal sperm production is achieved if the temperature of the testes is maintained a level below normal body temperature. The bull has 3 primary systems in place to ensure that the desirable testicular temperature is maintained. Any environmental effect or management approach which alters the ability of the bull to regulate testes temperature, or increases testes temperature through other means, will negatively influence seminal quality.

Nutrition is a key environmental influence that may alter seminal quality. Variable results have been obtained regarding the influence of nutrition on seminal quality. The most detrimental effects have been observed through the feeding of high energy diets to yearling bulls. Some reports indicate substantial reductions in sperm production, storage, and motility and an increase in abnormalities in bulls fed high versus medium energy diets, while others have observed little effect of dietary energy on seminal quality. Some of the variability between studies can be explained by the actual levels of energy provided and/or the final body condition of the animals tested. Circumstantial evidence suggests that the mechanism for reduction in seminal quality with high energy feeding is the result of impaired thermoregulation of the testes. The impaired thermoregulation thought to be caused by increased fat deposition in key areas of the scrotum that are responsible for thermoregulation. The practical implications are reflected in results from a field study in which 277 bulls were evaluated for backfat and fertility. A significant negative relationship between backfat thickness and fertility were observed in the natural breeding season; as backfat thickness increased, bull fertility declined. Thus, nutrition, through its influence on body fatness, may contribute to reduced seminal quality thereby decreasing the bull:cow ration that can be obtained with an individual sire.

Other factors that can affect temperature of the testes also may influence seminal quality. Elevated body temperature has clearly been shown to reduce seminal quality. An insult in the scrotal area such as frostbite or insect infestation can increase the local temperature, and influence seminal quality. Thus, maintenance of the thermoregulatory mechanisms of the testes are critical to defining the mating capacity of a bull.

In summary, seminal quality is the result of both the genetics of the sire and the environment in which he is reared and maintained. Proper selection and management are critical to maintain high levels of seminal quality. This characteristic is most often assessed through the use of the BSE. Seminal quality is one of the primary factors that influences mating capacity.

The influence of libido on mating capacity: In addition to seminal quality, the mating capacity of a bull is determined by his libido and mating capability. In other words the willingness and competence of a bull to service a female is critical to his mating capacity. For simplicity, the term “libido” or “serving capacity” will be used hereafter to describe sex drive and competence to mate, collectively.

1. The libido test: Several systems have been developed to evaluate and quantify libido of bulls. The basic premise of most tests is to expose a group of bulls to either females in heat or restrained females, and measure the willingness (# mounts) and competence (#successful matings) to mate in a set time period. Multiple evaluation periods (>2) are best for accurately categorizing bulls as high, medium or low libido. Virgin bulls often require a learning period, and perhaps an offered sexual experience in order for them to demonstrate their true potential for libido. In conclusion, libido tests can be used to accurately rank bulls according to libido score. However, the necessity for multiple tests, and providing the opportunities for young bulls to learn, increases the efforts necessary to successfully perform this type of evaluation.

2. Effect of libido on mating capacity: The importance of adequate libido or serving capacity on the mating capacity of a bull, and the resultant fertility of the females serviced, has been demonstrated. Some scientists have shown clear differences between bulls with high, medium, and low libido in terms of the conception rate of females to which they were exposed. However, others have demonstrated no difference between higher and lower libido bulls in terms of fertility, although usually the number of services was increased with increased libido. That factors other than libido are important can be easily assessed by comparing herd fertility among mature and yearling bulls that demonstrate equal mating activity. Typically, herd fertility, at the same bull:cow ratio is lower with the younger bulls.

The variations cited above for the effects of libido on herd fertility can be explained as follows. The largest effects of libido score on herd fertility can be detected when one compares bulls of different libido scores that have equal BSE scores (i.e. seminal quality) and in which mating capacity is challenged (i.e. high bull:cow ratio). Under these conditions, seminal quality is not an issue, and differences in libido are expressed due to the large number of females that are to be serviced. If BSE scores are not considered, or if bull:cow ratios are low enough that even a bull of marginal libido can complete the task, then libido effects may not be detected, or in the case of low bull:cow ratios, be unimportant.

In order to define the mating capacity of a bull, then, it is essential that both willingness and competence to mate (libido) and the ability to produce viable semen in effective quantities (seminal quality-BSE) are considered. This is important since these traits do not appear to be genetically related. In other words, bulls with high libido do not necessarily have high seminal quality, and vice versa. After consideration of both of these characteristics the appropriate bull:cow ratio of an individual bull can be more clearly identified. While it is possible to quantity seminal quality through the BSE, determination of libido is much more difficult. In the typical situation in which a formal libido score is not available the only other viable option is through observation of the willingness and competence of the bull when exposed to females in heat. Determination of competence to mate should be relatively simple, whereas, quantification of libido would be much more difficult. Determination of the ability of the bull to identify the females that are in heat, and the aggressiveness to which he pursues these females should provide at least some subjective measure of libido.

In summary, mating capacity, or bull:cow ratio for an individual bull is a function of both seminal quality and libido. While BSE scores can be obtained, the evaluation of libido is usually a subjective determination. These characteristics can be used to optimize bull power in a natural breeding situation. However, caution and careful management is critical since the risks association with bull infertility increase substantially as bull:cow ratio is increased.