Management and Economics of Natural Service Sires on Dairy Herds

Introduction

Reproductive efficiency in a dairy herd improves when the percentage of pregnancies resulting from artificial insemination (AI) increases. As described by Bartlett's equation of reproduction, four factors determine the percent of resulting pregnancies from AI [1]. These factors are; cows detected in heat and inseminated, fertility level of the herd, semen fertility level and inseminator efficiency. The percentage of pregnancies resulting from AI is the product of these four factors and not their average. When these factors are multiplied, their product or percent pregnant is less than the lowest factor. The effect of a low factor has a cumulative effect on the percentage of pregnancies and is never averaged out. Examining this equation provides producers with a working concept of dairy cattle reproduction. Of the factors included in the equation of reproduction, the inability to detect estrus efficiently and accurately is the major impediment in attaining an optimal percentage of pregnant cows from AI. Estrus detection efficiency is expressed as the percentage of estruses observed over a given time period [2]. Accuracy of heat detection is the percentage of estruses observed that are true estruses [2]. Collectively, errors in efficiency and accuracy of heat detection result in high semen cost and an increase in the interval from calving to conception, reducing cow production and net returns.

To eliminate inefficient heat detection practices the use of natural service (NS) bulls in dairy farms appears to be increasing. The perception is that pregnancy rates improve when NS is used because more cows are detected in true estrus and serviced; that is the intensity and accuracy improves in the herd. A 1984 survey of dairy herds in Florida showed that 50 percent used AI, 38 percent used a combination of AI and NS, and the remainder used mostly NS [3]. A Pennsylvania study involving 329 dairy farms, evaluated the method used in breeding heifers. Here, 11.2 percent bred their heifers once with AI then used a bull, 8.5 per cent bred their heifers twice with AI then used bulls and 20.7 percent bred their heifers with bulls only [4]. Estimates from large dairy herds in Florida, and Texas indicate that the use of NS is still widespread particularly in dairy herds employing a rotational grazing management. A 1995 survey from the National Association of Animal Breeders reported that less than twenty percent of dairies use artificial insemination exclusively [5]. Dairy herds that predominantly use NS bulls usually do not raise replacement heifers; the genetic balance of the herd can be maintained by purchasing replacement heifers from breeders who are using AI with semen from proven bulls. The use of NS can reduce the negative effect that people can have on cattle reproduction by eliminating errors in estrus detection. However, when the equation of pregnancy rate is considered using NS, it becomes apparent that the fertility of the bull, and his ability to service cows that are in estrus greatly determines the pregnancy outcome. Therefore, in order to adequately exploit the use of NS in dairy herds, proper selection and management of these bulls should be included in the overall herd health program of the dairy. In addition, to prevent the disastrous economic consequences of sub-fertile bulls, periodic evaluation of their reproductive performance must be performed.

Economics of Heat Detection (AI vs NS)

The economics of NS bulls versus AI are usually determined by calculating the cost of semen, equipment, personnel and cost of bull maintenance. The major argument against the use of NS is the predicted difference in milk yield of AI daughters compared to NS daughters [3]. However, a major economic consideration for using NS bulls is the potential for the improvement in the herds reproductive efficiency by maximizing heat detection and conception rates. Use of NS bulls may result in higher conception rates when compared to AI because errors in heat detection are removed. In general, as heat detection rates improve, net revenues increase as a result of higher milk yields per cow. To illustrate this concept, the projected effect of three different heat detection rates on milk production and economic return is shown in Table 1 [3]. The model uses a seasonally adjusted conception rate of 30 per cent [6]. This model shows that an increase in heat detection rate from 47 per cent to 57 per cent produces an increase in milk yield of 370 lb of milk per cow per year and $39.97 per cow per year when modeled over a 10 year period. In addition, the replacement rate of a herd is also reduced as heat detection rates improve (Table 2) [3].

By evaluating the prevailing heat detection and conception rates in a herd, the effective pregnancy rate (heat detection rate x conception rate) of that herd can be determined. The effective pregnancy rate can then be used to determine if the use of NS bulls would improve reproductive efficiency in the herd. Pregnancy rate exert their greatest effect on herd production, profit and replacement rate when it is between 25 and 30 percent (Table 3) [3]. Each percentage point increase in effective pregnancy rate between 15 to 25 percent, gives an increase of 1301 lb/cow/yr and $140 net/cow/yr. Between 25 and 35 percent, each percentage point of effective pregnancy rate is worth 355 lbs/cow/yr and $37 net/cow/yr. Conversely, if a dairy herd has an effective pregnancy rate greater than 25 per cent, the use of NS bulls would not result in a significant improvement in net income per cow. Therefore, the expected change in yield per cow based on increased heat detection, through the use of NS bulls or improved heat detection management, can be utilized to calculated the net value of high predicted difference AI semen as compared with the value of higher heat detection rate [3].

The male effect, or biostimulation, on female reproduction is well known [7]. Rams introduced early in the breeding season induce, and appear to synchronize, estrus in ewes. In sheep this biostimulation effect occurs in ewes that are in transitional cyclicity from the non-breeding to the breeding season and the males are introduced as a novel stimulus. Natural service bulls are often used in dairy herds in cows which already have cycled and have received multiple AI. These are the so called repeat or problem breeders and in this scenario biostimulation would probably not play a role. However, in those herds that use natural service only, bulls are introduced to cows in early lactation when the cows are in a transition cyclicity state. In this situation it is possible that some form of biostimulation may occur. An advantage seen for NS over AI suggested a possible male effect [8]. It should be kept in mind that comparison of NS versus AI are difficult to make. Researchers in this field conclude that biostimulation effects in cattle due to bull presence remain unclear. In addition there is anecdotal evidence from private practitioners that NS bulls reduce ovarian cystic degeneration in dairy cattle.

Selection of Bulls

Regardless of the genotype used for natural service in a dairy herd bulls must be carefully selected. The selected bulls should have the capability to detect females in estrus and be able to impregnate them. The ability of the bull to perform this task is dependent of his semen quality, libido, mating ability and social ranking among other bulls and females [7]. Therefore, in common with the recommendations made for beef bulls, selection of bulls for natural service in dairy herds should be subjected to a breeding soundness examination, as recommended by the Society of Theriogenology [9]. Only those bulls that successfully pass their BSE should be used. This examination should be repeated on an annual basis. It is recommended to use younger bulls that are less than 2 to 2.5 years old [3]. Young bulls should have achieved full puberty and sexual maturity which occurs around 14 months of age and should not be undersized in relation to mature cows.

Economic losses that occur from use of NS bulls due to lower milk production in daughters from these bulls are high. The sire-of daughter pathway was the weakest area of genetic improvement in the United States because of extensive use of NS bulls with low genetic merit [10]. Lost revenue represents the value of 695 lb of milk in each lactation over each generation. To help reduce these losses from genetically inferior NS bulls, producers should consider using bulls for natural service that are good enough for AI sampling. The genetic merit of young bulls used in AI sampling is as good as that for the average active AI bull. The typical bull in AI sampling would be at percentile 50, similar to the typical active AI bull [10].

Measuring Efficiency of NS Bull Breeding Programs

Adequate records and their proper analysis and interpretation are fundamental to effective reproductive management. Dairy herd improvement association (DHIA) records are widely used by dairymen throughout the U.S., and are frequently analyzed by veterinarians in North America [11]. In dairy herds that use NS bulls in their reproductive program, DHIA records can be used to evaluate the overall herd reproductive performance, which includes breeding for both AI and NS bulls. However, these records are not designed to access the efficiency or performance of NS bulls used in the herd. Therefore, accurate evaluation of the dairy's NS reproductive program is difficult to make. A case in point is the practice by many Dairy Records Processing Centers to enter just a single service in the record for successful bull breedings regardless of the number of services. This practice gives an inaccurate evaluation of NS bulls reproductive performance. It is important to accurately monitor NS bull performance in order to make correct and prompt decisions to replace sub-fertile bulls.

The reproduction committee of the American Association of Bovine Practitioners in 1991 recommended reproductive indices for herds using NS bulls [12]. These indices have been summarized by Chenoweth and Larsen [3] and Upham [13].

The above calculations require that diligent records be kept by the producer and analysis of these records are conducted on a timely basis. In those herds that use only NS and do not record when bulls are introduced to cows, the author has had success in monitoring bull fertility by monitoring percent of cows pregnant at the end of a herd check and cows open >150 days in milk. Except for the summer months these figures should remain constant on a monthly basis. Accurate information on NS breeding efficiency is difficult to obtain because in many situations NS bulls are used with cows that have failed to conceive after various AI attempts [14].

Management of Bulls

In many situations, NS bulls fail to participate in the health programs designed for the cows. Bulls used for NS should receive the same vaccinations for cows with the exception of Brucellosis.

Venereal diseases such as vibriosis and trichomonosis are an important consideration when using NS bulls. For vibriosis vaccination of females affords the best protection with best timing at several weeks prior to breeding. It is recommended that cows are vaccinated at least 3 weeks prior to bull exposure. Some success also has been attained with the vaccination of bulls [3]. Vaccination for trichomonosis is also available for females only.

The clinical picture for both of these venereal diseases is similar. The herd picture is one of repeat breedings which contribute to an increase to the interval from calving to conception in the herd. In addition, abortions may occur in the second trimester. In trichomonosis, pyometra may occur in some cows. Diagnosis is best done with appropriate sampling from both bulls and cows. In cows affected with pyometra as a result of trichomonosis, culturing is often unrewarding. In the majority of dairy farms bulls are purchased and have an unknown history of vaccination. It may be worthwhile for the veterinarian to design a protocol for the management and care of bulls used for NS that includes a physical examination, BSE, vaccination and deworming program.

Bulls being used for natural service should not be allowed to become overconditioned or to develop feet and leg problems. These undesirable traits negatively affect the breeding soundness of bulls. Rations which are balanced for middle to high producing dairy cows contain higher energy, protein and calcium levels than those required by the bull [15]. The excess in energy intake can predispose the bull to overconditioning and laminitis. Feeding bulls a high level of dietary calcium has been associated with lameness in conjunction with bone lesions in the spine and hip regions [3]. The fact is that when one examines the dietary requirements for mature bulls regardless of genotype, they are similar to requirements of a dry dairy cow . To avoid problems related to a lactating cow ration, evaluation of body condition and lameness should be conducted frequently in NS bulls.

During the past years there has been a concern over the effect of gossypol from diets containing cottonseed products on bull fertility. In many dairy regions of the United states as much as 8 pounds (15 percent DM basis) of whole cottonseed is fed in total mixed rations balanced for high producing dairy cattle. A mature Holstein bull with an dry matter intake of 13 kg [15] could consume as much as 13 g of free gossypol per day. Whether or not gossypol intake at this level has a detrimental effect on bull fertility is not definitively known. An increase in sperm midpiece abnormalities and erythrocyte osmotic fragility in Brahman bulls fed 2.75 kg of cottonseed meal (8.2 g of free gossypol per day) has been reported [16]. In contrast, Hereford bulls ingesting 7.6 to 19.8 g of free gossypol daily from whole cottonseed showed no significant sperm cell abnormalities [17]. In the above study [16], it was suggested that the mineral content of the drinking water contained sufficient minerals to bind with the free gossypol.

The type of cottonseed product (meal vs whole seed), and gossypol enantiomer (+ or -) may determine the extent of the toxicological effect that will occur and may explain the variable results obtained in research trials [18]. It has been suggested that detoxification of gossypol in the rumen is more efficient with whole seed diets than with cottonseed meal diets [18]. Brahman bulls fed 1.8 g/day of free gossypol from cottonseed meal had similar damage to seminiferous epithelium when compared to bulls fed 16 g/day of free gossypol from whole cottonseed [19]. The spermicidal effect of gossypol may also depend on the predominant + or - gossypol enantiomer present in the cottonseed product. Due to its stereospecific binding properties, the (-) gossypol enantiomer is less bound to plasma proteins and appears better able to cross the blood-testis barrier in vivo and inhibit the biological activity of some proteins [20]. The type of cottonseed used and gossypol enantiomer present, has contributed to the variable results obtained in gossypol related studies.

Recommendations in terms of gossypol intake in the total diet for bulls used for breeding is 200 mg/kg for diets composed of cottonseed meal and 900 mg/kg for diets composed of whole cottonseed [21]. However, the relevance of gossypol studies to commercial cattle operations needs to be carefully considered. The free gossypol content in the cottonseed meal study rations cited above, were obtained from solvent extraction methods, which accounts for less than 2 percent of the oil extraction method used today. In addition, males in gossypol related studies have not been subjected to actual fertility trials.

Reproductive Management of Cows

In herds that use only NS the advantages of a fresh herd and a voluntary waiting period of 60 days used in AI herds should be considered. Fresh cows can be monitored daily for postpartum complications and sick cows treated promptly without the nuisance of having a bull present. Concerns from practitioners that work in herds that use only NS bulls is that the opportunity to observe cows daily for sickness is lost because of the reluctance by herd personnel to enter a pen with a bull in it. The benefits of a well balanced postpartum transition diet in order to reduce metabolic or digestive disorders is also warranted. Further, the use a prostaglandin regiment to promote multiple cycle and uterine involution in an attempt to increase pregnancy rate at first service can be advantageous . To help reduce the interval from calving to first service cows can be treated with prostaglandin prior to being exposed to the bull.

Pregnancy diagnosis can be performed in cows 40 to 60 days after bull exposure. Cows that are found open on examination can be re-examined 30 to 60 days later. Cows that are found to be cystic can be treated with GnRH and use of prostaglandin should be limited to cows with pyometra only. Because of the inaccuracy of breeding dates in many NS herds, it is necessary to accurately estimate gestation length in order to allow the cow an appropriate dry period. The author has found it to be beneficial to re-examine pregnant cows again at 60 to 90 days. In herds in which trichomonosis has been diagnosed, the incidence of early abortions at 30 to 75 days, has been reported to be between 10 and 30 percent. In addition, the diagnosis of pyometra related to trichomonosis is more likely at this time. This will help diagnose this problem early and minimize the economic loss due to this venereal disease. Pregnant cows should also be confirmed prior to dry off similar to the practice used in A.I. herd.

In most herds that use both NS and AI, cows are turned with the bull after they are found open at a predetermined day in milk (130 to 150 days) regardless of service number. These cows can then be palpated for pregnancy as previously described after they are turned in with the bull.

Conclusion

Despite the tremendous evidence supporting the economical advantage of AI compared with NS bull, many dairy producers consider that the use of natural service is advantageous to their reproductive management. Considering the prevailing heat detection and pregnancy rates on a dairy farm, the use of NS becomes a valid option when the effective pregnancy rate (heat detection rate x conception rate) falls below 25 per cent. This option will only be maximized if bulls that are able to impregnate an estrous cow are used. Therefore, bulls should pass a BSE prior to use and should be repeated frequently. With the exception of brucellosis, bulls should undergo the same herd health procedures as the cows. Particular attention should be made to the prevention of vibriosis and trichomoniasis. Reproductive performance monitoring of NS bulls should be conducted on a periodic basis. Attention should be given to the recommended indices for monitoring NS bull performance. Veterinarians should be vigilant with dietary and managerial factors that may impair bull fertility.

A tremendous amount of time is spent by dairy consultants in convincing producers not to use NS bulls in their operations. As is often the case, these producers have made certain financial considerations that are unknown by the consultant in order to arrive at the decision to use a bull. Nevertheless, because of the danger involved in using bulls, the possibility of introducing venereal diseases and the loss in genetic progress, use of NS service should be recommended only after all methods to improve pregnancy rate with AI have been exhausted.