Whether the producer uses a limited or year-round breeding season, he needs to understand the factors that affect the fertility of the bull and what strategies he can use to improve or maximize the fertility of the bull or bulls in his herd. This improves overall herd performance, individual calf performance and longevity of the bull's usefulness in the herd.

Fertility in a livestock enterprise is 5 to10 times more important economically than any other production measure. Cows bred to high-fertility bulls bear more calves earlier in the season, resulting in more pounds of beef weaned and marketed per cow, which is a direct measure of profit. Bulls with identical semen quality in terms of physical assessment vary in actual fertility. The capability to identify bulls on the basis of fertility potential could result in higher pregnancy rates, leading to larger calf crops.

The following article will discuss some of the factors that affect fertility and steps the producer can take to make improvements.

Breeding Soundness Evaluation

A very useful and important tool to producers is the Breeding Soundness Evaluation. Chenowith and co-workers at Kansas State University have work extensively in this area and have developed a list of important considerations.

1) A Breeding Soundness Examination (BSE) is the best assurance of satisfactory bull fertility.

2) A bull should have a BSE done every year. It is good, inexpensive insurance.

3) Utilization of a BSE has the potential to return at least $20 for each $1 invested in the procedure. A bull may be big and masculine in appearance, but may not be a satisfactory breeder. A BSE provides the best evaluation of the bull's reproductive potential. Normally, a BSE should be done 45 to 60 days before the start of the breeding season. For the breeding season starting April 1, the BSE should be done between February 1 and March 15.

4) Bull fertility affects the pregnancy rate, average calving date, average weaning weight, uniformity of calf crop and age of puberty in replacement heifers � all of which have a significant effect on economic importance.

5) Bull scrotal circumference is positively related to both bull and heifer puberty as well as semen quality.

6) Surveys report that 15-20 percent of bulls have problems which adversely affect fertility.

Other Testing Programs

A new test determining bull fertility has been developed through collaborative research between the University of Arizona and industry. It gives results on-site in 20 minutes (compared to two days for the previous standard method) and costs about $45 per test, with a potential net return of $140 per bull tested.

In the 1990's University of Arizona animal scientists assisted in developing a color-based diagnostic test to identify a specific protein on bull sperm. The antibody used to detect presence or absence of that protein is referred to as fertility associated antigen (FAA). Bulls with FAA on their sperm are 17 percent more fertile than their herd-mates lacking the FAA over a 60-day breeding season. Heifers inseminated once to bulls with FAA had a 16 percent higher pregnancy rate than herd-mates inseminated to bulls without FAA on their sperm. The original diagnostic test, released for commercial use in 1998, was licensed for any rancher or veterinarian in the world to use. Since then it has been adopted by breeders across the U.S., and in at least a dozen countries worldwide, in the Pacific Rim, South America, Canada and Europe.

In January 2004, the next generation of that test, a lateral-flow cassette containing reagents to detect FAA, was released on the market after UA scientists conducted extensive research on it. The 2nd generation test takes 20 minutes to run and works just like a pregnancy test that can be conducted at home: a purple line in the cassette window indicates the presence of FAA.

Detecting high fertility bulls is the key to increasing livestock production and thus profitability. The new bull fertility test is not only accurate, it is also user-friendly, faster than the previous two-day laboratory method, and cost-effective. Housed in a small plastic cassette about the size of a stick of chewing gum, it has a three-year shelf life, requires no special storage, and can be used chute-side with results in about 20 minutes. If a few tests are purchased at $45, at a $50 profit per calf, and if a bull remains in a herd for four years breeding 25 cows per year, the return on investment is 13-fold per each cow bred. For bulls, a $45 test yields a net return of $140 per bull tested.

A one percent increase in fertility in the U.S. beef industry would return a net profit of $55-60 million to U.S. producers. Obviously, on a global scale, billions of dollars of income could result from identifying higher fertility bulls and males of other livestock species. On a local scale, a rancher who bred FAA-positive bulls to heifers in a higher ratio than usual�one bull to 20 heifers instead of the industry standard rate of one bull to 15 heifers�found that he got the same 92-95 percent pregnancy rate during the first 45 days while using a third fewer bulls and no additional feed.

Stress, Nutrition, Etc.

On going research by the KSU group is searching for ways to detect not only infertility in bulls but also to maintain fertility and maybe even improve fertility. The vast majority of cattle are still being bred naturally, and bulls are obviously an important part of the equation. Polls of producers and veterinarians have shown that about twenty percent of bulls have some sort of fertility problem. Unfortunately, relatively few bulls are tested for fertility so some (many) ranchers may not recognize that a problem exists or where the problem may be.

Fertility issues don't necessarily mean that bulls with these types of problems are sterile. They just have poorer calf crops than should be. Bull infertility can result from stress, caused by the weather, transport, handling, and so on as well as nutrition.

In some areas of the country extreme combination of cold temperatures and blizzard conditions during winter months is always a concern to cow-calf producers because of the added difficulty in feeding and caring for the cattle herd. Winters such as the one we are currently experiencing can be especially hard on bulls.

The first priority for most cattle producers is the well-being of the main cow herd. Herd bulls, which are generally kept separate from the main cow herd in those with limited breeding seasons, may experience equal hardship if proper nutrition and shelter are neglected. The future reproductive success of the herd will suffer if bulls are not prepared for or protected from winter weather. Like the cowherd, bulls need to be maintained in a body condition score of 5-6 in order to be in ideal breeding condition. Low temperature and windy conditions can easily increase feed requirements 25-30 percent above normal maintenance requirements. Also, lack of wind protection and proper bedding will increase the chance of frost damage to the scrotum and testicles. During normal winter conditions frostbite is not a common problem with breeding bulls, but prolonged exposure to extreme cold and wind increases the incidence of frostbite and is a problem that must be considered when planning for the breeding season. Evidence of frostbite to the scrotum is usually apparent a few days after freezing in the form of noticeable inflammation and swelling. The heat generated from the inflammation directly affects the sperm that are maturing and stored in the epididymis, which surrounds the testicle at the lower end of the scrotum. The resulting damage may cause temporary or, in more severe cases, permanent sterility in the bull. A scab may appear on the lower portion of the scrotum as healing occurs. However, the absence of a scab does not indicate that frostbite injury has not occurred. Severe frost damage to the testicle and epididymis may cause tissue adhesions, affecting mobility and circulation within the scrotum.

Likewise, external heat or internal heat can be detrimental to bull fertility. Here are some ways to manage this problem. Bulls depend on a number of mechanisms to keep the testicles in optimal operating temperature. The mechanisms include the tunica dartos, cremaster muscle, counter current function of the pampiniform plexus and the testicular arterial branching. The important thing for producers and veterinarians to realize is when a heat related insult occurs, it is important to consider several things, including the severity of an insult, prognosis and what processes, whether pathologic or management-induced, are involved. These factors can affect the efficiency at which any of these reproductive related processes function, resulting in higher testicular temperature that can lead to increased problems with sperm formation and function. Increases in testicular temperature have resulted in reduced semen quality. Sources of thermal insult include high environmental temperature, fever and excessive lying down. Any of these conditions can negatively impact semen quality. The breeding soundness examination can help identify these potential problems.

Nutrition and feeding practice can have a profound affect on fertility. Fertility can be negatively affected by feeding certain materials such as excessive gossypol, the pigment in cotton products (cottonseed meal, cottonseed hulls, etc.). This does not mean that cottonseed by-products cannot be fed to bulls, only that they should be fed at moderate levels � no more than 3-4 lbs of cottonseed by-product feeds per head per day. The good news about this situation is that it is reversible. Research has shown that bulls with poor fertility due to the consumption of excessive gossypol will regain normal fertility several weeks after the gossypol is removed from their diet. Additionally, several studies have been done with vitamin E, an antioxidant and commonly included nutrient in feeds and mineral supplements, in combination with gossypol. Not only did the vitamin E protect against gossypol, it appeared to be favorable toward bull fertility. Other research into alleviating stress effects are being done with vitamin A, C and the trace mineral selenium.

Energy intake can have a great deal of effect on the breeding soundness of bulls. Ideally a bull should be in a body-condition score of 6 to 7 prior to the beginning of the breeding season. In many cases young bulls purchased at production sales are in excessive body condition. If this is the case, it is recommended that young bulls carrying excess flesh should be "let down" from the time of purchase until they are turned out with the cow herd. It is suggested that the producer, when buying a new bull get information on the type of diet the bulls have been eating and then slowly step down the amount of concentrate and increase the percentage of forage over a period of 60 days. Avoid any major feeding changes. Additionally, exercise is important for bulls. For example to increase the exercise level of these bulls feed them on a hill and provide water at the bottom of the hill. This type of exercise not only pulls the weight off these bulls, but it also strengthens the feet, legs and back.

Good management practices and providing balanced nutrients are critical for maintaining cow and bull fertility. In addition to protein and energy, adequate trace mineral intake and absorption is required for reproduction. Reproductive performance may be greatly affected if zinc, copper, manganese or selenium levels are marginal to deficient. Common copper deficiency symptoms in cows include delayed estrus, decreased conception rates, infertility and early embryo death resulting in more open cows in the fall. Inadequate zinc levels are associated with decreased fertility and increased dystocia and retained placentas. In bulls, zinc deficiency causes lower fertility due to poor sperm quality and reduced scrotal circumference. Likewise, male reproductive performance is affected by low manganese status which can inhibits libido and lowers spermatozoa numbers. Another consideration for bull fertility is bone soundness and the ability to travel. Zinc, copper and manganese are needed for skeletal development and maintenance as well as hoof integrity. A bull suffering from lameness or joint problems will breed fewer cows. Selenium deficiency in bulls decreases spermatogenesis, the development and maturation process for sperm.

Feeding beef cattle complexed copper, zinc, manganese and cobalt such as ZinPro 4-Plex or Availa-4's has been shown to enhance reproductive performance early in the breeding season. Additionally, use of injectable trace minerals such as MultiMin Cattle 70TM three to four times per year can help overcome absorption issues and stimulate trace mineral-dependent reproductive processes.

As producers look at their mineral programs, two important considerations exist. First, the best approach to supplementation is to include a balance of all minerals that impact production and especially reproduction. Increasing the level of a single mineral due to identified deficiencies can create an imbalance, an antagonistic effect, on another mineral. For example, zinc and copper become antagonistic if either mineral is supplemented at extremely high levels. Optimum copper: zinc ratios are 1:3 to 1:5. Secondly, the strategic period for use of complexed trace mineral to improve reproduction and breeding performance in bulls is 60-80 days prior to the start of breeding season. Injectable trace minerals should be administered about 30 days prior to onset of breeding season and every three months thereafter.

Conclusions

Bull management to insure fertility and maximum productivity requires a considerable amount of understanding and input. The benefits are significant, however, when bulls perform optimally and provide the maximum genetic influence.