Reproductive efficiency is fundamentally the most important trait for profitability of a cow-calf production herd. This article addresses bull selection and timing of yearling bull sales strictly from a perspective of evaluating and utilizing bull fertility as a value-added trait. There are two major areas where bulls contribute to herd reproductive efficiency. One is producing offspring daughters with the ability to become pregnant at 14 months of age. Second is the ability to achieve a high pregnancy percentage at every mating. These two traits are from a combination of bull maturity and the genetics that directly contribute to the quality of semen. The ability to effectively assess these traits is dependent on the age and sexual maturity of the bull, and the ability to perform a valid bull breeding soundness examination (BBSE). A valid BBSE is a must if it is to be used in bull selection. Without a valid BBSE we cannot improve the genetics and the contributions of a bull on herd reproductive efficiency.
Yearling bulls are usually sold between 11 and 15 months of age. Puberty is reached at 9 – 10 months of age, at this time semen quality is very poor with only 10% normal sperm morphology. Over the next 4 months semen quantity and quality will progress rapidly. The percent of yearling bulls mature enough to be classified as satisfactory can range from as low as 40% and up to 85%. This variation is contributed to the age, weight, breed, time of year, pre-weaning nutrition, environmental stressors, and genetics. Any bull less than 15 months of age that has a semen test showing poor semen quality needs to be retested after allowing the bull more time to mature. While many younger bulls will mature and eventually “pass” a quality BBSE, 30% to 50% of retested bulls will not pass.
A BBSE’s purpose is to identify subfertile and infertile bulls, and the physical attributes of these bulls that are known to impair the bull’s ability to mate or lead to decreased fertility in the future. These physical attributes include the feet and claw set, legs, the prepuce, and scrotal shape and size. In every case the negative traits will often be difficult or impossible to detect in young bulls. It takes increased maturity and age before physical defects such as screw claw, severely pendulous prepuce, and a wedge-shaped scrotum will be noticeable.
A bull’s spermiogram, which is a combination of sperm progressive motility and a complete differential morphological assessment, does not usually stay the same throughout the year. The spermiogram and therefore the fertility of a bull, is influenced by the season of the year, including daylight hours, cold and heat stress, nutrition, bull use, environmental stressors, genetics, and the combined interaction of two or more of these elements. Higher sperm progressive motility and higher sperm normal morphology generally correlates directly with increased fertility. On average a bull’s fertility is highest in May and June then lowest at the end of the summer during August and September. The second lowest time of fertility is during the short and cold days of winter. This is of course from the perspective of living in the northern hemisphere where there are hot summers and cold winters. Ironically it is during or shortly after the shortest days and coldest time that we begin testing bulls, many of which are still in the process of maturing sexually. This can lead to frustration from a bull producer wanting to sell a high percent of bulls in an upcoming sale and the number of bulls classified as satisfactory is less than expected. We often don’t plan to use the bull for another 2 or 3 months. The more time that passes between testing and use the less reliable the results of the BBSE. It is safe to say that the average yearling bulls spermiogram will improve and be better in May than it was during late winter, both due to season of the year and maturity of the bull. However, we cannot assume this to be the case with every bull, some bulls may even become less fertile after the original test. The decrease in fertility can be due to disease, injury, stress, toxins, and others.
Epigenetics is an important consideration of bull fertility; this is the interactions of genetics and environment. Many of the sperm morphological abnormalities we see are due to the interaction of an environmental stressor and the genetic predisposition for expressing the abnormality. This means that some bulls fertility will decrease due to a stressor where other bulls will not. In these cases a bull may test normal only to have a stressor trigger the production of abnormal sperm, then within a few weeks the bull is back to normal production. The beginning of the breeding season can be a significant stressor for some bulls and may lead to lower fertility. We can too often assume that once we receive the desired classification of satisfactory the bull will naturally continue with that classification indefinitely or at least for several months to a year. While this idea is correct for many bulls, there are enough exceptions that we need to be aware of these possibilities and remove any affected bulls from the gene pool.
Identification of subfertile bulls can sometimes be a challenge for cattle producers. Currently BBSEs are a bit of a crap shoot with some real B.S. thrown in. Many tests performed today tend to place an emphasis on how many bulls can be ran through the chute in an hour, the person or business we use to test our bulls is often based on price and speed rather than quality. Occasionally the evaluation can be so bad that there isn’t even a morphological assessment performed. A quality test takes time and attention to detail. It also takes quality equipment. An evaluator can purchase a light microscope for $300 from Amazon, or a quality phase contrast microscope that will cost a few thousand dollars. The ability to pick up some of the more subtle abnormalities with a quality scope can be significant. This can be frustrating. The financial rewards tend to go towards performing a quick cursory evaluation with cheap equipment. In these cases, more bulls are classified as satisfactory which also financially favors bull producers.
To create a positive change pressure needs to be placed on bulls for genetic improvement, on veterinarians to purchase quality equipment and perform a test with high validity, and on bull producers to support and favor the two needs listed above. Changes will only be created by demanding it. This can best be done by retesting every bull purchased regardless of who sold you the bull or who performed the original evaluation. This will hold the seller accountable for the choice of evaluators and the bull for possible epigenetic traits that don’t show up until after a little stress is applied. As a cow-calf producer, choose the evaluator based on the completeness of the exam, including both the physical exam and the microscopic evaluation. The best microscope is a DIC microscope, next phase contrast, and then a high-quality light microscope with stains. Any results where the number of morphologically normal sperm cells drops below 80% and above 60% needs to have a complete differential morphological evaluation to know the types of abnormalities present. A simple division of head, midpiece, and tail abnormalities is not sufficient to declare a classification in these cases. Some veterinarians may have a difficult time justifying the cost of a quality microscope. In these cases, the samples should be collected and evaluated for motility, then the sample can be preserved and sent out for evaluation from a lab that has quality equipment.
Bull producers can postpone their bull sales even one month, this can improve the chances of bulls being sexually mature, the time of year more favorable, and a valid evaluation more reliable. Bull buyers are wanting quality bulls, you can improve the quality of the bulls available by obtaining a high quality BBSE, performed with the best equipment. This information can be invaluable for the improvement of future bulls and to show the quality of bulls available in your offerings. There is naturally going to be bulls that do not meet the minimum BBSE standards, use this information to show potential buyers the value offered through rigorous culling, not something many bull producers can brag about. Through good records and rigorous selection, it is possible to improve the reproductive potential of bulls offered, something few other producers have to offer.
Special Note: When considering differential morphology of bulls post stress, I am unaware of any specific studies that can help guide our decision process for assigning a classification. The cut off of 70% normal morphology could be used, however we need to be as fair as possible to the bull breeder. If, for example, a bull receives a classification of satisfactory prior to the sale and his normal morphology was in the 70’s prior to sale, then 3 weeks after the normal morphology is in the 60’s or about a 10% drop, this is most likely not going to significantly affect herd fertility. I would look for a greater than 20% drop which is more in line with studies that shows fertility difference of bull fertility when comparing 80% to 60% and insignificant fertility difference comparing 80% to 70%. A bull with a greater than 20% drop in normal morphology will likely recover over time, however future stressors will often once again cause an drop in normal morphology. These stresses are common at the beginning of breeding season. One of the difficulties will be comparing normal morphology between two practitioners using different types of microscopes. Phase contrast microscopes have been shown to identify a higher percent of sperm abnormalities, especially vacuoles and mitochondrial sheath defects. This concern of discrepancies can be minimized by selecting high quality phase microscopes especially if we really want to improve the genetic gene pool for traits that directly influence bull fertility.
Bull breeders will naturally want to sell as many bulls as possible and some will push back or may allow the bull to be returned only to sell it again after morphology levels once again increase. The goal is to place pressure on genetics and improve the overall gene pool. Therefore, these bulls should never be used as genetic sires in an AI program or herd where the purpose is to raise more bulls or heifers. Bull producers should use this as an opportunity to improve the genetics of their herd and create added value to their product.
By The Way: I wounder and am quite curious, would selecting bulls that tend to cope well with stress and do well by not having major effects on sperm morphology also have progeny that would cope better with shipping to feed yards and decrease the chances of BRD?