Sometimes it’s seems unfair beef cows and replacement heifers get more attention paid to their nutrient requirements, and bulls often are left to fend for themselves. Although, they make up less than five per cent of the beef herd, bulls provide 50 per cent of genetic material to make calves and 90 per cent of the genetic progress in most beef herds. Infertility of even one bull in a group rotation can have a significant detrimental effect on the breeding season.
So it is important bulls are provided with proper nutrition at all times in order to prepare them for a successful breeding season, even though it may be several months away. As part of a well-balanced bull diet, it is best to check even the smallest nutritional details such as supplying sufficient dietary zinc. It has been known for a long time that zinc is essential for good bull fertility.
Zinc’s role is pivotal in good bull reproduction for two microscopic reasons. First, it is involved in the production, storage and secretion of male hormones involved within the inner circle of male reproductive activities, namely; testosterone, insulin and adrenal corticosteroids. For example, testosterone is responsible for puberty, sex drive and is needed to maintain the overall functions of the bull’s reproductive organs. Second, dietary zinc plays a crucial role at the cellular level in spermatogenesis or sperm production, and thus vital semen production.
Subsequently, there is little debate as to the importance of dietary zinc in bull fertility. Over the last 50 years, university and extension research has clearly proven a zinc deficiency in cattle will lead to delayed puberty in intact young beef calves and low sperm count in older bulls, regardless of their breed and body condition score.
One of the best field trials on the importance of zinc nutrition for bulls in recent years was performed by Kansas State University (KSU)’s animal science department, where different levels of dietary zinc was fed to fertile breeding bulls in both inorganic- and organic- zinc forms.
The KSU experiment entailed feeding three groups of angus yearling bulls, one of the three diets for 126 days: (1) 40 ppm zinc supplied by inorganic zinc sulphate, (2) 40 ppm zinc supplied by 2/3 zinc sulphate and 1/3 supplied by an organic-zinc form or chelated zinc, and (3) 60 ppm zinc provided by zinc sulphate. The results of this study showed the percentage of viable normal sperm cells was the highest in the treatment group fed the second diet of inorganic- and organic-zinc (68.9 per cent), followed by zinc groups of 60 ppm zinc (62.5 per cent) and 40 ppm zinc (55.8 per cent).
The researchers drew two concise conclusions from the study. The NRC-recommended dietary zinc level for beef cattle of 30 ppm might be too low for bovine breeding stock and more bio-available organic zinc sources can improve bull fertility fed at relatively low levels in the diet. The research shows it is important for producers to apply some of these Zn-principles to their own bull feeding programs to help optimize bull fertility.
Producers feeding organic zinc might start to notice other growth and healthy benefits in their cattle. Because zinc is part of specific enzyme systems involved in epithelial (skin) formation and repair, feeding extra dietary zinc is theorized to strengthen cattle hooves, particularly under most harsh field and housing conditions. Zinc is also needed for strong bone calcification, involved in tissue healing, and part of circulating enzyme systems that regulate cell-mediated immune functions as well as vitamin A metabolism in the body.
Regardless as to how much dietary zinc a fertile bull ultimately needs, it should be supplied in a well-balanced commercial mineral-vitamin product that contains adequate levels of macro-minerals such as calcium, and phosphorus that compliment the rest of the diet; namely the forages and grains fed to young and mature bulls.
Furthermore, other trace mineral aside from high zinc levels, which are also essential for bull health, growth and fertility such as copper, manganese, iodine, and cobalt should be formulated into a bull mineral at relatively high levels and be assured of a high degree of bio-availability. Selenium should also be provided at three mg/head/d as well as recommended levels of vitamins A, D, and high vitamin E (i.e.: +1000 iu/head/d). Finally, salt and a good source of fresh clean water should round out the bull diet.
Likewise, a practical Zn-recomm-endation for breeding bulls is as follows; feed a standard $30 bag of well-balanced cattle mineral (with 40 ppm zinc on a complete feed basis) for much of the overwinter-early spring period (180 days) and then switched to a $40 cattle breeder mineral (a significant portion of zinc coming from organic Zn-sources) for about 60 days prior to the start of the breeding season (re: it takes about 60 days for new sperm to be produced and mature).
In one respect, zinc is no more or less important than those dozen or so above mentioned essential minerals required by breeding bulls. However, zinc is frequently singled out because of its strong nutritional association to good male reproduction. A good bull diet that follows this zinc concept should always assure that there is adequate dietary zinc to meet the beef bull’s specific zinc requirement, and be put alongside the other essential nutrients (including energy, protein, other minerals and vitamins), so they can all work together for the successful cause of developing fertile bulls that breed fertile beef cows and produce future profitable calves. †