Reproductive rates are falling at an alarming rate in high-milk-producing dairy cows. The latest estimates from many university and extension studies show there is a decline of 0.5 to one per cent per year among North American dairy herds. Many of these studies suggest a rise in milk yield is to blame, while other research shows that high milk yield is only incidental to poor reproduction. Regardless of the debate, most people will agree good nutrition in milking dairy cattle is crucial to good milk and good reproduction.
When dietary nutrients are well balanced for milk production and reproduction, soon after calving they help return the dairy cow to a natural reproductive cycle soon (21 days) after calving. Initially, they (along with other internal hormones and metabolites) play a complex role upon the endocrine system of the cow, which stimulate the pituitary gland in the cow’s brain to produce special hormones that initiate the entire reproduction process.
The non-pregnant cow has follicles; blister-like structures that appear on the ovary. These follicles grow from microscopic cells producing the growing ovum (unfertilized egg) and produce more reproductive hormones. During maturation, the follicle ruptures to release the ovum to be picked up by the oviduct for fertilization by the bull’s sperm. Even under ideal conditions, of the thousands of eggs produced by the ovaries, only some of the ovulated will develop normally and have a chance to be fertilized.
ENERGY IS KEY
Of many nutrients important to reproduction, energy status is the largest and first limiting nutrient that seems to affect fertility the most; despite the actual energy requirement for reproductive activity such as follicular growth, ovulation, and early embryonic development is very low. A lactating dairy cow requires less than 3MJ of metabolizable energy (ME) compared to 60 MJ for basal body needs and up to 250 MJ for milk production during early lactation.
Based on these low energy needs, one might assume it would be easy for a high-milk-producing cow to resume strong estrus cycles. Unfortunately, reproduction takes a very low priority in obtaining this nominal amount of energy, compared to energy needs for body maintenance and milk production. It means that early-lactation dairy cows that can’t consume enough dietary energy to meet high levels of milk production are put in a “negative energy balance” (NEB) for about six weeks after calving. As a result, they are more susceptible to suffer from silent heats and the onset of ovulation.
Reproductive research shows NEB can adversely affect the normal development of follicles by disrupting the involvement of local and systematic production of hormones. It has been shown when a cow has poor energy intake there is a significant decrease in the level of specific hormones that trigger normal estrus cycles after calving. Other similar studies demonstrate the release of large amounts of fatty acids from the breakdown of body fat is also poisonous to fertile egg cells, even if they are released during the ovulation process.
PROTEIN ALSO A FACTOR
Similar to dietary energy, prolonged protein deficiencies in dairy cattle have been shown to reduce reproductive performance. But an imbalance of too much protein formulated into the diet is likely to be the respective culprit. If excess dietary protein is fed above the cow’s protein requirements with the imbalance being wasted, rumen-soluble protein will result in high blood-urea nitrogen (BUN) and high milk-urea levels (MUN), which are associated with high ammonia levels in the uterus (originating from the bloodstream). These unfavourable conditions are thought to lead to failure of the fertilized egg to attach to the uterine wall and therefore responsible for many early embryonic deaths.
MINERALS AND VITAMINS
Mineral and vitamins also play a significant part in healthy dairy cow reproduction. For example, a calcium imbalance or milk fever after calving is also associated with lack of smooth muscle tone in the reproductive tract, which may delay uterine involution, and increase the incidence of retained placentas that often lead to resumption of nature estrus cycles. Phosphorus is called the “breeder mineral” involved in energy metabolism and a dietary imbalance with calcium is cited in situations of inactive ovaries, delayed sexual maturity and low conception rates. Trace minerals such as copper, zinc, manganese and selenium (along with vitamin E) activate enzyme systems that drive reproductive processes as well as play antioxidant roles, which promote good cell health in reproductive tissues.
Meeting the current National Research Council (NRC) requirements for energy, protein, minerals, vitamins and water are generally recommended by dairy nutritionists to supply the necessary nutrients in dairy diets, not only for high milk production, but for good reproduction in dairy cattle.
KEY POINTS TO FOLLOW
Here are some specific recommendations that also help promote good milk and reproduction in dairy cattle:
- Set up a proper transition cow diet (three weeks before cows calve and three weeks post-partum) in order to promote good dry matter intake and a body condition score of 3.0-3.5 in susceptible dairy cows. Otherwise a similar close-up group may be created.
- Increase the energy density of the early-lactation cow diet. Research shows by adding “bypass fats” to the dairy diets of dairy cows within the first 100 days of lactation reduces NEB and significantly increases pregnancy rates.
- Review protein intakes. Cornell researchers demonstrated that dairy cows fed excessive protein as little as 10-15 per cent above natural requirements required more services per conceptions and had longer calving intervals than control cow groups.
- Feed a full complement of macro-, micro-minerals and vitamins. Total calcium and phosphorus needs should be provided at required levels and often in a 2:1 ratio for lactating dairy cows. Insure that proper levels of trace minerals and vitamins are fed and balanced. It is known that zinc fed in excess will inhibit the absorption of manganese and copper that could lead to “silent heats” in dairy cattle.
The above recommendations of sound dairy nutrition upon dairy reproductive performance boils down to avoiding too little or too much of any particular nutrient formulated in the dairy diet. However, the perfect diet that promotes both high milk production and excellent reproduction at the same time might not even exist, but we certainly should try our best in the formulating one. †