The lactating dairy cow has an absolute requirement for nutrients, which is normally greater than nutrient requirements for vital body functions. These nutritional requirements are also linear — as she produces more milk, she needs to consume more dry matter feed containing the essential nutrients. In contrast, the need for the same nutrients for good reproductive performance seems to be a matter of strict nutrient balances — avoiding too little or too much of the same nutrients in the milking dairy diet.
When dietary nutrients are well balanced in a dairy ration, they support the natural process of the estrus cycle of the cow. Every 21 days, the non-pregnant cow has follicles; blister-like structures appear on the ovary. These follicles grow from microscopic cells producing the growing ovum (unfertilized egg) and produce many 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.
While the reproductive process concerning the cow’s follicular development is well understood, applied nutrition principles, which are thought to influence ovum production in the cow, remain elusive.
Regardless, it is often thought that many delayed post-partum heats, low conception rates and other infertility conditions in milk cows is a result of either low dietary energy intakes or inadequate body condition (to be mobilized for energy) when the cow really needs it.
Case in point: researchers at the University of Alberta tested this relationship between negative energy balance and the interval to first ovulation in dairy cattle. Their study showed declining physiological energy balance in early-lactation cows delayed the onset of ovulation after calving. Interestingly, it was also demonstrated that reproductive performance improved in test animals due to an improvement in their body condition, rather than to any specific level of energy supplemented in their diets.
Likewise, studies at other institutions have shown balancing dietary protein in the milking cow diet seems to be just as important as making sure the right amount of energy is being fed at all times.
TOO MUCH PROTEIN
In a normal cow rumen, microscopic bugs digest the soluble portion of feed protein into basic ammonia. A small amount of leftover ammonia that does not get used in microbial protein synthesis is absorbed across the rumen wall and transported to the liver where it is converted into urea. The liver releases this urea back into the bloodstream where it gets transported to different parts of the body, and ultimately much of it is excreted. Excess dietary protein fed above the dairy cow’s protein requirements with the imbalance being wasted rumen-soluble protein will result in high blood urea nitrogen (BUN) levels, which are associated with reproductive problems.
Unused ruminal ammonia has been linked to lower conception rates in dairy herds that record high BUN values. Barry Harris, a retired professor from the Dairy Science Dept., University of Florida, compared eight well-researched dairy studies on the subject. He reported that lactating dairy cattle fed excessively high- (19-21 per cent) protein diets resulted in raised BUN concentrations and lower conception rate percentages compared to cows consuming more standard (15-16 per cent) protein rations. He concluded although the BUN concentrations are not linear with conception rates, high BUN values do indicate potential reproductive problems. He stresses the importance of feeding the right amount and protein types, as well as balancing non-fibre carbohydrates, effective fibre levels and other nutrients.
VITAMINS AND MINERALS
Other nutrients that generate a lot of interest in the study of dairy cow reproduction are minerals and vitamins. Although, it is believed a variety of macro and trace minerals and vitamins play an essential role in good dairy reproduction, the main interest in good dairy cow fertility often centres around copper, zinc, manganese, and the unique selenium-vitamin E interactive relationship. Breeding dairy cows which do not obtain a sufficient level of these minerals, or are somehow bound by some antagonistic compound, may cause subtle deficiency symptoms such as “silent heats” (aestrus), failure to conceive, infertility, early embryonic death or even late-term fetal abortion.
Similarly, researchers have proven that trace minerals (including vitamins) play an essential role in follicular development, ovulation, and ovum survival as well as are facilitators in fertilization, uterine health, and embryo survival. Specifically, they have demonstrated feeding elevated levels of nutrients such as vitamin E and selenium in a close-up cow diet is an effective means of preventing many common post-calving problems such as retained placentas, and uterine infections.
It’s not exactly known on how essential trace minerals and vitamins encourage good reproduction, but it is known that trace minerals such as copper, manganese, zinc, and selenium are part of enzyme systems involved with good cell integrity.
For example, selenium is an activator of the enzyme glutathione peroxidase, which destroys “free radicals” naturally produced in the body and if left unchecked will respectively damage healthy reproductive cells. Vitamin E plays a more physical role by guarding the cell membrane surface from the same free radical compounds. Feeding more trace elements above the dietary requirements does not stimulate significant improvements in dairy cow fertility, however.
Most people agree adequate minerals and vitamins, as well as the other nutrients mentioned earlier, are needed for good reproduction. However, there is a knowledge gap because the science of good reproduction in dairy cows does not exactly follow the same nutritional laws as those for cow body maintenance and subsequent milk production. It may be time for more research in order to design better reproductive nutrient concepts that can one day be applied to hungry dairy cows on the milkline. †