Fertility problems and nutrition
Nutrition can have remarkable influences on fertility. Inadequate energy intake and/or inadequate body fatness early postpartum can retard resumption of ovulatory cycles, which extends the interval from calving to first estrus, reduces conception rates at first postpartum AI and increases late embryonic losses after first postpartum AI.
In addition to energy status, there are some well-characterized effects of nutrition on fertility. Nutritional deficiencies, such as antioxidant nutrients (vitamin E and Se), influence immunity status of the uterus and the ability of white blood cells to kill bacteria that invades the postpartum uterus (see sidebar at right). “This makes the cow more likely to develop retained fetal membranes and metritis, both of which reduce fertility,” explains Santos.
Another important example is nutritional management during the transition period to prevent subclinical and clinical hypocalcemia. Hypocalcemia can be minimized by manipulating the mineral composition of the prepartum diet (feeding low Na and K diets and including acidogenic salts). Hypocalcemia results in increased blood cortisol concentrations after calving, impairs uterine involution, is associated with increased uterine infections and reduces fertility.
On the other hand, excess of nutrients or dietary components containing toxins can have negative effects on fertility. One example is a diet too high in protein. “In order to get milk production of 25,000 pounds per lactation, some farms formulate diets too high in crude protein, such as 19 to 20 percent,” says Melendez. But he adds that high protein levels lead to high levels of urea and ammonia in the blood, and studies have demonstrated that high levels of urea will affect the uterine environment and can introduce toxic effects to gametes and the embryo.
High-producing lactating dairy cows do not require diets with 19-20 percent crude protein to achieve productions above 26,000 pounds, says Santos. “What these cows really need is a diet that is balanced for the energy and protein components and provides adequate amounts of metabolizable protein with an adequate amino acid profile.”
Jose Santos, DVM, PhD, says ketosis and associated diseases have been negatively associated with conception rates.
Photo credit: Jose Santos, DVM, PhD
However, producers and nutritionists often conclude that cows need very high crude protein diets. “In fact, well-balanced diets that contain 17 to 18 percent crude protein are enough to provide all the amino acid needs of high-producing cows even in early lactation,” explains Santos. “The problem is that high-producing cows that consume a lot of feed will also consume a lot of protein, although the diet is presumptively balanced. Even in those cases, urea nitrogen in the blood will be high as a consequence of the high protein intake.”
Feeding excessive amounts of gossypol is another example, adds Santos. “We have shown that feeding cottonseed that results in plasma gossypol concentrations above 5 micograms/mL reduces conception rates and increases fetal losses, and this is, in part, mediated by reduced embryonic development.”
When pregnancy gets beyond 25 days, the cow is fairly adept at protecting the conceptus from nutrition-related problems, but even still factors, such as excess copper or iodine, toxic plants, mycotoxins or fungal-infected silage, can produce abortions.
Feeding heifers versus cows
There’s a vast difference in how heifers utilize and respond to nutritional factors than cows because heifers are still growing, even when calving at 24 months. Because of that, heifers need a diet rich in protein but with moderate energy to avoid getting too fat and risking dystocia when calving.
Additionally, the pre-calving cow differs from the heifer in that because she’s not still growing, in the transition period she’s more likely to develop hypocalcemia. “Those animals can be fed acidogenic salts to prevent hypocalcemia,” says Melendez. “Heifers are less likely to develop hypocalcemia and don’t need acidogenic salts. The take-home message is that there should be two different rations – one for heifers and one for cows.” Melendez adds that first-lactation heifers should continue to be fed in a separate group until the second calving when they become mature adults.
Santos agrees that heifers prior to calving should be fed differently than cows. In the last three weeks prepartum, heifers have lower dry matter intake relative to their body weight than cows do. Because they are still growing, their requirements for metabolizable protein is greater and, as opposed to cows, increasing the concentration of crude protein in close-up rations above 13 percent benefits postpartum performance.
“The key here is to feed diets that provide adequate metabolizable protein according to the expected intake of the animal,” says Santos. Furthermore, prepartum heifers should not be fed acidogenic salts to prevent hypocalcemia as cows are because heifers do not experience this problem and those salts reduce feed intake and might impair the secretion/activity of some regulatory hormones, such as insulin, that are important during transition. “Whenever possible, cows and heifers should be fed separately pre- and postpartum, and prepartum, they should be fed diets that are tailored to their nutrient needs.”
Cows that are obese at calving with body scores of 3.5 to 4 or more can have more problems with retained fetal membranes and ketosis, which impact cyclicity.