INTRODUCTION
Water, energy, protein, minerals and vitamins are required for normal reproduction. These nutrients are the same as those required for other body processes: maintenance, growth and milk production. The impact of cows’ nutritional status on their reproductive performance may affect their ability to:
- Conceive (initiate a new pregnancy);
- Provide the proper amount and balance of nutrients to support the growth of a normal fetus;
- Deliver a calf without complications (retained placenta, milk fever, etc.).
HEIFER GROWTH AND NUTRITION
The heifer’s weight, rather than her age, determines when puberty occurs and the heat cycle begins. The first signs of heat usually appear when heifers have reached about 40% of their mature body weight. In well-fed heifers, sexual maturity usually occurs at about 11 months of age. However, heat stress and underfeeding of calves and young heifers delays sexual maturity and prevents initiation of heat cycles.
In tropical regions, sexual maturity of heifers may not occur before 14 to 15 months of age. Heifers should weigh about 60% of their mature body weight at the time of insemination (14 to 15 months of age). Thus if cows average about 600 kg, heifers should weigh about 360 kg (600 x 60/100) at the time of insemination (Table 1).
COW NUTRITION
Nutrition and Pregnancy
Malnutrition during pregnancy may lead to:
- Premature birth, malformations and weak calves that result from maternal deficiencies in energy, protein, vitamins and minerals;
- Abortion, which is rarely due to poor nutrition, except in severe cases of deficiency, the ingestion of moldy feed, or when feed contains high levels of estrogen (a hormone).
Nutrition and Post-Calving Complications
Many complications at calving are related, at least in part, to nutritional imbalances:
- Fat cow syndrome is a condition resulting from excess energy during late lactation or during the dry period leading to obesity, loss of appetite and excess body mobilization in early lactation.
- Milk fever is due to the drain of calcium from the blood to the milk the first days after calving. This condition is caused, in part, by excess calcium or an imbalance between calcium and phosphorus in the diet. Paralysis and death may occur if not treated immediately.
- Displaced abomasum is a condition in which the abomasum is twisted to the left or right of its normal position. The primary cause of this problem may be an excess of concentrate (lack of fiber) in the diet in association with the increased space in the abdominal cavity after calving.
- Ketosis is a metabolic disorder occurring in cows with insufficient or excess body reserve at calving—cows lose their appetites, and milk production and fertility decrease.
Cows suffering from milk fever and fat cow syndrome also have a much greater chance of suffering from retained placenta, metritis, dystocia and reduced conception.
Lactation versus Conception
In early lactation, milk production has the highest priority for the available nutrients. In addition to the nutrients found in the diet, cows tend to mobilize their body reserve (primarily energy) to support milk production. Cows cannot eat sufficient amounts during early lactation; thus they are in a state of energy deficiency, they lose weight and their ability to conceive is drastically reduced. It is only at a later stage of lactation, when the energy ingested is in balance with the energy required for milk production, that the ability to initiate a new pregnancy increases.
Energy Balance and Fertility
One of the most common causes of low fertility in dairy cows is the deficiency of energy relative to the animal’s need, or negative energy balance. Depending on milk production in early lactation, negative energy balance may last for the first two to 10 weeks of lactation (two and a half months). The effect of body weight change during the month of conception (two to three months after calving) is illustrated in Table 2.
Conception rates are lower for cows inseminated during negative energy balance (cows losing weight) compared to cows inseminated during positive energy balance (cows gaining weight). There is no evidence that higher producing cows have inherently lower reproductive ability. However, it is clear that cows with a negative energy balance have low fertility regardless of milk producing ability.
Protein and Fertility
The effect of dietary protein on reproduction is complex. In general, inadequate amounts of protein in the diet reduce milk yield and reproductive performance. Excess protein may also have a negative effect on reproduction. Sometimes, however, higher amounts of protein in the diet are associated with higher fertility.
Some of the following effects have been demonstrated to explain the poor reproduction sometimes observed with excessive levels of protein in the diet:
- High levels of blood urea may occur, which has a toxic effect on the sperm, the ova, and the developing embryo;
- The balance of hormones may be altered—progesterone levels are low when the blood contains high levels of urea.
In the early lactating cow, high levels of protein may exacerbate the negative energy balance and delay the return of normal ovarian function.Feeding protein and urea, such that early lactating cows have a diet containing 16% protein and late lactating cows have a diet containing 12% protein, should optimize the fertility of the cows.
Minerals, Vitamins and Fertility
Minerals and vitamins play an important role in reproduction. The effects of severe deficiencies are usually well understood. However, it is difficult to establish possible effects of long term marginal deficiencies or excesses. In addition, there are many interactions between minerals, especially the microminerals. In general, almost all required vitamins and minerals (except iron) have either a direct or indirect effect on the cow’s fertility (Table 3) and the cow’s ability to give birth to a healthy calf (Table 4).
Phosphorus deficiency may greatly delay sexual maturity in heifers and decrease the fertility of dairy cows. A deficiency or excess of either calcium or phosphorus in the diet may lead to milk fever at calving. A calcium-to-phosphorus ratio of from 1.5:1 to 2.5:1 is desirable. However, a ration should always be balanced for the amount of calcium and phosphorus that is required rather than the calcium-to phosphorus ratio.
Genetic Selection and Reproduction
Among breeds of dairy cows, the intense selection for higher milk production during the last 20 to 30 years has accentuated the problem of negative energy balance in early lactation. Consequently, as milk production has increased, reproductive efficiency has decreased. Although a conception rate of 50% is considered to be a poor level of reproductive performance today, it is probably above average in the U. S. dairy industry.
Some believe that the decrease in reproductive efficiency is due to genetic selection for milk production. However, research indicates that the conception rate of heifers has remained unchanged for the last 25 years, suggesting that genetic selection for higher milk production is not the cause of lower fertility. The heritability of reproductive traits, such as days open, is very low.
Thus improving reproduction through selection would be very inefficient. It is likely that cows selected for higher milk yield were also selected (indirect selection) for their ability to mobilize body reserve and to ingest more feed. Cows with higher intake in early lactation are likely to have fewer reproductive problems than cows that mobilize large amounts of body reserves. Thus it is probable that selection of cows with higher intake capacity in early lactation may allow for higher milk production with minimal negative effects on reproduction
