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Sub-fertility in high-producing dairy cows is a known problem in most countries, also in South Africa. The question is do we understand the causes and can farmers limit the consequences? The status of current knowledge and understanding has been reviewed recently by L.M. Chagas and co-authors in a paper published in the Journal of Dairy Science (2007), Volume 90, pages 4022-4032; the title being: New perspectives on the roles of nutrition and metabolic priorities in the sub-fertility of high-producing dairy cows.
Reproductive performance world-wide has declined in modern dairy production systems concurrently with strong genetic progress for high milk yield, and the problem occurs in both total mixed ration (TMR) and pasture-based systems. This suggests that reduced fertility is not caused simply by changes in management but also by changes in the genetic make-up of the cow and the associated metabolic processes that result during the course of selection progress toward higher levels of milk production. The genetic change, however, is not the direct cause of low fertility, but rather the demands imposed by lactation which interact with the genetic make-up of the cow to result in a major negative effect on the reproductive system. These demands require high levels of energy and nutrient intake and they introduce altered patterns of metabolism, the outcomes of which appears to be associated with the reduced fertility dairy farmers experience in their herds.
Reduced fertility is related to a number of health, physiological and management factors, themselves the result of modifications to physiological processes such as glucose manufacturing, hormonal interferences and appetite control. The modifications have come about because they provide some advantage to milk production, but they also prove detrimental to reproduction. One of these is because in the high-producing cow a major portion of the feed energy and nutrients as well as body reserves during critical phases of the lactation cycle, are shifted toward milk production at the expense of reproduction. This implies that even with higher feed intake, the outcome is simply an increase in milk yield without necessarily an improvement in reproductive performance.
To improve reproduction while maintaining high milk yield, nutritional strategies are needed that stimulate reproductive processes without compromising the shifting of energy and nutrients destined for milk production. These strategies will differ, depending on the genetic make-up of the cow, production and management system, and how these interact. To be successful scientists in research and farmers and their supporting advisors in management strategies must try to increase the chance of a successful pregnancy, the influencing stages and processes being after-calving immune function, uterine health, ovarian function, oestrus detection, ovulation and care to maintain pregnancy and embryonic viability. To find solutions to the problem of reduced fertility scientists, farmers and advisors will have to integrate the disciplines of cow management, nutrition, immunology, molecular biology and physiology.
A management tool that should be used more meticulously is body condition score (BCS). It is well-known that the less than optimum feed intake by the cow after calving is associated with body reserves that are mobilized to support milk production. Therefore, the relationship between BCS at calving and loss of BCS in early lactation is very strong. It is also well-known that energy balance of the cow is highly correlated with reproductive success. Therefore, management of body reserves is critical for reproductive success and requires an accurate assessment of the body condition of the cow. BCS is easy to measure and can be a valuable asset to the experienced farmer in his management program. However, ideal BCS profiles (together with other measures) have still to be developed for key phases where BCS and BCS change are known to affect indices of fertility, namely BCS at calving, BCS between calving and the lowest level of negative energy balance, BCS and breeding, and BCS or BCS change after the point of lowest level of negative energy balance.
Although crucial, energy balance is not the sole nutritional-related factor that affects reproduction. Specific nutrients that act independently of energy balance have been reported to directly or indirectly alter reproductive efficiency and fertility. Relationships have been found for protein, starch, monopropylene glycol, certain minerals and trace elements, and specific lipids. The modes of action, however, are not well-understood and it is unlikely that any of these specific dietary components will provide simple solutions to the problem of subfertility. Further research is required to improve the understanding of their individual effects and their interaction with others in their association with reproductive success. The same applies to the complex integration of endocrine/hormonal and metabolic processes controlling metabolism and reproduction. In the latter instance, although ovarian and other reproductive functions are primarily controlled by brain-induced hormonal influences, there is substantial evidence that the ovary can respond directly to metabolic signals, particularly those affected by high milk production in the early after-calving period.
Closing remarks: Reduced fertility of the high-producing dairy cow is, in part, the result of intensive selection for milk production traits and intensification of management systems with little consideration for the associating impact on reproduction. The consequence to some extent is conflict between milk production and fertility. This conflict is by no means inevitable since traits for fertility can be included successfully in selection indices and this should become standard practice. The authors is of the opinion that it may also be feasible to select for more persistent lactation or smaller after-calving changes in BCS to improve fertility. Overall though fertility is multi-trait controlled with metabolic and hormonal tissue targets and signals which will require decades to solve.