Impact of nutrition on fertility in dairy cattle, with particular reference to grass-based diets

Dairy cow fertility has declined in recent decades: cows take longer to return to oestrus, display poorer signs of oestrus, have greater early embryo loss, and may have poorer conception rates. The problem is considered, by many, to be largely nutritional, and, therefore, can be corrected through dietary adjustment. High quality pastured forages are acknowledged as highly digestible; however, they tend to be relatively low in nonstructural carbohydrates and high in rumen degradable protein, and the temporal supply may not adequately meet cow requirements at key times. Diet adjustment is, therefore, often recommended to overcome these limitations. However, the interactions between nutrition at a cow level and on farm reproduction outcomes are poorly defined. Experimental results are often contradictory and in vitro relationships between individual nutrients and oocyte and embryo development are not always represented in reproductive success and failure. The key nutritional factors implicated in reproductive failure in pasture-based systems are reviewed and their implications discussed.