THE EFFECTS OF LACTATION ON THE FERTILITY OF DAIRY-COWS

Lactation has been negatively associated with fertility because pregna ncy rates in maiden heifers exceed those obtained after first or subse quent calvings. The extent of this difference is less in pasture-fed d airy cows (< 10%) than in American Holsteins (> 20%) fed grain and con served forages. The latter cows have pregnancy rates to first insemina tion and oestrus detection rates of only 40 to 45%. This suggests that the subsequent fertility of inherently fertile Holstein heifers may b e severely compromised by high levels of milk production. Internationa l comparisons show that pasture-fed dairy cows may experience extended periods of anovulatory anoestrum but have normal fertility (60% pregn ancy rate to first insemination) once cycling, The high-producing Amer ican Holstein may ovulate within 4 weeks postpartum but is more likely to continue ovulating without being detected in oestrus. Both situati ons are associated with negative energy balances (NEB) during early la ctation. The severity and duration of this NEB may vary with body cond ition at calving, age or parity, ration formulation, production level and environmental factors. Relative daily milk yield is not an absolut e indicator of NEB, because some lower producing cows within a herd ha ve lower feed intakes and more severe energy deficits. NEB is not simp le to measure; nonetheless, it is correlated with genetic improvement for milk yield. A positive energy balance, greater weight gain and hig her body condition score have all been shown to be positively correlat ed with plasma progesterone concentrations in early lactation. No stud ies have investigated the possibility that the rapid increase in metab olic rate at this time may also alter steroid concentrations with cons equent effects on oestrous behaviour and fertility. Studies to more pr ecisely define the effects of increasing milk yields in early lactatio n, especially in Holsteins, may need to be completed in Australia and New Zealand. Oestrus detection rates and pregnancy rates for American Holsteins of less than 50% are accepted widely in the USA. Such low de tection rates confound studies on fertility. The objective should be t o increase these 2 rates to at least 80% and 60% respectively. This ma y involve the use of controlled breeding, especially if oestrous behav iour is less overt in high-producing Holstein cows.