Nutrition is key for transition cows
Nutritional management of cows during the transition period can have substantial effects on reproductive success. This was concluded from a large literature review.
Poor reproductive performance of lactating dairy cattle is a complex disorder that reflects associations with intensification of production and increased milk production. However, it is difficult to determine a causal basis for the decrease in fertility, as genetics and environment have changed markedly over the last decades. Nutritional influences on fertility have been examined and frequently reviewed, but difficulties and inconsistencies in study design occur. Studies must have large numbers of experimental units to identify biologically and economically important differences in proportion of cows pregnant. Nutritional influences during the transition period may be of particular importance, but it is clear that the effect of diet on fertility during this period is complex and multifactorial. The objective of the current study was to use carefully described dietary information from the available literature to explore the effects of the diet during the transition period on measures of pregnancy and calving to pregnancy interval as well as identifying factors that may explain variation in these responses. This research looked at 118 diets contained within 39 experiments to see what the effects on nutritional interventions fed during the early postpartum period.
Higher milk output, lower fertility
The extensive literature has observed that cows with greater milk production generally have poorer fertility, and that genetic selection for increased production can reduce fertility. Whereas genetic differences were not examined in this data set, the researchers identified associations between increased milk fat (kg/d) and protein production (kg/d) with reductions in the proportion pregnant, actual milk yield (kg/d), and milk protein yield (100 g/d) with longer calving to pregnancy interval. They also found that protein yield in very early lactation (first 3 weeks of lactation) was positively associated with the proportion of first services that resulted in pregnancy, and others identified positive associations between milk protein percentage and improved reproductive performance. These findings, overall, highlight possible differences between experiments conducted at the level of the individual and those at the group level. As milk protein production increases in a group of cows, it may be expected that nutrient intakes will need to be more closely aligned with nutrient losses, whereas the individual within the herd with greater production may have better phenotypic adaptation to the environment allowing greater milk protein yield and percentage.
Focus on nutrient balance and BCS
The availability of nutrients that can be allocated to reproduction is not just determined by immediate diet (i.e. the immediate intake of nutrients as dry matter intake (DMI)), but also by endogenous body tissue reserves, reflected in labile body weight (BW) and body condition score (BCS). Hence, DMI before and after calving is a key determinant of exogenous nutrient availability. The irreversible loss of nutrients in milk production and use of nutrients for maintenance and growth diminishes the nutrient pool available for reproduction. The difference between dietary intake and expenditure determines the nutrient balance, and if a negative balance occurs endogenous reserves are depleted. Many studies have examined the effects of estimated negative energy balance on fertility. The length and severity of a negative energy balance at the onset of lactation is largely determined by DMI around calving and milk yield. Estimated energy balance (MJ/d) was, as anticipated, positively associated with improved proportion pregnant and shorter calving to pregnancy interval. A better energy balance during the first 3 to 4 weeks of lactation reduces the interval to first ovulation and increases the probability of pregnancy at the following breeding. Excessively low or high BCS at calving, or extreme losses of BW or BCS in early lactation, are usually associated with impaired reproductive outcomes.
This article is a short summary of the original paper: Effects of nutrition on the fertility of lactating dairy cattle, by R.M. Rodney, P. Celi, K. Breinhild, J.E.P. Santos and I.J. Lean, published in the Journal of Dairy Science, Volume 101, Issue 6.