Developmental Programming of Fertility in Cattle-Is It a Cause for Concern?

Abstract

Simple SummaryPoor fertility is the main reason for premature culling of cattle. Dam factors and external variables can both influence how the calf fetus develops, known as fetal programming. Dam factors include age, parity, body condition, health and milk yield. External variables include nutrition and the environment during pregnancy. These all affect placental growth and the nutrient supply to the fetus, which in turn influence the size, shape and body composition of the calf. Postnatal growth rates, organ structure and immunity can all be affected. The impact on organs such as ovaries, liver, pancreas, lungs, spleen and thymus is dependent on the stage of pregnancy during which the fetal environment is sub-optimal. Regulatory systems which influence growth, metabolism and fertility can be permanently reprogrammed. Most changes affecting fertility are probably indirect. For example, calf health, the timing of puberty, age and size at first calving, and the ability to adapt to lactation after calving can all alter future reproductive potential. The size of these effects is hard to quantify due to practical difficulties in obtaining data over a sufficient time period. Nevertheless, there is compelling evidence that the fertility of some cows is compromised by events happening before their birth.Cattle fertility remains sub-optimal despite recent improvements in genetic selection. The extent to which an individual heifer fulfils her genetic potential can be influenced by fetal programming during pregnancy. This paper reviews the evidence that a dam’s age, milk yield, health, nutrition and environment during pregnancy may programme permanent structural and physiological modifications in the fetus. These can alter the morphology and body composition of the calf, postnatal growth rates, organ structure, metabolic function, endocrine function and immunity. Potentially important organs which can be affected include the ovaries, liver, pancreas, lungs, spleen and thymus. Insulin/glucose homeostasis, the somatotropic axis and the hypothalamo-pituitary-adrenal axis can all be permanently reprogrammed by the pre-natal environment. These changes may act directly at the level of the ovary to influence fertility, but most actions are indirect. For example, calf health, the timing of puberty, the age and body structure at first calving, and the ability to balance milk production with metabolic health and fertility after calving can all have an impact on reproductive potential. Definitive experiments to quantify the extent to which any of these effects do alter fertility are particularly challenging in cattle, as individual animals and their management are both very variable and lifetime fertility takes many years to assess. Nevertheless, the evidence is compelling that the fertility of some animals is compromised by events happening before they are born. Calf phenotype at birth and their conception data as a nulliparous heifer should therefore both be assessed to avoid such animals being used as herd replacements.