Influences of nutrition and metabolism on reproduction of the female ruminant

Abstract

Beef cows and ewes grazing native pastures are exposed to cycles of undernutrition that reflect the seasonal variations of biomass production. In grazing dairy cows, the physiological undernutrition during early lactation due to increased demands for lactation and low dry matter intake is exacerbated by the need to get sufficient intake from pasture and the extra grazing energy costs. Undernutrition has profound impacts on reproduction by affecting multiple reproductive processes at different levels of the reproductive axis. The objective of this paper is to review the influence of undernutrition on reproductive events of the adult female ruminant, with emphasis on both grassland and mixed rain-fed grazing farming systems. The comparative endocrinology and reproductive biology among ewes, beef and dairy cows may provide a comprehensive knowledge of the metabolic and reproductive adaptation to feed restriction. Understanding the critical underlying physiological mechanisms by which nutrition affects reproduction is the base of focus feeding strategy to improve the reproductive performance of the female ruminant.

Keywords: metabolism; reproduction; ruminant..

Figure 1. Simplified model of metabolic influences on reproduction of the female adult ruminant. While undernutrition in grazing beef cows and ewes is the result of energy intake below their requirements associated with the seasonal variation in pasture mass, in grazing dairy cows the negative energy balance (NEB) is the consequence of the increased demands for lactation and the low dry matter intake exacerbated by the need to get sufficient intake from pasture and the extra grazing energy costs. Grazing management interacts with parity in the metabolic adaptation to restricted dry matter intake or NEB that also depends on body reserves (metabolic memory). Reproductive events are affected by how and when this adaptation takes place. The nutrient flux affects all tissues and nutrient partitioning is modulated by the endocrine signals. Peripheral tissues (liver, adipose tissue, pancreas and others) secrete signals that not only regulate metabolic pathways, but also inform of the energy status. Hormones interact with each other by regulating their synthesis/secretion to ensure a coordinated regulation of energy partitioning. A complex system in the hypothalamus senses nutrient restriction and regulates voluntary feed intake, nutrient partitioning and reproductive function accordingly. The reproductive axis will respond both hierarchically by regulation of hypothalamus-pituitary gland and at follicular level (i.e., hormone receptors of the metabolic hormones) sensing the metabolic status to overcome the first reproductive limitation in the adult female, which is the pospartum/seasonal anestrus. The endocrine signals have also direct effects on the reproductive tract where they may tip the balance towards pregnancy success or failure. Broken lines: decrease, i.e., negative feedback IGF-I/GH, which is diminished and contributes to higher GH concentrations. Solid lines: increase (positive effect).

Figure 2. Proposed model of reproductive responses to negative energy balance (NEB) and environment in dairy and beef cows and ewes. Although dairy cows usually present a more profound NEB due to lactation requirements than beef cows and ewes grazing pastures, the later may suffer a more prolonged restricted nutrient availability. Beef cows and ewes present low frequency of LH pulses (suckling or seasonal inhibition, respectively) and undernutrition prolongs pospartum or seasonal anestrus, respectively. Although severe NEB also prolongs pospartum anestrus in dairy cows, is comparatively shorter. Body reserves affect the length of pospartum (dairy and beef cows) and seasonal anestrus (ewes). Once ovulation has occurred, the sequential preparation of the oviducts and uterus by E2 and P4 sustains embryo growth. Clearance of E2 and P4 is affected by liver metabolic flux: in dairy cows the high hepatic flux (intake-lactation) is related to low steroid plasmatic concentrations, whereas in ewes undernutrition is associated with high steroid circulating concentrations (no data as such was found in beef cows). In both dairy cows and sheep, data indicate that uterine sensitivity to P4 (PR) is diminished in NEB, as well as uterine sensitivity to other metabolic hormones. These findings may explain the embryo losses. Embryo mortality seems to be more important in the high producing dairy cow than in beef cows and ewes. The schematic representation of follicular waves in dairy and beef cows has been taken from Crowe et al. (2014).