Digestive and metabolic adaptations of ruminants to undernutrition, and consequences on reproduction.

In response to undernutrition, short- (days) and medium-term (weeks) adaptations are more pronounced for splanchnic organs than for other tissues. For the latter, the long-term response involves a sequential mobilization (fat > muscle > bone) with relative priorities differing among anatomical sites. Among chemical components, the body lipids are extensively used (up to 80%) in underfed animals, while the range of protein utilization is limited (up to 15-20%). The decrease in energy expenditure during undernutrition is mostly due to a short- and medium-term decrease in feeding activity, and in the mass and activity of splanchnic tissues. In the long-term, energy expenditure and tissue masses decrease concomitantly, but there does not appear to be a significant decrease in expenditure per unit tissue weight. Nitrogen losses decrease in response to decline in nitrogen fluxes and also due to sparing by renal activities and sometimes by urea recycling. However, ruminants do not seem to be able to compensate for a low level of intake (below maintenance) by an increase in digestive efficiency. Numerous hormones (insulin, growth hormone, insulin-like growth factor I, catecholamines, thyroid hormones, cortisol, leptin, etc.) are involved in the changes during undernutrition of nutrient fluxes between tissues, either through control of synthesis and/or degradation in peripheral tissues (adipose tissue and muscle), or through hepatic conversions of substrates (gluconeogenesis, ureagenesis and ketogenesis), in order to maintain the constancy of the internal environment (homeostasis) and/or to sustain productive functions (teleophoresis). However, reproductive process may be blocked in underfed animals.