The effects of food restriction on the perception of hunger, metabolic state and stress responsiveness in sheep

Abstract

Sheep in temperate regions may be exposed to periods of limited food availability and a range of environmental conditions. Insufficient dietary intake may change live weight (LW) and body condition score (BCS) resulting in an increase in hunger and alterations in metabolic state, which may influence the welfare of sheep. In Chapter 2, a behavioural demand methodology to measure feeding motivation was refined and improved; five different walking distances (costs) were used and the number of rewards consumed, the total distance walked, Pmax (maximum price) and Omax (maximum expenditure) were used as measures of motivation. Sixteen non-pregnant ewes were subjected to two feeding levels (24 h fasted and ad libitum fed control); fasted ewes consumed more rewards and walked a greater total distance compared to control ewes. The methodology provided good measures of feeding motivation and was a potential indicator of hunger. However, the range of costs used was too narrow for accurate assessment of Pmax and Omax. In Chapter 3, the effects of body condition score (BCS) on feeding motivation and endocrine and metabolic responses in twin-bearing ewes were investigated to identify and quantify indicators of hunger and to assess the metabolic state of twin-bearing ewes with different BCS. Twenty-two ewes were divided into low BCS (LBC), medium BCS (MBC) and high BCS (HBC) treatments. Feeding motivation was assessed between day 91 and 105 of pregnancy and blood samples were collected once every 2 weeks between day 35 and 133 of pregnancy. The number of rewards consumed and Omax were significantly lower in the HBC ewes compared to LBC and MBC ewes, while the MBC ewes tended to show a lower motivation compared to the LBC ewes. LBC ewes were in a metabolically catabolic state and had low plasma concentrations of leptin, insulin and insulin-like growth factor-1. In Chapter 4, it was investigated whether a fast loss in BCS would increase hunger and alter the metabolic state more compared to a slow loss in BCS. Twenty-six twin-bearing ewes were assigned to Slow Loss (1 BCS loss in 13 weeks, SL), Fast loss (1 BCS loss in 10 weeks, FL) or Control (maintain BCS) treatments. Feeding motivation was assessed between day 42 and 67 of pregnancy and blood samples were collected once every 2 weeks between day 35 and 140 of pregnancy. SL and FL ewes had not lost much BCS at the start of the motivation period and no effects on feeding motivation were found. SL ewes were in a similar metabolic state as Control ewes. However, FL ewes were in a catabolic state in mid-pregnancy. In Chapter 5, it was investigated whether BCS affects the ability to maintain energy homeostasis and induce adaptive metabolic and stress responses to an environmental challenge. Eighteen shorn single-bearing ewes were divided into low, medium and high BCS treatments. Ewes were exposed to a 6 h acute cold challenge between days 85-87 of pregnancy. LBC ewes were able to maintain body temperature, but were slower to mobilise energy substrates and had a delayed and reduced cortisol response compared to MBC and HBC ewes. In conclusion, feeding motivation appeared to be an indicator of hunger in ewes; fasting and a loss in BCS markedly increased hunger. A loss in BCS induced a metabolically catabolic state. However, ewes were metabolically better able to adapt to a slow than a fast loss in BCS. Finally, low BCS ewes may have a reduced ability to respond to acute stressors.