Motivation to obtain a food reward of pregnant ewes in negative energy balance: Behavioural, metabolic and endocrine considerations

Low food availability often coincides with pregnancy in grazing animals. This study investigated how chronic reductions in food intake affected feeding motivation, and metabolic and endocrine parameters in pregnant sheep, which might be indicative of compromised welfare. Ewes with an initial Body Condition Score of 2.7 ± 0.3 (BCS; 0 indicates emaciation and 5 obesity) were fed to attain low (LBC 2.0 ± 0.0,), medium (MBC 2.9 ± 0.1) or high BCS (HBC 3.7 ± 0.1) in the first trimester of pregnancy. A feeding motivation test in which sheep were required to walk a set distance for a palatable food reward was conducted in the second trimester. LBC and MBC ewes consumed more rewards (P = 0.001) and displayed a higher expenditure (P = 0.02) than HBC ewes, LBC ewes also tended to consume more rewards than MBC ewes (P = 0.09). Plasma leptin and glucose concentrations were inversely correlated to expenditure (both P < 0.05) and appear to be associated with hunger in sheep. LBC ewes were in negative energy balance, with lower muscle dimensions, plasma glucose, leptin, insulin, cortisol, and insulin-like growth factor-1 concentrations and higher free fatty acids concentrations compared to HBC ewes; metabolic and endocrine parameters of the MBC ewes were intermediate. The high feeding motivation and negative energy balance of low BCS ewes suggested an increased risk of compromised welfare. Imposing even a small cost on a food reward reduced motivation substantially in high BCS ewes (despite high intake when food was freely available). Assessment of a willingness to work for rewards, combined with measures of key metabolic and endocrine parameters, may provide sensitive barometers of welfare in energetically-taxed animals.

► Under-nourished pregnant ewes show an increased motivation to walk for a food reward.
► Imposing even a small cost on a food reward reduces motivation in overweight ewes.
► Plasma leptin and glucose concentrations are associated with feeding motivation.
► Feeding motivation combined with physiological parameters are sensitive barometers of welfare.