A review on the effects of part-time grazing herbaceous pastures on feeding behaviour and intake of cattle, sheep and horses

Part-time grazing (PTG) is the grazing technique based on the time-restricted access to pasture of farmed herbivores, usually supplemented indoors. This review evaluates the effects of the duration of access to pasture on the functional responses of grazing time and herbage intake rate in cattle, sheep and horses and the implications of these responses on diet selection, diet digestibility, energy expenditure, animal welfare, the performance of ruminants and the quality of their products (milk and meat). Ruminants with restricted access time to pasture display compensatory behaviour through increased intake rate, achieving similar levels of intake and performance compared with 24 h-grazing ruminants, particularly if access time is in the range 6-8 h/d. This can depend on the reduction of locomotion energy expenditure, and, sometimes, on the selection of a better quality diet than that on offer. Nevertheless, due to lower ingestive fibre trituration, fibre digestibility could be reduced, particularly with access time <4 h/d. Moreover, milk content of FA regarded as beneficial for consumers'health, such as n-3 PUFA and rumenic acid, is usually higher in PTG than stall-fed ruminants, with a minimum access to grass pasture of 6 h/d in cows supplemented with total mixed rations or 4 h/d in sheep supplemented with concentrate and hay. Timing the grazing session of ruminants in the afternoon and evening hours is a good strategy to match pasture quality and animal attitude to forage intensively and efficiently, favouring intake, performance and produce quality. Horses show on average lower intake rates scaled to metabolic weight than ruminants, probably due to their lower energy requirements but also for the need to spend part of the time outdoor performing physical activity and social behavior. Therefore, they probably need longer access time than ruminants. However, access should be time restricted or avoided during periods of the year and day hours (from midday to evening) when herbage content of non structural carbohydrates (sum of starch and water soluble carbohydrates) is high (> 15 % DM) since it can be conducive to equine metabolic syndrome and laminitis. In general, PTG can improve ruminant and horse welfare as compared with stall-feeding with reference to appropriate behavior and freedom from some pathologies, although further research is needed to quantify these effects on a wider range of animal species and welfare indicators. To conclude, PTG in ruminants and horse offers some benefits when properly managed, compared to 24 h-grazing and stall-feeding.