The dynamics of energy supply and demand of foaling mares managed at pasture: a deterministic modelling approach

Abstract

Context. At pasture, the nutritional status of mares will change due to a dynamic association between their energy requirements and the energy supply. Change in the plane of nutrition can influence reproductive performance measures in the mares. To help optimise nutritional and operational management of mares on stud farms, the nutritional status of Thoroughbred broodmares and the factors influencing the energy supply and demand were investigated. Aims. To assess nutritional status of the Thoroughbred broodmare herds under ‘normal’ New Zealand commercial stud-farm conditions. Methods. The energy intake, energy requirement and energy balance during the last 3 months of pregnancy and the first 5 months of lactation were modelled for mares with an initial bodyweight of 450 kg, 500 kg, 550 kg and 600 kg, and foaled at 0, 15, 50, and 90 days after 1 September. Key results. For all foaling dates and bodyweights, mares were in positive energy balance during the last 3 months of pregnancy (6.1–8.5%). Energy balance decreased as pregnancy progressed, followed by a large and acute energy deficit that was initiated soon after foaling and continued during lactation. The energy deficit during lactation varied between –8.6% and –12.4%, depending on the foaling date modelled. Mares foaling later in the season (50–90 days after 1 September) had greater and longer (30–50 days) postpartum energy deficit than did mares that foaled earlier in the season, who recovered within 20 days postpartum. The modelled changes in the energy balance would be large enough to initiate mobilisation of the fat reserve (body condition), and could, therefore, explain the observation of delayed postpartum to conception interval observed with later-foaling mares on commercial farms. Conclusions. There is a large and prolonged energy deficit soon after foaling and throughout 150 days of lactation in Thoroughbred mares managed under New Zealand commercial grazing conditions. Foaling later in the season can cause a mismatch in energy supply and demand that would increase and prolong the energy deficit. Implications. These findings suggest a need for stud farms to actively manage the pasture supply and monitor the mare’s BWT and BCS changes in an attempt to optimise the mare’s nutritional status and reproductive performance.