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.

Condition‐dependent maternal sex allocation in horses can be demonstrated using a biologically relevant, multivariable condition measurement

“…(a) The percentage (Mean, 95% CI) of males born by New Zealand Thoroughbred mares mated <15 days (FCI group 1), 15–35 days (FCI group 2), >35 days (FCI group 3) post‐partum in either August (Red), September (Blue), October (Green) or November (Purple) and (b) the estimated energy balance during 0–100 days of lactation in New Zealand Thoroughbred mares gave birth on either 0 (Red), 15 (Blue), 50 (Green), 90 (Purple) days after 1 Sep. (b) Adapted from Chin et al. (2020). …”

“…Foaling to conception interval indicates the stage of lactation when conception occurred. The energy balance data of Thoroughbred mares were adapted from Chin et al (2020) and overlaid with the sex ratio of lactating mares in this study.…”

“…The energy balance data of Thoroughbred mares were adapted from Chin et al. (2020) and overlaid with the sex ratio of lactating mares in this study.…”

“…The commercial breeding season is short (September-November) and is displaced from the natural breeding season due to the commercial pressure to produce foals close to 1 Aug (universal birthdate of Thoroughbred racehorses in Southern Hemisphere) (Chin et al, 2021a(Chin et al, , 2021bGee et al, 2017). Under pastoral management, the feed availability and environment conditions can be different for mares mated earlier versus later in the season (Chin, 2018).…”

“…Thoroughbred breeders in New Zealand aim to have foals born as close to the official birth date of August 01. To achieve this, breeders will attempt to mate mares as early in the breeding season as possible, which differs from the natural, or evolutionary programmed, reproductive pattern of the horse to have reproduction coincide with summer solstice (increased day length/photoperiod) when pasture/feed supply would be at its greatest (Chin et al., 2020; Williams et al., 2012). Therefore, the date of last service (conception date) provides a measure of the deviation of conception from the biological ideal conception date near summer solstice.…”