9th International Deer Biology Congress

Impacts of Nutrition on Growth and Reproduction in Female Red Deer: Phenotypic Flexibility within a Photoperiod-mediated Seasonal Cycle
Geoff Asher, AgResearch Ltd, Invermay Agricultural Centre, Mosgiel, New Zealand

Red deer (Cervus elaphus) are widely distributed throughout cold northern temperate latitudes, where they have evolved to cope within highly seasonal environments generally characterised by warm summers and cold winters. IN addition to pronounced seasonality these regions are also characterised by substantial annual variation in climate that strongly influences seasonal feed supply (Clutton-Brock et al 1989). Naturalisation of this red deer to the more moderate seasonal (but  variable climatic) environment of New Zealand has been spectacularly successful, and they are widely farmed in the country’s pastoral environment for venison and antlers. Red deer are genetically programmed to exhibit photoperiodic control of voluntary feed intake (VFI), growth and reproduction, ensuring that energy demands are aligned with seasonally available resources and offspring are born in summer when climate is favourable for survival (Lincoln and Short 1980). However, despite genetic control of their endogenous seasonal cycles, phenotypic variation of a number of seasonal traits may have contributed to their successful naturalisation to a wider range of seasonal environments than would be expected within their ancestral range. While precise timing of conception and duration of gestation length are the two fundamental mechanisms by which the strict seasonality of birth is maintained in seasonally breeding mammals, red deer exhibit considerable variation in both these traits.

Impacts of lactation on conception date are well described for red deer. Energy expenditure on lactation over summer months can reduce both body mass and body fat reserves by the start of the breeding season in autumn, as often measured on farmed hinds prior to the mating season by assessment of standardised 5-point ‘body condition score’ (BCS) system. While lactation per see does not appear to influence ovulatory activity (i.e. no lactational anoestrous), body fat reserves impact strongly on the timing of the initiation of ovulatory activity, accounting for phenotypic variation in conception date of up to 12 days (Pollard et al. 2002). Typically, hinds of very low BCS (<2) will remain anovulatory, while those of moderate BCS (2-3) will exhibit delayed conceptions. Removal (weaning) of calves 3-4 weeks before the expected mating season is often associated with earlier conception date in hinds facilitated through rapid increase in BCS following cessation of lactation (Pollard et al.2002).

More recently, research has focussed on the influence of BCS at the time of parturition on the ability of the red deer hind to undertake successful lactation, and the downstream impacts on BCS (and hence, conception date) around the time of mating. The study demonstrated that hinds entering lactation at a higher BCS maintained improved lactation outputs (as measured from offspring growth performance) and were better able to buffer against their own body mass (BCS) loss under experimentally constrained nutritional conditions over summer lactation. This impacted on subsequent conception timing and/or management intervention required to improve BCS in time for mating (Stevens et al., 2017).

Recent studies have demonstrated that gestation length in red deer (which ~233 days) can be highly variable and influenced by various environmental factors. In one study, red deer hinds between days 150 and 220 of gestation, which were individually penned indoors under a range of nutritional allowances, expressed unexpectedly wide variation (27-day range) in gestation length, with low-nutrition (50% diet restriction) hinds exhibiting longer gestations (>240 days) than hinds on ad libitum diet (<230 days), but there was no effect of level of nutrition on calf birth weight (Asher et al. 2005). It was concluded from this study that variable nutrition to hinds during late pregnancy may strongly influence fetal development, but under conditions of under-nutrition, variation in gestation length compensates to ensure optimisation of birth weight (and, hence, calf survival). Subsequent studies have shown an apparent effect of conception date on gestation length in red deer, with earlier conceiving hinds exhibiting longer gestations that late conceiving hinds, thus potentially moderating the impact of conception date on birth date. This was initially described as a putative ‘push/pull’ control of gestation length to promote with-in herd birthing synchrony (Scott et al., 2008) in the face of phenotypic variation in conception date. However, studies on the relationship between conception date and gestation length in red deer have yet to factor the potential effects of level of nutrition during various phases of pregnancy on birth date outcomes, and it is possible that extremes in conception date merely differentially align key phases of pregnancy with level of nutrition, and that birthing synchrony may be a coincidental, rather than intended, outcome.

A third area of interest is the impacts of nutrition on growth and subsequent puberty of young red deer hinds. Hinds normally enter puberty (first fertile ovulation) during their second autumn at about 16 months of age. However, there is a permissive body mass threshold (PBMT) for attainment of puberty which equates to approximately 70% of ultimate expected adult body mass (which differs between sub-species/genotypes).  Ultrasonographic pregnancy scanning data of >20,000 young red deer hinds on NZ deer farms has shown that the PBMT , calculated from the probability function of pregnancy at various pre-rut live-weights, can vary within a given genotype. Analysis of early life growth records (e.g. 3 month weaning live-weight) and puberty attainment records of a sub-set of >6000 individual hinds of similar genotype across several farms demonstrated that the nutritional environment from birth to weaning significantly influenced the PBMT, with hinds of lower weaning weight exhibiting a higher PBMT than hinds of higher weaning weight. This raises the intriguing hypothesis that early-life somatic growth influences ‘set-points’ (e.g. PBMT) for future reproductive processes in red deer (Asher and Cox 2013), hinting at an adaptive strategy for optimising life-time reproductive performance (i.e. delaying puberty under adverse environmental conditions). The mechanisms behind this remain to be elucidated but it is hypothesised that it may relate to rates of body lipid accumulation (Asher and Cox 2013).

These studies demonstrate that while red deer are assumed to be under fairly rigorous genetic control of seasonality traits, they have a repertoire of phenotypic variation at various points of the reproductive cycle that may potentially allow adaptation to climatic variation that influences annual feed supply.  This may explain the success of this cervid species in colonising a range of new environments that differ seasonally to their ancestral environment.