J.A. Archer

Effect of melatonin implants on the incidence and timing of puberty in female red deer (Cervus elaphus)

A study was conducted to test the hypotheses that exogenous melatonin treatment of 11-13 month-old red deer hinds: (1) advances the timing of first ovulation, (2) increases the proportion of individuals attaining puberty at ∼16 months of age, and (3) reduces the live-weight threshold for attainment of first pregnancy. A total of 3901 rising-2-year-old (R₂) hinds within two herds (A and B) across two years either received single melatonin implants on two occasions in summer (n=1399) or were untreated controls (n=2502). Hinds were joined with stags from mid January to mid May, and were subjected to real-time rectal ultrasonography in early June to assess pregnancy status (proxy for puberty attainment) and foetal age for conception date assignment. Live-weights were recorded for each hind in January (12 months of age) as a proxy for weight at puberty. Melatonin treatment of hinds was associated with a significant advancement in mean conception dates in both herds in both years (P<0.05), with a cohort difference in mean dates between treated and control hinds ranging from 9 to 17 days. Analysis of the temporal distribution of conception dates for each cohort revealed bi-modal or tri-modal patterns of conception indicative of conceptions to first or subsequent ovulations (oestrous cycles). Across all cohorts, melatonin treatment was associated with higher conception rates to first ovulation (P<0.05) resulting in greater overall synchrony of conceptions. Regression analysis demonstrated a significant negative slope for conception date against live-weight (P<0.001), but there was no evidence that this slope varied with treatment, herd or year (P>0.05); for every 10kg increase in live-weight conception date was advanced by an average of 1.3 days. In Herd A, melatonin treatment was associated with significantly higher pregnancy rates in both years (90.3% vs. 78.0% in Year 1 and 84.4% vs. 57.1% in Year 2; P<0.05). The principle effect of melatonin treatment was to increase the pregnancy rate of hinds of low body-mass. In Year 1, at 60kg live-weight a logit regression model indicated a pregnancy rate of 52% for untreated hinds and 83% for treated hinds. At 105kg the rate for both cohorts was 90%. In Herd B, melatonin treatment was associated with higher conception rates in both years but these differences were not significant following correction for slight differences in mean live-weight (P>0.05). The study has demonstrated that factors influencing puberty attainment in R₂ red deer hinds can vary between populations. In Herd A, in which body mass of hinds immediately prior to their first potential breeding season may have been the principle limiting factor, melatonin treatment appears to have instigated the pubertal process in hinds that would otherwise be of insufficient body mass.

Evaluation of management variables to advance conception and calving date of red deer (Cervus elaphus) in New Zealand venison production systems

The ability to shift the supply of New Zealand chilled venison from farmed yearling red deer stags to obtain premium prices in seasonal European markets necessitates early calving of hinds combined with high growth rates of their calves. Two studies over a three-year period evaluated three management variables that offer potential to advance calving date. Under the conditions of the studies there was no consistent evidence that the management practices of early stag introduction, early weaning and enhanced hind nutrition prior to conception (lactation) and pre-calving (third trimester of pregnancy) advanced conception date and calving date in red deer hinds. However, the nutrition effect was diminished by the difficultly in achieving the dietary contrast necessary for the targeted 5kg differentiation in hind live weight at strategic times of the year. Across all hinds there was a significant pre-mating (mid-March) live weight effect on conception day in the one year in which a 5kg difference between nutritional regimens was achieved, but the driver was live weight and not nutrition. There were significant effects of nutrition on calf growth, with the growth rates of calves weaned in mid-March significantly higher when their dams grazed a high plane of nutrition pre-conception. There were significant and consistent inverse relationships between conception day and calving date that implied variation around gestation length, with early- and late-conceiving hinds exhibiting longer and shorter gestation lengths, respectively. Across all treatments, calving date was predicted to advance by approximately 5 days for every 10-day advance in conception date. However, there was a significant carry-over effect of nutrition pre-conception on calving date, with hinds on a high plane of nutrition pre-conception exhibiting shorter (2-4 days) gestation lengths. There were also indications that hinds may manipulate gestation length in response to live weight gain pre-calving. These findings suggest that fetal growth trajectory may be the principle driver of gestation length and calving date. Although there were no direct effects of hind nutrition pre-mating on conception dates, nutrition remains an important component of the management of hinds and their calves in venison production systems. The outcomes of the 3-year program suggest that there are limited opportunities to manipulate calving date through manipulation of management variables.

Effect of conception date and hind nutrition on fetal growth trajectory and gestation length of red deer (Cervus elaphus)

This study tested the hypothesis that the negative association between gestation length and conception date in red deer is mediated by nutrition. Twenty-eight pregnant red deer were randomly allocated to four groups according to a 2 × 2 factorial design, with the factors conception date (14 March, E; 28 April, L) and level of nutrition (ad libitum, H; restricted, R). Animals were housed indoors in individual pens from early winter until calving and offered daily an ad libitum pelleted ration. The daily ration was then restricted from late winter in ER (134 days post-conception) and LR (89 days post-conception) groups, so that these hinds did not experience a seasonal increase in food intake. X-Ray computed tomography scans were taken at Days 120, 150, 180 and 210 of gestation (mid–late gestation) to estimate weight of various conceptus components. Growth rate of the total fetus was significantly higher in LH than in other treatments (P 0.05) between treatments or calf sex. Birthweight was associated directly with change in hind liveweight (P = 0.03) and body condition score during the third trimester of pregnancy (P = 0.01), but was not significantly associated with gestation length (P = 0.34). Gestation length was 4.4 days longer in LR than LH hinds (P = 0.03) and was negatively associated with both food intake (P = 0.03) and LW gain (P = 0.02) during the final trimester of gestation. Feeding late-conceiving hinds an ad libitum diet of high-quality food during the third trimester of gestation maximises fetal growth and shortens gestation length.

Seasonal luteal cyclicity of pubertal and adult red deer (Cervus elaphus)

Reproductive failure of rising-two-year-old (R(2)) hinds and seasonal misalignment between calving and pastoral feed production are two factors limiting reproductive productivity of farmed red deer hinds in New Zealand. This study aimed to better understand processes around female puberty and breeding seasonality by describing the potential breeding season (i.e., oestrous cyclicity) of three red deer genotypes. A total of 27 hinds born in December 2005, representing Eastern European (Cervus elaphus hippelaphus), Western European (C.e. scoticus) and F1 crossbred (C.e. hippelaphus×scoticus) red deer, were blood sampled thrice-weekly for 7-8 months (February-September/October) across two years spanning the potential breeding seasons as R(2)s in 2007 (i.e., puberty) and as adults in 2008. Plasma progesterone profiles were used to construct breeding cycle histories for each hind. Four R(2) hinds failed to initiate oestrous cycles (i.e., puberty failure). The remaining R(2) hinds, including all F1 hinds, exhibited between two and seven oestrous cycles. F1 hinds were significantly earlier to initiate, and later to terminate, cyclic activity, resulting in a longer mean pubertal breeding season (139 days) than for Eastern (86 days) and Western hinds (86 days). However, the data for R(2) hinds are confounded by live-weight, with the F1 hinds being significantly heavier than other genotypes. There were significant correlations between live-weight and seasonality parameters in 2007. All hinds were cyclic as adults in 2008, exhibiting between four and nine oestrous cycles, and a mean breeding season duration of between 132 (Western) and 137 (F1) days. For adult hinds there were no significant genotype differences in cyclic onset and cessation timing, and no observable relationships between live-weight and any reproductive parameter. However, the mean dates for the onset of the breeding season for all genotypes in 2008 were 2-3 weeks later than normally expected for adult hinds in New Zealand. The reasons for this are unclear but may relate to chronic stress of frequent animal handling. The study has demonstrated that puberty in red deer hinds is associated with a shorter potential breeding season than for adult hinds, and that perturbation of breeding activity appears to be quite common, leading to incidences of puberty failure and possibly other aberrant cyclic events. Live-weight×genotype interactions may influence puberty but do not appear to be strongly expressed in adults. However, the relatively late onset of oestrous cyclicity in the adult hinds may be an artefact of the study that has masked genetic influences on seasonal breeding patterns.