Plane of nutrition during the rearing phase for replacement ewes of four genotypes: I – effects on growth and development, and on ovulation rate at first joining

Abstract

Plane of nutrition (PN) offered to ewe replacements during the rearing phase (8 to 17 months) affects BW at joining and may affect reproductive performance when joined at ~19 months. The effects of PN offered to ewe replacements during their first winter (winter_1) and second summer (summer_2) were evaluated over 3 consecutive years, using 287 spring-born ewe lambs, representing four genotypes (Belclare (Bel), Charmoise×Scottish Blackface (C×SBF), Bel×SBF (Bel×SBF), Bel×SBF that were heterozygous either FecGH or FecXG mutations that increase ovulation rate (OR) (BelMG×SBF)). During extended (deferred) grazing in winter_1 (November to March) the lambs were offered herbage DM allowances (HA) of 0.75 (L) or 1.75 (H) kg/day. During summer_2 (March to August) the replacements were set stocked to maintain sward heights of 4 (L) or 6 (H) cm, thus yielding a 2×2 factorial design for the nutrition treatments (71 to 73 animals/treatment). Increasing HA during winter_1, and residual sward height during summer_2, increased (P0.05) on OR. Mean BW at joining was 53.3, 56.8, 56.6 and 61.7 (SEM 0.74) kg for ewes offered the LL, LH, HL and HH treatments, respectively. Mean OR of C×SBF and BelMG×SBF differed (P<0.001) from that of Bel×SBF by -0.58 and +0.47, respectively. Correlations between linear measurements (LMs) of body size and BW at 7, 12 and 17 months were all positive and significant (P<0.001). The precision of the set of LMs as a predictor of BW at joining improved with age (R 2 0.46, 0.54 and 0.74 at 7, 12 and 17 months) but BW at a given age was a better predictor at all age time points. Chest girth was the best predictor, among the LMs, of BW at joining and its explanatory power increased with age. Equations developed using LMs at 7, 12 or 17 months, to predict BW at joining exhibited biases of -2.1, -1.5 and +0.9 kg, respectively. It is concluded that whilst altering PN during the rearing phase changed BW by 16% it had no effect on OR. There was no interaction (P>0.05) between the PN offered during winter_1 and summer_2, or PN and ewe genotype for BW at joining or OR. LMs, either at 7, 12 or 17 months of age, are useful predictors of BW at joining.

Effects of Age at First Joining and Ewe Genotype on the Performance of Two-Tooth Ewes and That of Their Progeny to Slaughter

“…Keady and Hanrahan [ 2 ] used the probability of rearing ≥ 1 lamb as an overall efficiency index since this encompassed ewe fertility and litter size together with ewe and lamb mortality. In the current study, the lack of an effect of BW at joining on the probability of rearing ≥ 1 lamb was expected, as earlier studies had shown that an 8.4 kg difference in BW when joined at 19 months did not affect ovulation rate, litter size or the number of lambs reared per ewe joined [ 8 , 9 ]. No published information was found on the effect of weight at 7 months on the productivity when joined to lamb at 2 years.…”

“…Ewe body condition score (BCS) [ 23 ] was recorded pre joining, post joining, at lambing, at approximately week 5 post lambing, and at weaning. Withers height, body length, chest (heart) girth and circumference of the cannon bone were recorded for all ewes post mating (early November), as described previously [ 8 ]. This collection of linear measurements will be referred to as body size.…”

“…However, Vavra et al [ 6 ] reported that age at first lambing had no effect on the number of lambs born to 2-year-olds but the number reared was higher for those that had lambed at 1 year whilst McMillan and McDonald [ 7 ] reported that ewes producing their second litter at 2 years of age reared more lambs per ewe joined. Keady and Hanrahan [ 8 , 9 ] reported that altering the weight of two-tooth ewes at joining by 8.4 kg did not alter the number of lambs born or the number reared per ewe joined. Most previous studies of the effect of age at first lambing on the performance at 2 years involved ewes of relatively low productivity and which lambed outdoors [ 4 , 5 ].…”