Lamb growth patterns following different seasons of birth

Fall-, winter- and summer-born lambs were compared for birth weight, 70-d weight and avg daily gain (ADG); 70-d to market). Data were collected on 457, 510 and 640 lambs born during the fall (October, November), summer (June, July) and winter (January, March), respectively. Similarly bred lambs born during the different seasons varied in birth weight (P less than or equal to .01) and for 70-d weight (P congruent to .07). Large year-within-season variation for post-weaning gain contributed to inconclusive results for seasonal effects. Winter- and summer-born lambs were 1.28 and .95 kg heavier (P less than .05) at birth than were fall-born lambs, respectively. Winter-born lambs were 2.93 and 3.79 kg heavier (P congruent to .1) at 70-d of age than fall- and summer-born lambs, respectively. An interaction between sex and season was observed for ADG (P less than .01). There was a smaller average difference (40 g/d) between ram and ewe lambs born in the summer than between ram and ewe lambs (90 g/d) born in the fall and winter. Results suggest that differences in birth weight and 70-d weight existed between fall-, winter- and summer-born lambs under conditions where lambs were creep fed the same diet and ewes were well fed. Sex and birth (birth-rearing) type adjustment factors appeared to be similar among seasons for birth weight and 70-d weight. Only the adjustment of ADG for sex appeared to depend on the season of birth of the lamb.