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Data on 4,754 lambs born from Pelibuey (PB) and Barbados Blackbelly (BB) ewes collected over 9 years at the Mococha Experimental Centre in Yucatan, Mexico, were used to determine environmental, dam-breed and sire-breed effects on birth weight (BW), weaning weight (WW) and average daily gain (ADG). Effects of year and season of birth, sex of the lamb, type of birth, breed of dam, breed of sire, their interaction and ewe body weight were included. Heritabilities were estimated for each breed. Season of lambing was a significant source of variation on WW and ADG. Male lambs weighed more than female lambs at birth and weaning. Single born lambs were heavier than twins and triplets. Lambs born from PB dams were heavier and grew faster than those born from BB dams. Heritabilities of BW, WW and ADG for the PB breed were 0.16 +/- 0.04, 0.12 +/- 0.03 and 0.11 +/- 0.03, respectively and 0.04 +/- 0.07, 0.17 +/- 0.09 and 0.15 +/- 0.08 for the BB breed. Heterosis values were close to zero.

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