The objective of the study was to evaluate the effects of sire breed (Charollais, Dorset and Texel), utilizing Hampshire ewes, lamb sex and birth type, on carcass traits of the lambs. After weaning (74±8 d), 45 lambs from these crosses were fattened during 63 d, consuming a balanced diet with 14% CP and 2.82 Mcal of ME/kg of DM. Lambs were weighed and slaughtered at 137 d of age; later, measures on slaughter weight and yield of hot and cold carcass, rib eye area, subcutaneous fat, carcass length, leg length, and rump perimeter were taken and the carcass compactness index was calculated. In addition, the carcass regional composition (shoulder, chest-belly, anterior-loin, leg, neck, and rear-loin) was determined. Data were analyzed utilizing a mixed model. Charollais-sired lambs had the highest (P<0.05) rib eye area (20.1±0.5 cm 2 ), while Dorset-sired lambs were superior (P<0.05) in carcass length (66.3±0.9 cm) and leg length (34.8±0.3 cm). Ram lambs surpassed ewe lambs (P<0.05) in most traits analyzed. Lambs born as singles had higher means than twin lambs (P<0.05) in subcutaneous fat and leg length. In general, crosses with Charollais and Dorset showed superiority in carcass traits.

Selenium decreases cellular damage caused by oxidative stress and improves the organism physiological response. Leukocytes were evaluated during embryonic development in ewes with consumption of organic selenium. A total of 27 local ewes at the municipality of San Andrés Chiautla, Mexico State, with an average age of 2.5 years, were randomly assigned in two groups, 11 in control group (without Selenium, S-Se) and 16 with oral selenium (C-Se; selenium-enriched yeast) addition at dose of 0.3 ppm, for a period of seven days before Artificial Insemination (AI) consuming a diet based on alfalfa and corn stubble. White cell differential counting was performed on blood smears. Samples were taken before, during and after selenium treatment. Data were analyzed by mixed model, showing significant differences by treatment and time (p ≤ 0.05) in neutrophils; day 9 of embryonic development (5.91 ± 0.89 y 2.11 ± 0.46; S-Se y C-Se, respectively) and basophils amount was at day -3 (1.85 ± 0.31 y 1.05 ± 0.23). Regarding lymphocytes and eosinophils, they showed a difference in time (p ≤ 0.05) but not by treatment (p ≥ 0.05). It is concluded that selenium decreases the number of neutrophils and basophils; after blastocyst formation and four days after starting organic selenium treatment.leukocytes, organic selenium, ewes. RESUMEN El selenio contrarresta el daño celular causado por estrés oxidativo y mejora la respuesta fisiológica del organismo. En este estudio se evaluaron leucocitos durante el desarrollo embrionario de ovejas como respuesta al selenio orgánico. Veintisiete ovejas locales del municipio de San Andrés Chiautla, Estado de México, con edad promedio de 2.5 años, se asignaron al azar en dos grupos, 11 en grupo testigo (sin selenio; S-Se) y 16 con adición de levadura enriquecida con selenio (selenio orgánico; C-Se) a dosis de 0.3 ppm, vía oral por período de siete días antes de la Inseminación Artificial (IA) consumiendo una dieta a base de alfalfa achicalada y rastrojo de maíz. Se realizó conteo diferencial de células blancas; en frotis sanguíneos realizados antes, durante y después del tratamiento de selenio. Los datos se analizaron mediante modelo mixto, mostrando diferencias significativas por tratamiento y tiempo (p ≤ 0.05) en neutrófilos; el día 9 del desarrollo embrionario (5.91 ± 0.89 y 2.11 ± 0.46; S-Se y C-Se, respectivamente) y basófilos el día -3 (1.85 ± 0.31 y 1.05 ± 0.23). En cuanto a los linfocitos y eosinofilos mostraron diferencia por tiempo (p ≤ 0.05) pero no por tratamiento (p ≥ 0.05). Se concluye que el selenio disminuye el número de neutrófilos y basófilos; después de formarse el blastocisto y cuatro días después de iniciado el tratamiento con Selenio. leucocitos, selenio orgánico, ovejas.