Use of narasin in diets for lactating ewes.

• Over the past 5 years, narasin began to be used on a large scale in ruminant production systems in Brazil. • Narasin increases milk production in lactating ewes. • 13 mg/kg DM of narasin is the ideal concentration in diets for lactating ewes. The objective of the present study was to evaluate the effect of different doses of narasin on dry matter intake (DMI), milk production and composition in ewes and growth performance of their lambs. Forty crossbred lactating ewes Dorper vs. Santa Inês (54.4 ± 1.7 kg of initial BW; mean ± SD) were penned individually with their lambs and used in a randomized complete block design with 10 blocks and 4 treatments. Ewes were fed a basal diet (168 ± 0.003 g/kg DM of CP) containing 50% concentrate and 50% Brachiaria brizantha hay. The treatments were as follows: 0 N - control diet (basal diet without addition narasin); 13 N - addition of 13 mg/kg DM of narasin; 20 N - addition of 20 mg/kg DM of narasin; 27 N - addition of 27 mg/kg DM of narasin. Once a week, from the second to tenth week of lactation (weaning time), ewes were separated from their lambs, injected with oxytocin, and mechanically milked to empty mammary gland. After 3 hours, the same procedure was repeated, milk production was recorded and samples collected for milk composition analysis. Ewes were weighed for 3 consecutive days at the beginning and end of the experiment. Lambs growth performance was evaluated weekly. The data were analyzed using a linear or quadratic polynomial contrast in SAS. There was a quadratic effect (P = 0.03) of narasin on milk production, in which ewes fed the diet containing 13 mg/kg DM had the highest production (194.9 g/3 hours) Narasin inclusion decreased linearly the percentage of protein (P = 0.01), non-fat solids (P = 0.03), and SCC (P = 0.04) in milk. The fat, lactose and total solids content were not affected by treatments. There was a quadratic effect (P = 0.05) of narasin doses on concentrate intake by the lambs, in which the lowest intake was observed for lambs from ewes fed N13. The supply of narasin did not affect the ADG and final BW of the lambs. In conclusion, the use of 13 mg/kg DM of narasin was the best concentration in diets containing 50% forage and 50% concentrate for lactating ewes, due to positive effect on milk production. The inclusion of values higher than 20 mg/kg DM of narasin is not recommended, because this content drastically reduced DMI and milk production, resulting in greater concentrate intake by lambs with no effect on ADG and final BW. [ABSTRACT FROM AUTHOR]