ABSTRACT: Adding ionophores to ruminant diets is a strategy to manipulate ruminal fermentation and improve milk yield. This study evaluates the effects of narasin supply to lactating ewes on dry matter intake (DMI), milk yield and composition, and performance of the lambs. Thirty lactating Santa Inês and Santa Inês × Dorper ewes fed a basal diet containing 50 % coastcross hay and 50 % concentrate were randomly assigned to two treatments: control (CON; without ionophores) or NAR (addition of 13 mg narasin kg–1 DM). From the 2nd to 10th week of lactation, DMI of ewes was determined, and once a week, their milk production and composition was measured over a 3-h interval. At the 10th week of lactation, lambs were weaned and their average daily gain (ADG) and starter DMI continued to be evaluated for two more weeks. Narasin supply did not affect weight and DMI of ewes. Ewes fed NAR had greater feed efficiency for milk production and displayed tendency for higher milk yield. Narasin supply reduced milk protein levels, but it did not affect other milk component levels. Ewes fed NAR had greater production of milk urea nitrogen and showed tendency for higher production of fat and total solids. Starter DMI of lambs was not affected by treatments; however, there was a tendency for greater weaning weight for NAR lambs. At the end of experiment, no differences were observed in the performance of lambs. The supply of 13 mg narasin kg–1 to lactating ewes improved milk yield efficiency and tended to increase the weaning weight of their lambs.

ABSTRACT Two experiments were proposed to evaluate the addition of monensin for lambs fed diets containing a high level of mature ground flint corn. The experimental diets were as follows: no inclusion of monensin (M0) and inclusion of 8 (M8), 16 (M16) and 24 mg kg–1 of monensin (M24). In experiment 1, eight cannulated wethers were divided into a double 4 × 4 Latin square experimental design to evaluate nutrient digestibility, plasma parameters and rumen fermentation. The experiment lasted 112 days, divided into four periods of 28 days each. In experiment 2, ninety-two lambs were used in a randomized block design to evaluate the performance over 56 days. In experiment 1, doses of monensin had no effect on nutrient intake (p ≥ 0.07) and digestibility (p ≥ 0.09). There was a quadratic effect for acetate molar proportion (p = 0.01), acetate to propionate ratio (p = 0.04) and rumen pH (p < 0.01). However, there was no effect on the molar proportion of propionate and butyrate. The monensin decreased linearly the total SCFA concentration (p < 0.01). The inclusion of monensin increased glucose (p < 0.01) and decreased lactate concentration in plasma (p = 0.05). In experiment 2, monensin decreased dry matter intake (p = 0.04). However, there was a quadratic effect for average daily gain (p = 0.03) and feed efficiency (p < 0.01), with the greatest values observed for the M8 diet. Thus, the inclusion of 8 mg kg–1 of dry matter diet (DM) improves ruminal fermentation and plasma parameters, resulting in greater growth performance in lambs.