. 2000. Effect of a surfactant and exogenous enzymes on digestibility of feed and on growth perfomance and carcass traits of lambs. Can. J. Anim. Sci. 80: [35][36][37][38][39][40][41][42][43][44]. The effects of Tween 80 on the stability and substrate binding capacity of two commercially available enzyme preparations were studied in vitro. After 1 wk of incubation, endoglucanase activity was higher (P < 0.05) in the presence of 0.02 or 0.2% (vol vol -1 ) Tween 80 than without this surfactant. Tween 80 increased (P < 0.05) protein binding both to filter paper and to cotton fibres. Digestibility and feedlot experiments were conducted with Dorset and Romanov × Suffolk lambs, respectively, given forage-and concentrate-based diets. The diets were untreated or sprayed with the enzyme preparations, Tween 80 or a mixture of both. With the concentrate diet, treatment with enzymes + surfactant reduced (P < 0.05) intake of feed dry matter and N, acid detergent fibre digestibility, N digestion and microbial N production, relative to treatment with either additive alone. Treating the concentrate diet with enzymes increased (P < 0.05) N retention (g d -1 ) by the lambs, as compared to untreated diet. With the forage diet, neutral detergent fibre digestibility was higher with Tween 80 alone than with enzymes + surfactant. In the feedlot experiment, treating the forage diet with surfactant reduced (P < 0.05) dry matter intake and daily gain, relative to untreated diet, and increased (P < 0.05) the number of days to finish. Feed conversion efficiency (gain feed -1 ) of lambs fed the concentrate diet was lower (P < 0.05) when the diet was treated with surfactant than with enzymes. Treating the concentrate diet with enzymes + surfactant increased (P < 0.05) the number of days required to finish the lambs. Although Tween 80 appeared to increase the stability and substrate binding capacity of enzymes in the laboratory, it did not enhance the effects of the enzymes on nutrient digestibility or growth performance in animal feeding studies. Nous étudions in vitro les effets de Tween 80 sur la stabilité et sur la capacité de se fixer aux substrats de deux préparations enzymatiques commerciales. Au bout d'une semaine d'incubation, l'activité de l'endoglucanase était plus forte (P < 0,05) en présence de 0,02 ou de 0,2 % (en volume) de Tween 80 que sans le surfactant. En outre, la fixation de la protéine enzymatique au papier filtre et à des fibres de coton était accrue (P < 0,05). Des expériences en cages de digestibilité et en parquets d'engraissement étaient conduites, respectivement, sur des agneaux Dorset et sur des agneaux Romanov × Suffolk, recevant des aliments complets à forte proportion de fourrage (HPF) ou de concentré (HPC). Lors du mélange des ingrédients, les aliments étaient aspergés ou non d'une ou l'autre des deux préparations enzymatiques en solution, avec ou sans Tween 80 ou de Tween 80 seulement. Traité aux enzymes + surfactant, l'aliment HPC causait une diminution (P < 0,05) de l'ingéré de m.s. et de la digestibilité d...