Supplementation of exogenous beta-mannanase to low-protein diets improves feed conversion efficiency in lactating dairy cows

Context. Supplementation of exogenous enzymes has been used to improve nutrient utilisation and reduce environmental impacts of excess nutrient release to the environment in swine, poultry and, to a lesser extent, ruminant production systems. Aims. We aimed to determine effects of supplementation with a fibrolytic enzyme, beta-mannanase, on feed conversion efficiency (FCE) and milk yield in cows fed a diet relatively low in crude protein (CP) concentration. Methods. Twelve mid-lactation multiparous Holstein cows producing 40.5 +/- 3.6 kg milk/day were assigned to three dietary treatments in a three-period crossover design. Treatment sequences were balanced using 3 x 3 Latin squares to mitigate possible carryover effects. Treatments, fed in a total mixed ration, were high CP (16.1%), low CP (14.6%), and low CP supplemented with commercially available beta-mannanase at 0.1% of concentrate dry matter (DM). Key results. beta-Mannanase supplementation did not affect DM intake, milk yield, or milk component yield and composition. Milk urea-nitrogen was significantly lower in cows fed diets with low CP. Somatic cell counts were reduced in cows supplemented with beta-mannanase compared with the other treatments. Apparent total tract digestibilities of DM, organic matter, CP, acid and neutral detergent fibre, starch and ash were unaffected by treatment. Cows receiving beta-mannanase showed better FCE than those receiving high CP (13.4% improvement, P = 0.003) or unsupplemented low CP (11.0% improvement). Cows receiving beta-mannanase used dietary CP more efficiently to synthesise milk protein than cows receiving high CP (milk protein:CP intake, 0.34 vs 0.30). Nitrogen intake was significantly reduced in the low CP treatments, leading to reduced fecal and urinary nitrogen excretions. Conclusion. beta-Mannanase supplementation at 0.1% of dietary DM improved FCE and lowered somatic cell counts of dairy cows without affecting milk yield or component yield and composition, while reducing nitrogen excretion. There were potential daily savings of US$1.03/cow compared with a high CP diet.