Effects of dietary concentrate level and feeding length on nutrient digestibility, rumen hydrolytic enzymes activity, intermediary metabolites, and feeding behavior in growing fat-tailed lambs: Iranian feedlot system

High concentrate diets (feedlot) are commonly used in the intensive lamb finishing systems. However, the use of high concentrate diets increases the cost of production especially when feeding length (FL) is prolonged. Both levels of concentrate and FL need to be adjusted in order to get the best economic profit in feedlot systems. This experiment aimed to evaluate the effects of dietary concentrate level (CL) and FL on nutrients digestibility, activity of rumen hydrolytic enzymes, circulating intermediary metabolites, and feeding behavior of growing fattailed lambs. Twenty-seven male Lori-Bakhtiari lambs (26.3 +/- 3.3 kg body weight, age 90 +/- 15 days) were maintained in individual pens and fed diets with 55% (CL55), 70% (CL70), and 85% (CL85) concentrate level and the remaining with alfalfa hay. The experiment lasted 105d including a 15d of adaptation period. Daily feed intakes, feed conversion ratio (FCR), average daily gain (ADG), dry matter digestibility (DMD), feeding behavior were measured three times during the 90d feeding period. Intermediary metabolites in blood and rumen fluid volatile fatty acid, ammonia, protozoa number, alpha-amylase, carboxymethyl cellulase (CMCase), microcrystalline cellulase (MCCase), and filter paper-degrading (FPD) activity were measured at 30, 60 and 90 d of feeding. At the end of feeding, final body weight (FBW), carcass weight and relative weight of offal were recorded. Lambs fed CL85 had higher (P < 0.05) ADG, DMD, activities of alpha-amylase, CMC, MCC, FDP and greater FBW, carcass weight and subcutaneous fat and lower acetate/propionate (AP) ratio, and less blood protein as compared to lambs fed CL55 or CL70. Lambs fed CL55 spent more (P < 0.05) time on eating and chewing than those fed CL70 and CL85. Overall, toward the end of feeding period, DMI, DMD, CPD and activity of alpha-amylase, CMC, MCC, FDP significantly increased (P < 0.05), while circulating glucose, cholesterol and total protein were decreased(P < 0.05) when compared with early feeding period. Extended feeding worsened FCR values and decreased the utilization efficiency of energy and protein for gain by 31% and 37% respectively. In conclusion, increased feeding length more than 60 days significantly reduces feed efficiency per unit of gain presumably through a shift towards elevated dietary energy usage for fat deposition in the body, evidenced by greater subcutaneous fat content and fat tail weight. The study indicates that the use of concentrate level more than 70% in the ration along with extended feeding length may not be profitable in finishing of fat-tailed lambs.