Effect of protein supplements of varying degradability on growth rate, wool yield and wool quality in grazing lambs

Abstract

Two experiments were conducted to investigate productive responses in grazing lambs to supplemental protein of varying degradability. In Experiment 1, rapeseed cake (RSC), a highly degradable feed, was treated with formaldehyde (HCHO) for protecting against rumen fermentation. Different levels of formaldehyde, namely, 0, 1, 2, 3, 4 and 5 g/100 g crude protein (CP) were used to find an optimum level. Increasing levels of formaldehyde linearly decreased the in sacco protein degradability of RSC at 6, 12 and 24 h of incubation. Maximum response per unit of the chemical was found with 3 g/100 g CP and this level of formaldehyde was used for treating RSC for feeding lambs in the growth trial. In Experiment 2, 80 female lambs (Kaghani×Rambouillet, 10–12 months age) from four different private flocks (20 lambs per flock) were selected in one of the hilly areas of Pakistan. The lambs had free access to range grazing during daytime. In each flock, the lambs were randomly divided in five groups (four lambs per group) and assigned to the following protein supplements; control (no supplement), cottonseed cake (CSC), untreated rapeseed cake (U-RSC), HCHO-treated rapeseed cake (T-RSC) and wheat bran (WB) for a period of 80 days. The supplements were fed on an iso-nitrogenous basis equivalent to 20 g N/100 kg BW. All the lambs received a complete mineral mixture. Nutrient composition of range grasses grazed by the lambs remained the same throughout the experimental period and did not vary due to locations. CP in the grasses averaged 12.03% on a DM basis. The rate of protein degradation (%/h) was maximum in WB (25.55) followed by U-RSC (15.15), CSC (10.53) and T-RSC (5.32). Body weight (BW) gain ( P<0.01) and growth of clean wool ( P<0.05) increased to the same extent with all the four supplements. Daily BW gains were 30, 61, 56, 58 and 63 g in the lambs fed control, CSC, U-RSC, T-RSC and WB diets, respectively. Growth rate of clean wool averaged 3.4, 3.9, 4.1, 4.0 and 4.0 g per day on the above diets, respectively. Staple length and fiber diameter of wool did not respond to diet composition and remained the same on control and supplemented diets. Despite distinct variation in protein degradability of the supplements, the lack of difference in BW gain and wool growth among the lambs receiving these supplements were presumably due to the same quantity of total protein supplied by these feeds post-ruminally. We concluded that protein supplements of varying degradability supplying 25% of the total dietary protein intake had equal substitution value in grazing lambs.