To study the effect of Ca-salt of linseed oil (Ca-LSO) supplementation in diets with varying protein content, thirty-six 12-day-old Farahani lambs with an average BW of 8.9 kg were used in a completely randomized block design with a 2 × 2 factorial arrangement as follows: (1) no Ca-salt of linseed oil supplementation and 18% CP (NLSO-18CP); (2) no Ca-salt of linseed oil supplementation and 21% CP (NLSO-21CP); (3) starter diet supplemented with Ca-salt of linseed oil and 18% CP (LSO-18CP); (4) starter diet supplemented with Ca-salt of linseed oil and 21% CP (LSO-21CP). All lambs were weaned on d 72 but remained in the study until d 92 of age. The results showed that starter intake was similar among the experimental treatments; however, feed efficiency (FE) was improved in lambs supplemented with Ca-LSO during the post-weaning period. Higher concentrations of blood glucose, HDL-cholesterol, and insulin were found in the lambs supplemented with Ca-LSO. The 21CP diets improved FE (tendency) in the post-weaning period. The higher protein content in the starter diet increased ruminal concentrations of branched-chain fatty acid (d 45), ruminal NH3-N concentration (d 90), and improved MPY compared to the 18CP diets. Regarding the interaction effects between Ca-LSO supplementation and starter protein content, there was a higher average daily gain, body weight, and blood insulin concentration (d 46) for the LSO-21CP treatment (P < 0.05). In contrast, the greater blood urea nitrogen concentration (d 46) and urinary N excretion were observed in the NLSO-21CP treatment (P < 0.05). In conclusion, under the conditions of the present experiment, results showed that supplementation of the Ca-LSO with 21CP improved growth performance in lambs reared on an indoor feeding system, which can be attributable to improved N efficiency.
