Thirty-six male lambs (averaging BW = 24.4 kg) were assigned in a completely randomized design with the factorial arrangement (nine lambs per each treatment) to assess the effects of linseed processing method (ground vs. extruded) and dietary crude protein content (12% vs. 15%, DM basis) on performance, digestibility, ruminal fermentation, and protozoa population. Treatments were; (1) ground linseed with 12% CP (GLS-12CP), (2) ground linseed with 15% CP (GLS-15CP), (3) extruded linseed with 12% CP (ELS-12CP), and (4) extruded linseed with 15% CP (ELS-15CP). The study lasted 84 d, and the lambs had free access to experimental diets and water. Outcomes showed that dry matter intake, digestibility of organic matter, and crude protein were increased in lambs fed ground linseed compared to extruded linseed. Feeding extruded linseed decreased urinary allantoin concentration, total purine derivatives (PD), the ruminal proportion of propionate, and blood concentration of glucose and HDL-cholesterol compared to ground linseed diets. Regarding dietary protein content, results showed that dry matter intake, urinary concentration of allantoin and total PD were increased in 15CP diets compared to 12CP diets. Considering the interaction, results showed that average daily gain was improved in GLS-15CP treatment compared to other experimental treatments. In conclusion, results show that grinding the linseed is more beneficial compared to its extrusion in lamb nutrition. Moreover, 15CP diets had a beneficial effect when fed with either ground or extruded linseed and seem to compensate, to some extent, the negative effects of extruded linseed on the gain of growing lambs.
