Effect of dietary inclusion of rumen-protected flaxseed oil (Linum usitatissimum) on intake, ingestive behavior, diet particle selectivity, digestibility, nitrogen balance, and blood parameters of lambs

This study evaluated the effects of rumen-protected flaxseed oil (RPFO; 30% flaxseed free oil and 70% carnauba wax) on intake, digestibility, ingestive behavior, nitrogen balance, and blood parameters in crossbred male lambs (Santa Inês × mixed breed). Fifty lambs (20.5 ± 3.0 kg; five months old) were assigned to a completely randomized design with five treatments: control, 4% and 8% free oil, and 4% and 8% RPFO ( n = 10 per treatment). Flaxseed free oil inclusion, regardless of form, significantly affected intake, digestibility, feeding behavior, and nitrogen metabolism ( P < 0.05). Lambs fed 8% free oil showed the lowest ( P = 0.009) dry matter intake (DMI), whereas 4% RPFO resulted in the highest DMI. Crude protein and neutral detergent fiber corrected for ash and protein ( a NDFom) intakes decreased ( P < 0.05) with 8% free oil, while control, 4% free oil, and 8% RPFO showed higher intake and feed efficiency. Average daily gain was lowest with 8% free oil (169 g/day). Free oil reduced rumination time and fiber eating efficiency ( P < 0.01), while RPFO (4%) improved chewing activity and fiber utilization. Animals receiving free oil showed greater rejection of coarse particles ( P < 0.01), whereas RPFO maintained more uniform particle selection and physically effective fiber intake. Nutrient digestibility generally decreased with oil inclusion, especially with RPFO at 4% ( P < 0.05). Nitrogen intake was reduced with 8% free oil and 4% RPFO, while urinary N excretion increased with RPFO ( P < 0.001), resulting in lower N retention efficiency at 4% RPFO. Blood biochemical parameters were not affected ( P > 0.05). The RPFO mitigated negative effects of unsaturated lipids on rumen function, whereas high free oil reduced intake and performance. However, RPFO levels tested did not improve overall efficiency, indicating the need for further optimization.