Balancing compound lysine level and roughage intake: impacts on performance and protein utilisation in organic growing-finishing pigs

Ensuring sufficient lysine intake without crude protein (CP) oversupply is challenging in organic pig production. Although roughage is mandatory, its nutrient contribution is often overlooked. This study evaluated how compound-feed lysine concentrations affect performance and roughage intake in organic growing–finishing pigs. One hundred fifty pigs (31.3–112 kg) received 5 compound diets (6.1 to 7.7 g digestible lysine/kg) with ad libitum grass-clover silage. Performance and plasma metabolites were measured. While average daily gain was stable, dietary lysine linearly improved early-phase feed conversion efficiency (2.37 to 2.23; P < 0.01) and lean meat percentage (58.8 to 59.8 %; P = 0.03), suggesting that weight gain was energy-limited under the isoenergetic conditions while lean tissue deposition remained lysine-responsive. Silage intake reached 1 kg/pig/day, but its low dry matter content restricted its nutrient contribution, (1.5 to 2.2 % additional lysine and 3.1 to 5.8 % CP), with no compensatory intake at lower compound lysine levels. Plasma amino acid increased linearly with dietary lysine ( P < 0.01), reflecting greater nutrient availability. Concurrently, rising plasma urea/creatinine ratios ( P < 0.01) indicated that surplus protein was not fully utilised for muscle synthesis due to energy limits. Ultimately, optimising organic feed formulations requires balancing early-phase efficiency gains and leaner growth against high feed costs and metabolic load of unutilised protein. These findings suggest that a multi-phase feeding regime, with high lysine density during the early responsive phase (34.7–58.7 kg) and reduced density in later phases, would likely maximise efficiency while minimising N waste.