Dietary starch level determines the efficacy of calcium salts of palm fatty acids on nitrogen use efficiency and performance in feedlot lambs

High-concentrate diets supplemented with rumen-protected fats are widely used in feedlot lamb systems to improve energy supply and performance; however, the metabolic efficacy of these lipid sources may depend on the availability of fermentable carbohydrates, which influences ruminal fermentation and nutrient utilization. This study evaluated the interaction between dietary starch concentration and calcium salts of palm fatty acids ( CSPFA ) on nutrient intake, digestibility, nitrogen metabolism, ingestive behavior, serum and liver metabolite profiles, and growth performance of feedlot lambs. Thirty-two Dorper × Santa Inês lambs (25.0 ± 2.85 kg) were distributed in a completely randomized 2 × 2 factorial design. The treatments consisted of two starch levels (low: 220 g/kg DM or high: 420 g/kg DM) and two lipid supplementation levels (0 or 30 g/kg DM of CSPFA). A starch × CSPFA interaction (P ≤ 0.05) was detected for the intake of ether extract (P = 0.045), starch (P = 0.050), digestible organic matter intake ( DOMI ) (P = 0.029), starch digestibility (P = 0.025), nitrogen retention (P = 0.021), microbial N synthesis (P = 0.005), average daily gain (P = 0.015), and feed efficiency (P = 0.018). In low-starch diets, CSPFA reduced starch intake (P < 0.001) due to dietary dilution rather than physiological satiety, as the total dry matter intake remained unaffected. Consequently, CSPFA failed to improve N retention or growth in an energy-limited environment. Conversely, in high-starch diets, CSPFA maintained fiber digestibility, reduced serum urea by 43% (P = 0.005), increased microbial N flow by 29% (P = 0.007), and improved N retention by 22% (P = 0.021). Furthermore, high-starch diets promoted higher efficiency of microbial protein synthesis than low-starch diets (73.72 vs. 59.92 g/kg DOMI), regardless of CSPFA inclusion (P = 0.747). Systemic safety markers, including aspartate aminotransferase activity and lipid profiles, remained within physiological ranges, indicating no adverse effects on liver function or metabolic homeostasis of the lambs. As a result, CSPFA increased average daily gain (+35%) and feed efficiency (+22%) exclusively in lambs fed the high-starch diet. The metabolic efficacy of CSPFA depended on dietary starch concentration. In high-starch diets (420 g/kg DM), CSPFA supplementation improved nitrogen-use efficiency and microbial protein synthesis, resulting in improved nitrogen retention and growth performance in lambs. These benefits were not observed in low-starch (220 g/kg DM) diets, indicating that the positive effects of lipid supplementation are contingent on an abundant supply of digestible organic matter to support microbial protein synthesis.