LAUSR

Eighty-seven sows [average parity: 1.49±0.50; body weight on lactation day 9: 195.5±14.9 kg] were recruited to evaluate the apparent utilization of dietary Met sources for N retention during lactation. On day 9 of lactation, sows were assigned to one of six dietary treatments (n=~15): [1] 70% of estimated standardized ileal digestible (SID) methionine + cysteine (Met+Cys) requirements provided via whole-protein sources (0.34% SID Met+Cys), [2] and [3] 80 and 90% SID Met+Cys requirements supplied by DL-methionine (DL-MET), [4] and [5] 80 and 90% SID Met+Cys requirements supplied by the hydroxy-analog of methionine (HMTBa; equimolar), or [6] 110% SID Met+Cys requirements supplied by whole-protein sources and DL-methionine (CON). In all diets, no crystalline cysteine was included. All other indispensable amino acids were supplied at estimated requirements. Sows received dietary treatments between days 10 and 20 (weaning) of lactation, and feed was offered according to a standard feeding curve. Litters were standardized to 13±2 pigs within 48 h of farrowing. A N-balance was conducted between days 16 and 19±2 (peak) of lactation using total urine collection and fecal grab sampling. Milk N output (g/d) was calculated using estimated milk yield based on litter growth rate and size and analyzed milk N concentration. Contrast statements were used to assess linear effects of increasing dietary Met within source, and all treatments were compared to CON via a Dunnett test. Dietary Met source did not influence sow average daily feed intake or body weight and back fat thickness changes between days 9 and 20 of lactation, but piglet average daily gain increased as dietary SID Met content increased for both DL-MET (224 to 244±7 g/d; P<0.05) and HMTBa (224 to 251±9 g/d; P<0.05), with only 90% HMTBa not different from CON (277±11 g/d). During the N balance period, sow N intake tended to increase with SID Met for both DL-MET (162.4 to 170.3±5.4 g/d; P=0.098) and HMTBa (162.4 to 169.8±4.2 g/d; P=0.075). Whole-body N retention (N intake – N output in urine and feces) increased with increasing SID Met content for both DL-MET (100.5 to 112.1±3.7 g/d; P<0.05) and HMTBa (100.5 to 110.8±4.5 g/d; P<0.05), but all were less than CON (123.5±4.3 g/d; P<0.05). The N output in milk increased with SID Met for both DL-MET (69.3 to 87.7±3.7 g/d; P<0.05) and HMTBa (69.3 to 86.0±4.7 g/d; P<0.05), but only the 70% diet was less than CON (94.2±4.3 g/d; P<0.05). Neither dietary Met source nor dose influenced maternal N retention (overall average 25.5±6.4 g/d) or apparent N utilization efficiency (N retention/N intake × 100; overall average 65.2±4.1%). Therefore, both DL-Met and HMTBa were used effectively to improve N utilization for milk production during peak lactation when SID Met+Cys was provided below estimated requirements.