A total of 90 growing barrows (Camborough Plus × C3378; PIC Canada, Ltd., Winnipeg, MB) with an initial body weight (BW) of 20.4 ± 0.5 kg were used in an N-balance experiment at the Prairie Swine Centre, Inc. (Saskatoon, Canada) . Pigs were housed individually in metabolism crates (1.4 m × 1.5 m) in a temperature-controlled room (22 ± 1 • C) and were randomly assigned to 1 of 10 dietary treatments arranged as a 2 × 5 factorial in a randomized complete block design over nine blocks (n = 9 pigs/treatment). The factors were NPN inclusion (no ammonium phosphate (NAP) or supplemented with ammonium phosphate at 1.7% (AP)) and graded levels of standardized ileal digestible (SID) Lys content (0.8%, 0.9%, 1.0%, 1.1%, or 1.2%). The NAP diet was formulated to have an E:T ratio of 0.36 (considered to be relatively deficient in N) and the AP diet to have a ratio of 0.33 (N supplemented). While these ratios are lower than the suggested optimum in swine (Heger et al. 1998) (link), previous ratios were calculated using total AAs and only considering AA-N, ignoring the contribution of NPN (Heger et al. 1998; (link)Heger 2003) . As in our previous study (Camiré et al. 2023) (link), the E:T ratio in the current study was calculated using SID EAA-N to the requirement (NRC 2012) and total dietary N content. As such, all excess EAA-N and all other sources of N are considered as potentially contributing to N supply. Lysine was chosen as the test EAA based on our previous study (Camiré et al. 2023 (link)) and as it is generally first-limiting in diets for swine and is first limiting for lean gain. Diets were formulated to meet or exceed nutrient requirements according to NRC (2012), except for Lys, and contained titanium dioxide as an indigestible marker (Table 1). Only the diets containing the lowest and highest Lys content were milled under standard commercial conditions and then mixed in appropriate proportions to obtain the 0.9%, 1.0%, and 1.1% SID Lys diets (Canadian Feed Research Centre, North Battleford, SK, Canada). Daily feed allowance was provided at 2.8× metabolizable energy requirements for maintenance (determined as 110 × BW 0.75 ) and was adjusted to BW at the start of adaptation and collection periods. Diets were fed in two equal meals per day at 0800 and 1500 h with ad libitum access to water. Feed refusals were collected for each pig daily and weighed to determine daily feed intake.