Two swine feeding trials were conducted (initial body weight = 47 ± 2 and 41 ± 3 kg for Trials 1 and 2, respectively) to evaluate reduced crude protein (CP) and yucca (Yucca schidigera Roezl ex Ortgies) extract–supplemented diets on NH3 emissions. In Trial 1, nine pigs were offered a corn–soybean meal diet (C, 174 g kg−1 CP), a Lys-supplemented diet (L, 170 g kg−1 CP), or a 145 g kg−1 CP diet supplemented with Lys, Met, Thr, and Trp (LMTT). In Trial 2, nine pigs were fed diet L supplemented with 0, 62.5, or 125 mg of yucca extract per kg diet. Each feeding period consisted of a 4-d dietary adjustment followed by 72 h of continuous NH3 measurement. Urine and fecal samples were collected each period. Feeding the LMTT diet reduced (P < 0.05) average daily gain (ADG) and feed efficiency (G:F) compared to diet L. Fecal N concentration decreased with a reduction in dietary CP, but urinary ammonium increased from pigs fed diet LMTT (2.0 g kg−1, wet basis) compared to those fed diet C (1.1 g kg−1) or L (1.0 g kg−1). When pigs were fed reduced CP diets NH3emission rates decreased (2.46, 2.16, and 1.05 mg min−1 for diets C, L, and LMTT). Yucca had no effect on feed intake, ADG, or G:F. Ammonium and N concentrations of manure and NH3 emission rates did not differ with yucca content. Caution must be executed to maintain animal performance when strategies are implemented to reduce NH3 emissions.