The objective of this study was to determine the impact of protein supplementation on dry matter intake and ruminal hydrogen sulfide concentration ([H₂S]) in lambs consuming low-quality tropical grass hay and drinking low salt water (LS) or high salt water (HS). Twenty lambs in individual cages were used in a 10 treatments by 2 period (10 × 2) trial. First factor was water quality (WQ): LS (466 mg/kg TDS) vs. HS (8927 mg/kg TDS of which 6680 mg/kg as sulfates). Second factor was 5 levels of soybean meal (SBM; %BW/d). Water quality × SBM interactions were significant for forage (FOMI; P < 0.05) and total OM intake (TOMI; P < 0.05), but not for water (WI; P = 0.60) and sulfur water intake (SWI; P = 0.14). At the higher levels of SBM (>0.5%) there was no statistical difference for FOMI and TOMI. Water intake was not affected by WQ (P = 0.39) but increased linearly (P < 0.01) in response to SBM. Ruminal H₂S concentration was affected by WQ × SBM interaction (P < 0.01). Our results suggest further research is needed on the water quality × protein interaction to understand the mechanisms involved in H₂S production, to generate efficient supplementation strategies on low-quality forages. Table 1. Effects of SBM supplementation and WQ on sulfur water intake (SWI) and ruminal H₂S concentration in lambs fed low-quality grass hay

Water quality	SBM	SWI, mg/kgBW0.75		H2S (ppm)1
Low salt	0	5.10		1.23 (16.98)
	0.25	4.90		1.19 (15.49)
	0.50	5.26		1.24 (17.38)
	0.75	5.79		1.20 (15.85)
	1	6.48		1.70 (50.12)
High salt	0	328.67		1.59 (38.90)
	0.25	278.49		2.69 (489.78)
	0.50	359.00		3.32 (2089.30)
	0.75	383.06		2.99 (977.24)
	1	454.38		2.98 (954.99)
SEM		33.74		0.20
Effects		P-value
WQ		<0.01		<0.01
SBM		0.20		<0.01
WQ × SBM		0.14		0.01

¹Means were back transformed.