Effect of protein supplementation on tropical grass hay utilization by beef steers drinking saline water

An experiment was conducted to assess the impact of increasing levels of supplemental soybean meal (SBM; 45.7% CP) in cattle consuming tropical grass hay (Panicum maximum cultivar Gatton; 7.0% CP and 81.8% NDF) and drinking low salt water (LS) or high salt water (HS). Six ruminally fistulated beef steers (BW = 375 ± 43 kg) were used in a 6-treatment, 4-period crossover experiment. Treatments were arranged as a 2 × 3 factorial, with 2 levels salt in the water (LS and HS: 786 and 6,473 mg/kg of total dissolved solids [TDS], respectively) and 3 levels of SBM (0, 0.2, and 0.4% BW/d). After 15 d of adaptation to treatments, periods consisted of 5 d for intake and digestibility determination, 1 d for monitoring ruminal fermentation, 1 d for ruminal evacuation, and 1 d for blood sampling. Supplemental SBM × water quality interactions were significant (P < 0.05) for most measures of intake, except for total tract digestible OM intake (P = 0.38) and total tract digestible NDF intake (TTDNDFI; P = 0.32). At greater levels of SBM, forage OM intake, NDF intake, and water intake seemed to reach a plateau in LS while this was not observed in HS. Total tract digestible OM intake increased linearly (P = 0.01) and TTDNDFI tended to increase (P = 0.09) in response to increased SBM. Digestibility of OM and NDF were not affected by treatment (P > 0.21). Passage rate of acid detergent insoluble ash linearly increased (P < 0.01) in response to SBM, although it was not affected by water quality (P = 0.98). Total VFA concentrations and ruminal pH were not affected (P > 0.60 and P > 0.31, respectively) by treatment. Ruminal ammonia N levels were linearly increased by SBM supplementation (P < 0.01) but were not affected by water quality (P = 0.25). However, ruminal ammonia tended (P = 0.09) to be greater in HS at 0.2% of SBM supplementation. No interaction was observed for plasma urea N (PUN; P = 0.20). Plasma urea N was affected by SBM supplementation (P = 0.05) and water quality (P < 0.01). However, PUN did not differ for 0.4% SBM supplementation (P = 0.30) either at LS or HS treatments. In conclusion, a high level of SBM supplementation (0.4% BW) counteracted the detrimental effect of high TDS in drinking water on low-quality forage consumption by cattle.