The present study was conducted to assess the inclusion of liquid protein feed (maize steep liquor; MSL) in growing lambs fed low quality forage (400 g/kg wheat straw, dry mater [DM] basis) compared with two protein sources (soybean meal; SBM, and cottonseed meal; CSM). Eighteen Male Farahani lambs, average body weight 36 ± 3.3 kg, were allocated to individual pens for 9 weeks. Three protein sources were; 1) MSL; 2) SBM and 3) CSM. Feed intakes did not differ among treatments. Growth rate and feed conversion ratio were improved in SBM-fed lambs. Nitrogen efficiency was improved in MSL vs. CSM-fed lambs. Digestibility of fibre was enhanced in lambs fed SBM diet. Ruminal short chain fatty acid was highest in lambs fed SMB and lowest in lambs fed CSM. The urinary allantoin concentration was greater in SBM-fed lambs, with improved microbial crude protein (MCP) synthesis. Blood urea nitrogen (BUN) tended to be reduced in SBM-fed lambs. Dressing percentage improved in SBM-fed lambs. Lambs fed with SBM also grew faster and more efficiently than lambs fed either of the other two diets. However, regardless of the positive effects of SBM on performance and ruminal fermentation profile observed in the current study, the MSL diet could be a profitable protein source in comparison to CSM in growing lambs fed low quality forage. In conclusion, the present study indicates a positive outlook for feeding a liquid protein source to growing lambs fed low quality forage such as wheat straw.
