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 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 v. 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 synthesis. Blood urea nitrogen 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 the 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.

Starchy grain is usually supplemented to diets containing low-quality forage to provide sufficient energy for ruminant animals. Ruminal degradation of grain starch mainly depends on the hydrolysis of the endosperm, which may be variable among grain sources. This study was conducted to investigate the influence of endosperm structure of wheat and corn on in vitro rumen fermentation and nitrogen (N) utilization of rice straw. The 3×4 factorial design included three ratios of concentrate to forage (35:65, 50:50 and 65:35) and four ratios of wheat to corn starch (20:80, 40:60, 60:40 and 80:20). The endosperm structure was detected by scanning electronic microscopy and a confocal laser scanning microscopic. An in vitro gas test was performed to evaluate the rumen fermentation characteristics and N utilization. Starch granules were embedded in the starch–protein matrix in corn, but more granules were separated from the matrix in the wheat endosperm. With the increasing ratio of wheat, rate and extent of gas production, total volatile fatty acids, and ammonia N increased linearly (P<0.01), but microbial protein concentration decreased (quadratic, P<0.01), with the maximum value at a ratio of 40% wheat. The efficiency of N utilization decreased linearly (P<0.01). Rumen fermentation and N utilization were significantly affected by the concentrate-to-forage ratio (P<0.01). Significant interactions between the concentrate-to-forage ratio and the wheat-to-corn ratio were detected in total volatile fatty acids and the efficiency of N utilization (P<0.01). In summary, the starch–protein matrix and starch granules in the wheat and corn endosperm mixture play an important role in the regulation of rumen fermentation and N utilization under low-quality forage.