Supplementation of cattle fed tropical grasses with microalgae increases microbial protein production and average daily gain.

A series of 3 experiments were conducted to evaluate the use of microalgae as supplements for ruminants consuming low-CP tropical grasses. In Exp. 1, the chemical composition and in vitro protein degradability of 9 algae species and 4 protein supplements were determined. In Exp. 2, rumen function and microbial protein (MCP) production were determined in steers fed speargrass hay alone or supplemented with , , , or cottonseed meal (CSM). In Exp. 3, DMI and ADG were determined in steers fed speargrass hay alone or supplemented with increasing amounts of NPN (urea combined with ammonia sulfate), CSM, or . In Exp. 1, the CP content of and (675 and 580 g/kg DM) was highest among the algae species and higher than the other protein supplements evaluated, and sp. had the highest crude lipid (CL) content (198 g/kg DM). In Exp. 2, supplementation increased speargrass hay intake, the efficiency of MCP production, the fractional outflow rate of digesta from the rumen, the concentration of NHN, and the molar proportion of branched-chain fatty acids in the rumen fluid of steers above all other treatments. acceptance by steers was low and this resulted in no significant difference to unsupplemented steers for all parameters measured for this algae supplement. In Exp. 3, ADG linearly increased with increasing supplementary N intake from both and NPN, with no difference between the 2 supplements. In contrast, ADG quadratically increased with increasing supplementary N intake from CSM. It was concluded that and may potentially be used as protein sources for cattle grazing low-CP pastures.