The effect of <i>Ulva lactuca</i> on <i>in vitro</i> ruminal gas production kinetics

The aim of this study was to determine whether the inclusion of a marine algae found in South African waters, Ulva lactuca, can reduce total gas and methane production in vitro when it replaces lucerne hay in a mixed sheep feed at incremental levels. Four treatments were prepared and incubated using bovine rumen fluid as inoculum: (1) 0 g U. lactuca kg-1 feed dry matter (DM) (0U), (2) 25 g U. lactuca kg-1 feed DM (25U), (3) 50 g U. lactuca kg-1 feed DM (50U), and (4) 100 g U. lactuca kg-1 feed DM (100 U). Total gas and CO2 production was determined with the aid of an automated system and methane production was estimated by difference. Cumulative gas production data were fitted to a non-linear model (Y = b(1- exp-c(t-L))) to estimate values for total gas production (b, mL), rate of gas production (c, mL/h), and a discrete lag time (L, hours). The extent of total gas production was lower for 100U than for 25U, but neither differed from 0U or 50 U. The lag time observed was lower for 50U than 0U, but neither differed from 25U or 100U. No differences were found for the rate of gas production. No differences for any gas production values were observed between treatments. The ratio between methane and total gas production was highest for 100U, which differed from 25U, but not from 0U or 50U. The higher methane ratio observed in the 100U treatment may be attributed to the lower total gas production in this treatment due to the lower fermentability of U. lactuca compared to lucerne.