In vitro fermentation and in situ rumen degradation kinetics of summer forage brassica plants

The aim of the present study was to assess and compare the nutrient concentration, the in vitro fermentation and the in situ rumen degradation characteristics of Brassica rapa ssp. rapa L. (turnips) and Brassica napus ssp. biennis L. (forage rape). Five varieties of each species were established in three field replicates and were organised in a randomised complete-block nested design. All varieties were harvested and further analysed for chemical composition, in vitro gas-production kinetics, volatile fatty acid (VFA) production and in situ degradation kinetics of dry matter (DM) and crude protein. Turnips showed higher ash, total sugars, raffinose, sucrose, glucose and fructose concentrations (P < 0.001) than did forage rape. Turnip varieties differed in their sucrose, glucose, fructose and total soluble sugar concentration (P < 0.001), whereas rape varieties differed in their neutral detergent fibre concentration (P = 0.004) and digestible organic matter on a DM basis (P < 0.01). Regarding DM-degradation parameters, turnips had a higher soluble fraction 'a' (P < 0.01) and a lower insoluble, but potentially degradable fraction 'b' (P < 0.01) than did rape, but the fractional degradation rate 'c' (0.18/h) was similar to that of rape. Rates of gas production were slightly higher (P = 0.018) for turnip than for rape. No effects for brassica species nor for varieties within species were detected (P > 0.05) for total in vitro VFA production, as well as for the relative proportions of acetate, propionate, butyrate, branch chained VFA and the actetate: propionate ratio. Our study showed that most of the differences that were observed in terms of chemical composition and degradation kinetics did not result in differences in in vitro fermentation products.