The feeding value of forage brassica plants for grazing ruminant livestock

Forage brassicas fed to ruminants include swedes, turnips, kale and forage rape. All have high dry matter (DM) digestibility (0.81-0.89) and metabolisable energy (ME) (12.1-14.1 MJ/kg DM), which are higher than most grass based pastures or legumes. All brassicas contain the secondary S-containing compounds S-methyl-cysteine sulphoxide (SMCO) and glucosinolates. The SMCO is fermented in the rumen to dimethyl disulphide, which causes haemolytic anaemia and depressed voluntary feed intake (VFI). Glucosinolates are decomposed to iso-thiocyanate and nitriles, with the latter having the potential to depress VFI. When grazed by young ruminants as the sole diet, their growth was considerably less than would have been expected with high ME forages, and was variable. Nevertheless, growth of young sheep was higher on forage rape (225 g/d) and turnips (173 g/d) than on swedes (95 g/d) and kale (120 g/d), with swedes and kale having the highest concentrations of SMCO where initial growth was very low. However grazing lambs fed low SMCO kale, induced by low S fertiliser treatment, eliminated the initial 6-wk period of low lamb growth. Nitrate N concentrations in turnip leaves and kale were variable, with some in the range associated with reduced VFI. End products of kale digestion are low in absorbed amino acids in relation to ME, perhaps explaining responses to protein supplementation in sheep fed forage brassicas. In contrast, where brassicas have been fed as a supplement to supply 0.2-0.3 of DM eaten, notably to grazing dairy cows during dry summer conditions, there have been responses in milk yield equal to those with supplementation of chicory and barley grain. Low growth in sheep and cattle fed sole diets of brassicas may be due to combinations of degradation products from secondary compounds, low amino acid absorption in relation to ME, induced trace element deficiency in long term studies and perhaps high nitrate N concentrations in some crops. SMCO and glucosinolate concentration in kale, the highest yielding forage brassica, have heritabilities of 0.24 and 0.48. Low glucosinolate lines have been produced and should be evaluated as the sole diet of ruminants. It has not proven possible to breed low SMCO kales to date, but single trait selection is suggested for future research. Low S and N fertiliser treatments can reduce SMCO and nitrate N concentrations in kale. Protein supplementation should be evaluated in young ruminants fed sole diets of all brassicas and careful adaption to sole diets of forage brassicas over 5 wks is recommended.