Rape digestability

Recent research by the FutureDairy project has shown that forage rape (Brassica napus) holds its nutritional quality for more than three months after sowing, giving dairy farmers greater flexibility for grazing the crop. Forage rape, or brassica, is a useful way to fill the usual autumn feed gap on Australian dairy farms, due to its rapid growth, high quality, high potential yield, low establishment costs and efficient use of water and nitrogen. Grazing usually starts about seven weeks after sowing, when the plant has more then eight mature leaves. Cows’ access to forage rape must be restricted to a couple of hours a day to prevent nitrate poisoning. Restricting access also prevents over‐grazing and optimises regrowth. Ideally, cows should graze a new strip of forage rape every day once grazing starts. The area sown should be big enough to allow for about eight weeks of grazing (sow about 5ha/100 cows).   In most species, the nutritional quality will fall over a two month grazing period due to the normal maturation process which results in more stems and fewer leaves. Stems are more fibrous and have a lower nutritional value than leaves. FutureDairy researcher Dr Ravneet Kaur found that the nutritional quality of forage rape does not fall as rapidly with maturity as other plants, including perennial ryegrass. As it matures, forage rape continues developing new leaves which form the bulk of feed grazed by cows. Dr Kaur found that the rumen digestability was relatively similar for forage rape plants from seven to 13 weeks after sowing. “This higher nutritional value of forage rape, compared with perennial ryegrass, adds further value to the role of forage rape in filling the autumn feed gap in dairy systems,” said Dr Kaur. “Because forage rape holds its quality for longer, dairy farmers have more flexibility with the timing of grazing,” she said.