Rape forage is a valuable fodder for ruminants that provides green feed during summer, autumn and early spring when grasses are not available. Rape forage helps sparing grass hay or silage. However, due to several potential constraints, some precautions should be taken before introducing cattle to this forage. Animals should get used to eat rape forage gradually. This can be done through mixing the rape forage with other forages (Johnston, 2009). After the accustoming period, livestock should be offered sufficient dry feeds like hay or straw and free access to water (GNIS, 2019; Limagrain, 2019). Rape forage should not represent more than 35-50% of total DM intake (Limagrain, 2019).
Digestibility and degradability
Rape forage is highly digestible. In vitro DM digestibility values reported for different varieties ranged from 87 to 91% (Pearce et al., 1991; Lancaster et al., 1990; Guillard et al., 1988, Jung et al., 1988). In vivo OM digestibilities reported by different authors ranged from 73 to 90% (Dumont et al., 1978; Alibes et al., 1990; Emile et al., 1993; Sun et al., 2015). This high digestibility could result in acidosis if the ration is not adequately balanced with roughage (Meslier et al., 2014).
Cattle
In France, rape forage can be a valuable grazed resource for cattle. Strip-grazing of rape forage is recommended in order to prevent trampling. Dairy cows should be allowed to graze 20 m²/day to receive 25% to 33% of their daily ration. Grazing rape forage provides good supplement to maize silage and it can help saving 1 kg concentrate/day (Sagot, 2012). In Australia, it is recommended to supplement dairy cows with high fibre sources like grass or maize silage, kikuyu (Pennisetum clandestinum), or paspalum (Fulkerson, 2008). Dairy cows should preferably graze rape forage after the afternoon milking as the nitrate content of rape is lower and the sugar content is higher. They should never graze rape forage less than 2 hours before milking because of the risk of milk taint (GNIS, 2019; Fulkerson, 2008).
Milk thiocyanate content was shown to increase for dairy cows fed on about 50% rape forage. This higher thiocyanate content was shown to have goitrogenic activity on rats fed on milk only. This could be potentially an issue for newborns fed only on milk from cows grazing high-thiocyanate rape forage (Grongnet, 1982).
Sheep
Rape forage is not very palatable to sheep because of the cabbage taint. Neither gradual introduction to rape forage nor restricted feeding (strip-grazing) are necessary provided sheep have been fed prior to entering the stand. After a while, rape forage is eaten more readily and no concentrate is required (Sagot, 2012). In Australia, in a high-rainfall zone, dual-purpose rape forage grown during winter to feed Merino growing lambs (hoggets) was eaten readily and the sheep had 210 g/day growth rate (Kirkegaard et al., 2008).
In the UK, finishing lambs grazing on rape forage gained 273 g BW/day without requiring other forage. This reduced their concentrate needs dramatically in comparison to lambs fed on concentrates (5 kg vs. 66 kg per kilogramme of meat). For better economical returns, it was recommended to feed finishing lambs and culled ewes on rape forage (AHDB, 2015). In a comparative experiment, growing lambs were fed either on rape forage or on irrigated grass pasture. Growth rates for both diets were similar during the first 6 weeks, but after that the lambs fed on grass pasture had lower weight gains. At slaughter, lambs fed on rape forage were heavier and fattier, the carcass had higher pH, the meat had significantly lighter colour but there was no significant difference in cooking loss. Overall meat quality based on objective assesssments was the same for both groups of lambs but sensorial analysis through panellists detected a stronger, less acceptable flavour from the meat of rape-fed lambs (Hopkins et al., 1995).