Glucosinolates affect palatability, feed intake
Rapeseed is not exactly the most palatable feed ingredient. It belongs to the Brassicaceae family of plants, which it shares with mustard and horseradish. A bit of mustard is fine, but you would not make a whole meal out of it.
What makes these plants so pungent is a family of compounds called glucosinolates, which irritate the mucous membranes of the sinuses and eyes. Those of us who like wasabi know very well how little you need to get the punch. Once exposed to air or heat, glucosinolates lose some of their pungency, darken in color, and develop a bitter flavor. So, rapeseed meal, rich in glucosinolates, can become understandably undesirable at high concentrations. Not so, however, when newer (feed) varieties are used, like those that produce canola, for example. Here we will examine research demonstrating the difference between the two varieties.
In young calves, Ravichandran et al. (2008) fed canola meal versus rapeseed meal with differing levels of residual glucosinolates. Calves fed canola meal (20μmol/g of glucosinolates) had similar feed intake compared with control calves fed diets without canola (1.10 kg vs 1.08 kg/day, respectively). In contrast, calves fed rapeseed meal (> 100 μmol/g of glucosinolates) exhibited severely depressed feed intake (0.76 kg/day). This is not a surprising result as similar findings have been corroborated in other species. Apparently, nobody likes too much glucosinolates on their plate.
So, not all rapeseed meals are the same. Before committing to buying any quantity, it is best to know the source and, if possible, the level of glucosinolates. If too high, and the price is too attractive, then the quantity in question could possibly be fed at very low concentrations over an extended period of time. But, clearly, for high-quality feeds, it is best to opt for the best quality ingredients.