Feeding Grasses to Dairy Cows
Nutritional value of grasses for dairy cow diets
In our last article, we explored the increasing interest in grass production and some of the agronomic aspects that drive this interest. In this article, we will present some aspects related to the nutritional value of grasses for dairy cow diets.
Professor David Combs, of the University of Wisconsin, explains that diets formulated with excellent corn silage may have two problems: they may be low in fiber and at the same time they may be too high in non-fibrous carbohydrates (NFC - this includes most nutrients that ferment in the rumen, like starch and sugars). This combination can have negative consequences in terms of rumen health, namely acidosis or displaced abomasum. Alfalfa can be used to compensate for these problems, but often, high quality alfalfa is also very low in fiber. So how can we correct this?
There are a variety of approaches, but these normally rely on the use of fiber sources, either from other forages (like hay or straw) or from fibrous concentrates (like beet pulp, cottonseed or soy hulls). Straw is a very efficient way to reduce the level of rumen available carbohydrates in the diet, but for high production cows, it may end up being a limitation to milk production. However, in some diets, it may be the most efficient feed to help increase cow chewing and improve rumination. On the other hand, non-forage fibrous sources, like cottonseed or soy hulls, can be of great help in these diets. However, they may be expensive and/or not available and/or difficult to store.
This is where high quality grass has its place. As Table 1 indicates, on average, grass has much higher levels of fiber (NDF) than alfalfa (56.7% versus 43.7% in Table 1), but at same time the digestibility of that fiber tends to be higher in grasses than in legumes (63.3% versus 51.5% in Table 1).
Grasses can complement low fiber, high starch diets, so its value may be higher than simply comparing the numbers in the table above. What this means is that in this type of diet, we can replace some of the corn silage and some of the alfalfa with grass with some nutritional advantages.
Early maturity grass contains higher proportions of NDF than corn silage or alfalfa. Grass NDF is more digestible than alfalfa NDF, meaning that although when adding grass to the diet the total fibre of the diet increases and the amount of rumen available carbohydrates decreases (as was the objective), the digestibility of the diet will remain more or less the same (when we use only straw we may reduce diet digestibility).
If done carefully and with quality grasses, this replacement may allow us to end up with healthier diets, without loss of production.