Low-starch, low-forage diets can support normal rumen function

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Many factors, including ethanol production, have led to lower starch rations using more non-forage fiber sources. We know that lower starch does not generally lead to problems in the rumen, but the combination of low starch and lower forage has been relatively unexplored. This trial looked at the effect of feeding a lower-starch diet with different amounts of forage.

The starch level maintained in all diets was 21%. Some higher forage some lower. They monitored milk production, cud chewing, rumen fermentation, microbial N yield, and total-tract nutrient digestibility.  Forages were a mixture of corn and hay silages. As forage in the diet was reduced, chopped wheat straw was added in an effort to maintain cud chewing. Diets were adjusted to maintain similar amounts of starch, total carbohydrate and protein fractions. The cows were split into four groups, with forage being 52%, 47%, 43% and 39% of ration dry matter.

As expected, the lower forage group had a higher dry matter intake. However, reducing the forage in the diet did not change milk production, yield or composition.  Cud chewing did vary and was higher for the higher straw ration. Interestingly, reducing the forage percentage of the diet did not affect rumen pH, ruminal digesta volume and mass or rumen mat consistency or microbial N yield. However, Ruminal acetate-to-propionate ratio was reduced, turnover rates of NDF and starch were greater, and total-tract digestibility of fiber was lower as dietary forage content decreased.

This study shows that decreasing the forage of a diet while increasing the straw to maintain cud chewing is a viable option to maintain similar milk production with a lower starch diet. The cows fed 39% forage diet did consume 6.6% more dry matter than the cows fed 52% forage. This short-term study shows the minimal forage content needed to maintain rumen performance was between 39% and 43% of dry matter. It also shows that lower forage diets with lower starch content represent a viable strategy for feeding high-producing dairy cows under conditions of expensive or limited supplies of corn-based grain and high-quality forages.

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