Given carbohydrate and protein account for most of the nutrients that are digested in the rumen, it follows that in those situations where rumen function is optimal, between 85-90% of the total nutrients digested in the rumen should ultimately be either absorbed as VFA across the rumen wall, or transferred as microbial biomass for subsequent digestion in the small intestine.  Where efficiency is compromised, the usual cause is either insufficient or excess protein. In the former case, the extent of carbohydrate digestion in the rumen will be reduced with a concomitant decline in feed intake, whilst in cases of excess nitrogen supply, as much as 30% of the ingested nitrogen may be absorbed as ammonia from the rumen. Effects of this magnitude have been noted with fresh forages containing high levels of protein and provides evidence that in such situations supplementation with a suitable energy source could be beneficial.

The rumen is also a major site of starch digestion but both rate and extent of digestion will be influenced by a number of important factors. These clearly include the chemical characteristics of the starch where two types, amylose and amylopectin are known to have contrasting digestion profiles. As cereal crops mature, generally there is an increase in the level of amylopectin at the expense of amylose, with the former having a lower rate of degradation. With the exception of sheep, cereal grain need to be processed before they are fed in order to disrupt the seed coat and allow access to the rumen microbial enzymes. This can be achieved by grinding or rolling of dry or moist grains or in the case of cereal silages (whole crop or maize) by use of in machine grain crackers or a dedicated mill. However, the process of seed coat disruption does not need to be extensive and very finely ground grains should be avoided as these will accelerate the rate of starch digestion in the rumen – and may predispose the animal to rumen acidosis. In such situations, there is opportunity to treat grains with sodium hydroxide in the preparation of soda grain. This will effectively remove the seed coat and allow total digestion of the starch contained within the treated grain at controlled rates. It is imperative however when considering the inclusion of different starch containing feeds into rations, that their likely degradation characteristics are taken into account, thus allowing balanced rations containing both rapidly and slowly degrading sources of starch to be used.

There has been much conjecture about the value of ruminal protected starch that will by-pass the rumen and contribute directly to small intestinal digestion. Clearly, this can be seen to be advantageous in that it will supply glucose directly to the animal and thus avoid the attendant losses, albeit relatively small, associated with the conversion of starch in the rumen to propionate followed by the subsequent conversion of absorbed propionate in the liver to glucose. However, there are no specific feeds which contain high levels of ruminal protected starch and previous attempts to produce these on a commercial scale have not been successful. Thus in almost all situations, the rumen is likely to remain as the principal site of starch digestion. With the relatively low levels of starch consumed, when compared with a whole tract starch digestibility approaching 100%, ruminal starch digestion will account for over 95% of this achievement.

At higher levels of starch inclusion, provided rumen function is not compromised, ruminal starch digestion will still account for in excess of 90% digestion which occurs in the whole tract, which in turn should be of the order of 97%+. Even at starch intake levels approaching 7kg/day which have been measured for cows consuming in excess of 24kgDM/d of rations containing as much as 28% starch, the amount of starch entering the small intestines is unlikely to exceed 1.2 to 1.5kg/d, indication at least 80% of ingested starch has been digested prior to the small intestine. In this respect, one area of concern has been the effect of maize silage maturity on starch digestion in the rumen. As the maize crop matures, so starch levels will increase to over 30% DM basis and as indicated earlier the nature of that starch will change. This led to the suggestion that more mature maize silage may promote the post-ruminal digestion of starch and a series of in vitro studies carried out in Holland supported this claim. However, studies at CEDAR, which quantified the sites of starch digestion in lactating dairy cows receiving total mixed rations containing equal amounts of maize but provided from harvests of increasing maturity (23 to 38% DM), showed only small increases in the amount of starch which escaped during rumen degradation.

From this, it is concluded that in most situations the post-ruminal digestion of the starch component of maize silage is not increased by crop maturity and should not be considered as important when rations are being formulated. At the same time however, it is highly likely that the ruminal rate of digestion of the starch component of more mature maize silage will be slower than that seen with less mature crops.

Author: Denis Dreux