During the dry period, dietary levels of starch only need to be relatively modest, at best to provide some readily available energy for the rumen microbes whilst allowing some adaptation of the rumen microbes. Based on price, availability, as well as overall performance however, farmers recognise the value of including starch in lactation rations and are keen to introduce relatively high levels as soon as possible after calving. On this basis, including a small amount of the lactation ration prior to calving is advisable, as this will allow development of the rumen papillae as well as the rumen microflora prior to the cow being fed increased levels of starch in the lactation ration.

Starch occurs principally in cereals and some roots (eg potatoes) and most starch sources will be almost completely digested in the alimentary tract, provided some pre-processing has occurred where necessary. Furthermore, the rumen is the principal site of starch digestion, although digestion rate is affected by both chemical and physical form of starch. It follows that the amounts of starch escaping rumen digestion are usually modest, but can be affected by type of starch fed.

When high levels of rapidly degradable starch are fed, the ruminal production of volatile fatty acids (VFA) increases dramatically, with an associated increase in rumen VFA concentrations, due to VFA production rate exceeding the rate of VFA absorption from the rumen. As a consequence, rumen pH falls and below pH 6.0, fibre digestion rate will be reduced due to negative effects on rumen fibrolytic microbes. Furthermore, such dietary regimes are often characterised by increased levels of lactic acid which is a stronger acid then any of the major VFA, thus increasing the acidifying potential of the rumen.

Below pH 6, cows are considered to be suffering from subclinical acidosis, which may be manifested in general feed inappetence and mild diarrhoea. If not treated, conditions are likely to deteriorate further and clinical acidosis is generally assumed to occur at pH levels below 5.5. In this case, rumen digestion will have almost ceased, rumen contractions will be virtually non-existent and the cow will be totally off–feed and showing few signs of rumination. If such conditions are not dealt with rapidly, the cow may die, most probably due to the invasion of gut microbial toxins through the compromised intestinal wall of the cow.

Avoidance of all forms of rumen acidosis rely on improved nutritional management. Dairy cows can cope quite successfully with relatively high levels of starch in the total ration, but only if the form and rate of delivery of that starch to the cow are controlled. Feeding high levels of high starch containing concentrates in the parlour is not advisable as this will undoubtedly accentuate the problem. An obvious alternative is to include a significant proportion of the starchy feeds along with the forage components in a well-mixed ration. Even here however it is advisable to balance the forms of starch in the ration with respect to rapidly and slowly degradable sources but provided this balance is correct it is possible to feed relatively high levels of starch (24-26% DM basis), assuming that overall levels of sugar in the ration are not excessive. It is important to ensure that the ration is well mixed in order that cows cannot select the cereal components in preference to the forage, whilst ensuring adequate feed space for all cows, thus avoiding excessive bouts of feeding with large sized single meals.

Adopting this strategy and at the same time ensuring that the transition ration contains some starch prior to introduction of the full lactation ration should minimise the incidence of acidosis, whilst the prophylactic inclusion of sodium bicarbonate can be considered in some instances. It is noteworthy that the freshly calved cow is particularly prone to acidosis and may in part be attributable to a compromised mineral status at this time which prevents adequate buffering of the rumen. In studies at CEDAR to examine rumen acidosis, when rumen fistulated cows were fed the same amount of a starch rich ration, those cows with higher milk yields were less able to control rumen pH resulting in an overall difference of over 0.5 pH units.

Author: Denis Dreux