Effect of starch fermentation in the rumen on voluntary intake of roughage and kinetics of digestion*

Abstract

The effect of starch fermentation in the rumen on the kinetics of roughage digestion, was studied using 12 sheep fed three roughages, viz. lucerne hay, maize cob leaves and wheat straw. The amount of starch infused per day was increased from o to 600 g/d in steps of 20 g/d over 30 days. The amount of starch infused was delivered at two rates; either as the daily amount infused over 12 h, or over 24 h. The well known negative effect of starch fermentation on roughage intake and digestion was observed when the lucerne and maize cob leaf diets were fed, but not when the wheat straw diet was used. The rate at which starch was infused affected the intake of maize cob leaves in a variable manner. The slow infusion rate led to a very small negative effect, whereas the fast rate of starch infusion resulted in a large negative effect. Regardless of diet, the negative effects of starch on intake could not be ascribed to reduced rumen fill, nor to a reduced concentration of rumen ammonia. Furthermore, pH of the rumen contents was not lowered. Rate of passage of  non-fermentable OM was decreased by starch infusion on one diet (maize cob leaves) only, even though the mean retention time (MRT) of water was significantly influenced by starch infusion on all the diets. Although starch fermentation  negatively affected many aspects of roughage digestion, the paramount factor appears to be a reduced rate of roughage digestion.

Die invloed van fermentasie van stysel in die rumen op die verteringskinetika van ruvoer is met 12 skape ondersoek. Drie verskillende ruvoere, nl. lusernhooi, mieliekopblare en koringstrooi is gebruik. Stysel is in die rumen ge"infuseer teen hoeveelhede wat oor 30 dae vermeerder is vanaf 0 tot 600 g/d met stappe van 20 g/d. Die stysel is teen twee tempo's toegedien, sodat die daaglikse hoeveelheid oor 12 h of oor 24 h ge"infuseer is. Die welbekende negatiewe effek van styselfermentasie op ruvoerinname en verteringstempo is waargeneem met die lusern- en mieliekopblaardiete maar nie met die koringstrooidieet nie. Die tempo waarteen die stysel ge"infuseer is het 'n veranderende effek op die inname van mieliekopblare gehad. Feitlik geen effek is waargeneem met die stadige infusietempo nie terwyl 'n groot negatiewe effek waargeneem is met die vinnige infusietempo. Hierdie negatiewe effekte is nie veroorsaak deur 'n verlaging in rumenvulling, of deur 'n te lae rumen-ammoniakkonsentrasie nie. Die pH van die rumeninhoud is ook nie verlaag nie. Die tempo van deurvloei van nie-fermenteerbare organiese materiaal is slegs op een dieet (mieliekopblare) verlaag, alhoewel daar  betekenisvolle effekte op die retensietyd van water by al drie diete waargeneem is. Alhoewel daar baie faktore betrokke was by die negatiewe effek van  kragvoerfermentasie op ruvoervertering, blyk dit asof 'n verlaagde tempo van ruvoervertering die belangrikste faktor was.