Development of rumen metabolism and epithelium in heifers during transition to pasture

Abstract

After turn-out to pasture, total volatile fatty acid (VFA) concentrations in the rumen contents of heifers significantly decreased from 107.7 to 88.7 mmol.l-1 (p < 0.01). After subsequent increase the total VFA concentrations reached their highest value (117.0 mmol. l -1) in the 8th week of grazing. The proportion of the molar acetic acid concentration in the rumen content increased insignificantly whereas that of propionic acid decreased insignificantly. In the molar proportion of butyric acid no significant differences could be stated. The energetic yield of VFA production in the rumen of heifers decreased insignificantly from 73.6 to 72.7 %, the acetate : propionate ratio revealed an insignificant increase from 3.66 to 4.18. Cellulolytic bacterial counts in the rumen contents significantly decreased from 8.08 to 7.61 log10.ml-1 (P<0.01) and then a significant increase to 8.39 log10.ml-1 was observed again in the 3rd week of grazing (P<0.05). During pasture, a significant increase in the counts of lactate-utilizing bacteria was recorded. As to the numbers of lactobacilli, a significant decrease (P<0.05) with a subsequent insignificant increase during grazing could be observed. Scanning electron microscopy revealed the surface of the rumen papillae during winter feeding to be smooth, cocci presenting the dominant morphotype of adhering rumen microflora. In the first phase after turning out to pasture (week 4) roughened sites on the sides and base of the papillae as well as an increase in the free surface of the epithelial cells could be observed. The numbers of adhering bacteria were increased and none of the morphotypes prevailed. In the second grazing cycle (week 8) circular depressions became visible on the larger epithelial cells which were colonized by high numbers of adherent rumen bacteria.