The addition of 1.5 kg dry matter (DM) heat-treated full-fat rapeseed (HR), 1.5 kg DM untreated full-fat rapeseed (UR) or 0.7 kg tallow to a hay/concentrate mixed ration (12 kg DM) fed to dairy cows was studied for its effects on rumen metabolism and milk production in a 3 x 3 Latin square experiment. Rumen digestibilities of organic matter and neutral detergent fibre tended to be lower when tallow was fed than when either of the rapeseed forms were included in the diets. However, there were no differences in the total digestibility of organic matter (74-76%) and neural detergent fibre (63-65%) between treatments, indicating a compensatory digestion of fibre in the hindgut. Duodenal flow of nitrogen (N) tended to be inversely related to fibre digestibility. Duodenal flows of microbial N were the same for all treatments. Increased dietary fat did not affect the numbers of protozoa in the rumen, but did decrease the numbers of methanogenic bacteria when compared with those in the rumen of a dry cow fed a low-fat ration. A larger proportion of the fat in the rumen appeared as triglycerides and less as insoluble soaps when HR was fed than when either UR or tallow were fed. The fatty acid composition of the esterified, non-esterified and soap fractions in the rumen was almost the same for all treatments. The treatment effect was highly significant for total fat digestibility. The lowest digestibility was obtained when HR was fed and the highest when tallow was fed. The fatty acid composition of milk was affected by the inclusion of full-fat rapeseed. The proportions of C18:0 increased and those of C16:0 and C16:1 decreased in the milk fat with both rapeseed diets in comparison to the tallow diet. Generally, the differences between the two forms of rapeseed were small.
