Fermentation and Microorganisms in the Rumen and the Content of Fat in the Milk of Cows Given Low Roughage Rations

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Abstract
In five lactating fistulated cows the effect of progressively substituting flaked maize for part of the hay and concen- trates in a high roughage ration (8 kg hay; l0 kg dairy concentrates daily) on the rumen fermentation, tureen micro- flora, and fat content of milk was investi- gated. With 1 kg/day of hay, the amount of concentrates and flaked maize required to depress milk fat below 2.0% varied from 1 to 6 and from 4 to 9 kg/day, respec- tively. Cows which developed high-buty- rate fermentation during the change of ration maintained a normal milk fat con- tent. To assess the influence of rumen fermentation on milk fat content, molar proportions of butyric as well as acetic and propionie acids must be taken into account. There were low numbers of cellulolytic and fiber-digesting bacteria and large numbers of lactic and propionie acid- producing bacteria in the tureen of cows given rations that depressed milk fat. However, there were particularly large variations between eows in the propor- tions of individual laetogenie or propion- ogenie genera and in the numbers of ciliate protozoa. Unstable microbial populations devel- oped which may have caused variability in rumen fermentation and in milk fat content.

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