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Description: The rate and extent of ruminal digestion of feed constituents such as DM, OM, CP, or fiber components can be measured in sacco by digesting feedstuffs for various lengths of time in small dacron bags of known pore size that are placed in the rumen of fistulated animals. In this image, dacron bags are being inserted into the rumen of a dairy cow through a cannula in the left flank of the animal. The animal's head is thus on the left behind the researcher and the tail is to the right. (See also the image of a fistulated cow elsewhere in this gallery.) Duplicate or triplicate bags should be used for each time of fermentation, which could be 0, 2, 4, 8, 12, 16, 20, 24, 36 and 48 hours for concentrates or to determine protein degradability. Since fiber digestion is slower, longer incubation times are required for the digestion of forages, such as 0, 4, 8, 12, 24, 36, 48, 72 and 96 hours. Different researchers have used different frequencies and extents of incubation, different pore sizes, and various non-linear equations to calculate the rate and extent of digestion of feed constituents, with and without allowing for a lag time before digestion commences. Rumen degradation is affected by many factors including, but not limited to, the pore size of the bags, between animal variation, and the diet being fed, of which the last two factors affect the rumen environment and rate of passage. After incubation all the bags must be thoroughly washed to remove rumen fluid, rumen microorganisms, and the soluble products of digestion. The zero time bags are washed without incubation to estimate the water-soluble components, which are assumed to be instantly degraded. Unfortunately, washing the bags at zero time or after incubation also removes small particles that have not been digested yet, which causes an error in the parameters that will be calculated. In addition, washing does not remove all the microorganisms that are physically attached to the particles during digestion. This constitutes a relatively small error in the calculation of DM or fiber degradation but is of much greater consequence in determining protein degradability. If all the bags are inserted at zero time then bags must be removed after each time of incubation for washing before being dried, weighed, and the contents analyzed. If the bags are inserted in reverse order and left to ferment for the requisite time then all the bags can be removed together after 48 or 96 hours and washed in a washing machine before drying. The amount of soluble feed components, the amount of digestible insoluble feed components, and their rates of digestion can be determined by the use of appropriate models in non-linear regression. Digestion must be determined in multiple animals because of between animal differences in rumen environment. Ideally, when comparing feedstuffs they should be digested at the same time in the same animal to remove these differences. It is important that the bags do not become entangled as this can interfere with access to the bags by rumen fluid and rumen microorganisms, which reduces digestion. Different methods have been devized to prevent entanglement. In the image above the bags were tied to slits in a length of plastic tubing to maintain separation. Ideally, the bags should only be used once because of changes in their pore size with each use.

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