David L. Morris: Vet Column 2-7-11
Fort Collins, Colo.
Goats are ruminants as are sheep and cattle. Just because they are ruminants, however, does not necessarily mean that what is known about sheep and cattle diets is equally applicable to goats. Goats have different feeding behavior, intake, diet selection, and rate of eating from other ruminants. To measure some of these differences, investigators from China recently published the results of a trial investigating the effect of physically effective fiber on chewing activity, rumen fermentation, and digestibility in goats.
Sometimes referred to as “scratch factor,” physical form and/or particle length of ruminant diets is important to maintaining rumen health. A common health and production problem associated with diet occurring in ruminants is subacute ruminal acidosis, or low pH within the rumen. Diets low in fiber can contribute to such and is often a result of too much grain. This can be both a quantity of feed consumed as well as the physical form of the feed. Feed with shorter particle sizes usually results in reduced chewing time and an associated lowered rumen pH or acidosis.
To create the experimental diets, the investigators used alfalfa hay as the forage source for testing. Alfalfa hay was cut into 20 mm (around 3/4 of an inch) lengths and used as the forage component involving a ration of 45 percent concentrate (89 percent corn; 8.4 percent soybean meal; and minerals) and 55 percent alfalfa hay. From this initial proportioning of alfalfa hay to the diet, four experimental diets were developed that varied in the physically effective fiber of the alfalfa hay contribution. The original amount of 20 mm lengths of alfalfa hay was further processed by additional chopping and grinding to create low, moderately low, moderately high, and high levels of physically effective fiber. By passing the resulting alfalfa hay and mixed diets through a sieve designed by Pennsylvania State University, uniformity of particle size distribution was maintained.
In looking at feed left by the goats, the particle length was less than 8 mm. This suggests that long particles were preferred by goats in this study. A previous study of goats by other authors similarly indicated a preference to consume coarse particle feed to fine or very fine particle feed. This was consistent with a similar finding in sheep that ingestion of long fibers enhanced rumination.
Similar to other studies, the present study revealed that as particle size increased, the dry matter intake decreased. Reduced particle size of the diet decreased the amount of rumen fill with forage, increased rumen passage, and resulted in overall increased dry matter intake.
With regard to chewing time, this study reinforced the results of previous studies that found that increasing dietary physically effective fiber resulted in increased ruminating time. Correspondingly, increased ruminating time was associated with increased chewing time. Increased chewing time was associated with increased saliva buffering resulting in improved ruminal pH status. This study also found that long fiber in goats can stimulate extensive chewing in goats as compared to dairy cows.
This study reinforces the importance of balancing the amount of concentrate and forage in goat diets. Perhaps more importantly, the source of forage may not be as important as the amount of fiber contained in the forage source as presented to goats.
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