Effect of maintenance and ad libitum intakes on digesta kinetics was studied with six ruminally fistulated cows and six ruminally fistulated wethers to validate the use of sheep as a model of cattle. Complete diets were made up of ratios of alfalfa:cracked corn and soybean meal of 80:20, 55:45, and 30:70. The rate of passage of Cr-mordanted alfalfa and soybean meal in the reticulorumen was negatively related to percentage of concentrate in the diet in both species at low intakes. Passage values of particulate and liquid markers were faster at high than at low intakes in both species for all diets. Rumen liquid volume increased with intake only in the cows on the low and intermediate concentrate diets. No substantial differences were found in particulate passage values between sheep and cattle. However, liquid passage rates from the rumen and the differentials between liquid and particulate dilution rates were higher in cows than in sheep for all diets at both intakes. These results together with those for digestibility data reported in a previous communication suggest that caution should be exercised when extrapolating results from one species to the other.

Mechanisms exist in the ruminant to detect changes in osmolality and volume of plasma during feeding to maintain fluid and electrolyte homeostasis. Feed intake during a meal can be limited by the rise in osmolality of ruminal fluid, which is sensed in the wall of the rumino-reticulum. Ruminal microbes appear to be resilient to the short-term changes in ruminal fluid tonicity, but in vitro growth rates are inhibited when the tonicity of the culture medium is increased beyond physiological levels. Although mixing contractions of the rumen are not inhibited by the normal increases in tonicity of ruminal fluid, time to first rumination is increased. This aspect of motility requires further research. The tonicity of plasma increases toward the end of a large meal as a consequence primarily of absorption of VFA and Na+ from the rumen and fluid shifts into the gut. This hypertonicity is sensed centrally to inhibit parotid secretion by a reduction in the parasympathetic stimulation to the gland. Increases in animal production may result from future research directed toward developing ways of counteracting these negative effects of hypertonicity in body fluids on feed intake and ruminal function.

1. The rates of passage of markers of fluid and particulate digesta through the sheep alimentary tract have been described mathematically in single-injection experiments by analysing the concentration curves of marker excretion in faecal dry matter and then predicting these concentrations by means of an equation containing two exponential terms. Three components related to this equation are a transit time for digesta in the intestines, a half-time of marker in the reticulo-rumen and a half-time tentatively associated with the passage of marker through the caecum and proximal colon. With this approach no surgical interference with an animal is required to obtain meaningful information about gut function.2. Abomasal cannulation did not affect the rate of passage of 51Cr EDTA through the gut.3. The mean half-times (T1/2) for 51Cr EDTA in the reticulo-rumen and hind-gut, respectively, were 646 and 236 min for a roughage diet and 890 and 607 min for a diet consisting mainly of wheat grain, both diets providing approximately the same amounts of digestible dry matter. These results indicated that the changes in half-times were proportionately much greater in the caecum and proximal colon than in the reticulo-rumen. The mean transit time of marker through the digestive tract also increased from 721 to 1345 min when the concentrate diet was given.4. The complex of chromium-51 with EDTA was excreted faster in faeces than cerium-144–praseodymium-144 when both were given together as a single injection into the reticulorumen. This was caused mainly by differential rates of elimination of water and particulate matter from the reticulo-rumen.

Effect of maintenance and ad libitum intakes on digestibility of different feed fractions was studied with six ruminally fistulated cows and six ruminally fistulated wethers to validate the use of sheep as a model for cattle. Complete diets were made up of ratios of alfalfa:cracked corn and soybean meal of 80:20, 55:45, and 30:70. The regression coefficient of the line relating organic matter digestibility with proportion of concentrate in the diet was smaller for the cows at ad libitum intake than for the other groups. Increasing the intake caused a decrease in digestibility of different fractions. The depression in digestibility was greater for the 30:70 forage:concentrate diet than for the others. At high intake, digestion values in the cows were less than those in the sheep for all diets. An increase in intake depressed the digestion of cell wall fractions and cell solubles including starch in cows, whereas in sheep, an increase in intake reduced cell wall digestion and to a lesser extent cell solubles, without affecting starch digestion. The digestive physiology of these species is sufficiently different to preclude the use of sheep data in formulating nutrient requirements for cows.

This study compared the severity of ischemic injury to the equine jejunal mucosa caused by arteriovenous obstruction (AVO) or venous obstruction (VO) with that caused by reperfusion after ischemia. The degree of mucosal damage and regeneration was scored according to a modified version of an established light microscopic classification for ischemic injury. Biopsy specimens taken after 3 and 4 hours of obstruction, and after 3 hours of obstruction and 1 hour of reperfusion, were compared. There were no changes in the severity of mucosal injury (characterized by epithelial sloughing, loss of villus architecture, and necrosis of crypt cells) at 4 hours of ischemia when compared with 3 hours of ischemia. The mucosal injury score increased by one grade in three of six and five of eight segments during reperfusion for the VO and AVO models, respectively; however, only the scores for the AVO model were significantly different from the injury caused by ischemia alone. Modification of reperfusion injury was attempted by the administration of intravenous (IV) allopurinol, dimethyl sulfoxide (DMSO), or intraluminal oxygen insufflation at the time of release of the AVO and VO. Treatments did not significantly alter either the severity of injury noted after 1 hour of reperfusion or the degree of mucosal regeneration after 48 hours of reperfusion. In this group of ponies, the severity of mucosal damage was greater after 1 hour of reperfusion for both AVO and VO.