If genetically resistant and susceptible rabbits inhale a certain number of human type tubercle bacilli, no tuberculosis in the lungs of the resistant animals is seen, as a rule several months after infection, while there is a variable and often extensive disease in the susceptible rabbits. The analogy to the presence or absence of active tuberculosis in man infected with the tubercle bacillus is evident. The inhaled tubercle bacilli multiply for but a short time in the resistant rabbits and are usually rapidly and completely destroyed. In the susceptible rabbits, the bacilli multiply profusely for a much longer time and persist in large numbers even months after inhalation. Whatever be the cause of the more rapid destruction of tubercle bacilli in the resistant animal, the resulting more rapid release of the contained antigens enhances the development of allergic sensitivity and antibodies in these animals. The development of an acquired resistance against tubercle bacilli of the human type is sufficiently rapid to affect the genesis of the initial gross primary pulmonary foci that result from the inhalation of a given number of bacilli. The greater the genetic resistance, the fewer the initial primary foci. Variations in genetic resistance are essentially variations in the rate of development of acquired resistance. It is suggested that variations in genetic resistance to inhaled human type tubercle bacilli may affect the prevalence of alveolar phagocytes capable of acquiring adequate resistance to the growth of the bacilli in their cytoplasm. The prevalence of such cells is subject to hormonal and immunological influences.

The effect of infusing similar energy equivalents of starch into the rumen, or starch or oil into the abomasum was studied in four midlactation cows in a 4 x 4 Latin square design experiment; controls were ruminally infused with water. Cows were fitted with cannulas in the rumen, abomasum, and ileum, and nutrient digestion in the rumen and small intestine was evaluated with Cr as a digesta marker. Ruminal infusions of starch, or abomasal infusions of starch or oil, were associated with a decrease in voluntary feed organic matter intake. Overall energy intake was reduced in oil-infused, but not in starch-infused cows. Nonstructural carbohydrate digestibility in the rumen and in the small intestine was similar among treatments. In abomasally infused cows 3.4 kg/d of nonstructural carbohydrates was apparently digested in the small intestine. Milk production was reduced in oil-infused cows, but the efficiency of milk energy and protein yield was unaffected by treatments. Plasma glucose, insulin, and IGF-1 concentration, mammary glucose extraction rate, rumen ammonia and plasma urea, and arterial and mammary extraction rate of amino acids were all similar among treatments. Large quantities of starch can be digested in the rumen or small intestine of dairy cows. There appear to be no metabolic advantage to increasing the supply of starch to the rumen or the abomasum of mid-lactation dairy cows maintained on highly concentrated diets and exhibiting a positive energy balance.

A study was conducted to evaluate the effect of abomasal infusion of casein on post-ruminal digestibility of starch and on milk yield and composition. Six multiparous Israeli Holstein cows in mid lactation, fitted with ruminal and abomasal cannulas, were used in a 3 ✕ 3 Latin-square experiment. Each cow received 1600 g maize starch infused into the abomasum. Treatments were: abomasal infusion of sodium caseinate at 0, 350, or 700 g casein daily. Chromium mordant neutral-detergent fibre (NDF) was used as a digesta marker. Casein infusion was associated with increases in post-ruminal and total-tract digestibility of non-structural carbohydrates and protein. Concentrations of rumen ammonia and of plasma insulin, glucose and urea were higher in casein-infused cows. Concentrations of milk protein and lactose and milk protein yield increased with casein infusion. Results indicate that increased protein flow to the abomasum can improve the yield of milk constituents in dairy cows. Digestibility and yield responses to infusion of 350 g casein per day were similar to those at 700 g/day. There was a tendency toward reduced milk protein efficiency in casein-infused cows. It is suggested that part of the production response can be related to a direct effect of protein supply and the rest can be explained by the indirect effect of improved carbohydrate availability in the small intestine.

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