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A study of the digestion and metabolism of dairy cows differing in susceptibility to bloat

The digestive characteristics of cows of high (HS) or low (LS) susceptibility to bloat were investigated. Sixteen HS and 16 LS cows were offered pasture or hay when pregnant but not lactating, and pasture or 75% pasture plus 25% hay when lactating. The susceptibility groups did not differ in voluntary intake or apparent digestibility of dry matter, organic matter, nitrogen or energy on either occasion. LS cows produced more milk of a lower protein content than HS cows but yields of fat, protein and lactose were similar for both susceptibility groups. Subsequent experiments used 4 or 5 cows of each susceptibility group, each fitted with a fistula in the dorsal rumen and offered diets of hay or pasture. On 3 of 8 diets offered, non-lactating LS cows had less digesta, measured by removal through the fistula, in the reticulo-rumen before feeding than did HS cows. Increasing or decreasing the quantity of digesta in the reticulo-rumen after feeding correspondingly increased or decreased both the quantity of digesta subsequently lost and the quantity remaining 21 hours after feeding, or before the next feeding, for both HS and LS cows. On all diets LS cows lost less fluid from the reticulo-rumen during feeding and more after feeding, than did HS cows. Altering the concentration of Na in rumen fluid by addition of water or electrolyte solutions did not affect the loss of fluid and DM from the reticulo-rumen of LS cows relative to that of HS cows. When offered 3 pastures differing in maturity, an increase in maturity of the pasture resulted in an increase in the quantity of digesta in the reticulo-rumen before feeding, but HS and LS cows did not differ in quantity of digesta on each of the 3 pastures. The quantity of digesta in the reticulo-rumen was greater in cows when offered a diet of low K:Na ratio than when offered a high ratio diet. On both diets, however, LS cows had less digesta in the reticulo-rumen, a lower concentration of K in plasma and excreted less K in their faeces than HS cows. Lactating HS and LS cows offered pasture ad libitum did not differ in intakes of dry matter and water, in the quantities of fluid and dry matter in the reticulo-rumen or in the rate of digestion when estimated from the loss of dry matter from nylon bags incubated in the ventral sac of the rumen. HS and LS cows did not differ in capacity of the reticule-rumen. The rumen fluid dilution rate, estimated using Cr EDTA, was similar for both susceptibility groups in 3 experiments and higher for LS than HS cows in one experiment. Overall, these studies revealed few differences between HS and LS cows in the parameters measured, except that under some experimental conditions LS cows had less digesta in the reticulo-rumen than did HS cows.
Type of thesis
The University of Waikato
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