BLOAT INVESTIGATIONS. THE THRESHOLD FRACTION 1 (18S) PROTEIN CONCENTRATION FOR BLOAT AND RELATIONSHIPS BETWEEN BLOAT AND LIPID, TANNIN, Ca, Mg, Ni AND Zn CONCENTRATIONS IN ALFALFA

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Abstract
Incidence of lethal legume bloat in cattle was related to Fraction 1 (18S) protein concentration in mechanically grazed alfalfa over two grazing seasons. In the second year, lipid, tannin, Ca, Mg, Ni and Zn concentrations in the alfalfa were related to bloat incidence and to Fraction 1 protein concentration.Fraction 1 protein was most closely associated with bloat as the average bloat incidence increased from 2 to 35% over the bloating range of Fraction 1 protein. Comparable increases of bloat for Ca, Ni and Zn were 10.0, 10.0 and 8.5%, respectively. Bloat had a threshold level of 1.8% Fraction 1 protein, below which bloat did not occur and it was the only variable with a threshold level. This level of 1.8% can be used by the plant breeder or agronomist as a goal in producing a nonbloating alfalfa by selection or by management.Calcium, Ni and Zn were significantly associated with bloat and Ca was also associated with Fraction 1 protein. Nickel and Zn were associated with twice as much bloat as Ca when the effect of Fraction 1 protein was removed. However, there is no explanation for the relationship between Ni and Zn concentrations and bloat. Lipids and tannins were positively associated with Fraction 1 protein but they were not significantly associated with bloat (P < 0.05). Magnesium was not related to either bloat or Fraction 1 protein.This is the first report showing the threshold Fraction 1 protein content below which bloat did not occur. We have shown previously that rumen pH is also a trigger mechanism. In addition, Ca, Ni and Zn also have been found to have an important relationship to the amount of bloat that occurred. These interacting factors can explain much of the sporadic incidence of bloat and variable severity within and between both cattle and ranches.