0141658 



with low copper content (Hill and Matrone 1970). Campbell and 

 (Mills (1979) produced a severe copper deficiency in pregnant ewes 

 on diets of 750 ppm zinc. 



The form of zinc is another important factor in zinc toxic- 

 ity. Smith (1977) found that zinc sulfate was more rapidly 

 excreted in the urine of sheep than was zinc oxide. Zinc sulfate 

 has also been shown to accumulate less in tissues when given at 

 the same concentration as zinc oxide (Miller et al . 1970). The 

 sex of beef cattle has been shown to affect the amount of zinc ac- 

 cumulated in tissues, but the threshold level of zinc (900 ppm Zn 

 diet) necessary to produce toxicosis was found to be similar for 

 both heifers and steers (Ott et al . 1966b). 



It is apparent from this discussion that a given amount of 

 zinc, within limits, may or may not produce toxicosis. Many 

 studies have attempted to determine threshold toxic levels of zinc 

 in various animals. These studies are summarized in Tables 22-25. 



Excessive absorption of zinc is controlled up to a certain 

 dietary level by the body's homeostatic mechanisms. In lambs, 

 'this system is effective up to a dietary concentract ion of ap- 

 proximately 1000 ppm (Ott et al . 1966c). For calves, the level is 

 somewhat lower, as large increases in tissue zinc content have 

 been observed at dietary levels of 638 ppm (Miller et al. 1971). 

 Higher levels of zinc overwhelm -the homeostatic mechanisms and 

 significant increases of zinc have been observed in liver, kidney, 

 pancreas and blood serum (Tables 24 and 25). Miller et al. (1971) 

 found that zinc levels in whole blood did not correlate with 

 dietary zinc levels up to 638 ppm. Similarly, normal skeletal 

 muscle has been shown to be highly insensitive to dietary zinc. 

 These two livestock tissues would be of little use in" monitoring 

 zinc exposure. Zinc levels in blood serum, liver, kidney and 

 pancreas have been shown to correlate with dietary levels of the 

 element. These three organs tend to accumulate similar metal 

 levels and are about two orders of magnitude greater than levels 

 found in serum. Allen et al . (1983) found that the pancreas is 

 the only organ consistently affected by zinc toxicosis and 

 'suggested that pathological changes observed in the pancreas could 



58 



