In two experiments, individually penned growing pigs were exposed daily to two "tail models” (lengths of cotton cord about the size of a pig's tail), one of which had been impregnated with pigs' blood. When fed a balanced "control" diet, the pigs chewed significantly more on the blood-covered model than on the plain one, but with Iarge individual differences between animals. Four weeks of receiving a diet lacking all mineral supplements (iodized salt, dicalcium phosphate, limestone, iron, zinc, manganese, copper, and selenium) caused a pronounced increase in chewing the blood-covered model, and 4 wk of recovery on the control diet reduced, but did not completely eliminate, the enhanced attraction to blood. In a second experiment, a similar heightened response to blood was produced by omitting only iodized salt from the diet, whereas omission of all other mineral supplements led to a much smaller and statistically nonsignificant change. Although the causes of tail-biting are undoubtedly complex, the results suggest that heightened appetite for salt could make pigs particularly attracted to pen-mates with injured tails.
Fraser, D. (1987). Mineral-deficient diets and the pig's attraction to blood: implications for tail-biting. Canadian Journal of Animal Science, 67(4), 909-918.