In two experiments, 60 individually penned growing pigs were exposed daily to two sections of cotton cord, one of which had been soaked with pigs' blood and subsequently dried, while the other was plain. The animals' preference for chewing-on the blood-impregnated cord was quantified by direct observation. When fed a standard "control" diet of corn, barley, and soybean meal with mineral and vitamin supplements, the pigs had a clear but modest preference for chewing the blood-impregnated cord. Omission of the protein supplement (soybean meal) from the diet for 4 wk led to a major increase in attraction to blood and a significant reduction in body weight gain. In the second experiment, supplementation of this "negative" diet with (a) synthetic lysine, or (b) synthetic lysine and other synthetic amino acids, led to weight gains that were intermediate between those seen with the control and negative diets. Attraction to blood was also intermediate on average, although not significantly lower than that seen with the negative diet. Pigs on the diets supplemented with amino acids had highly variable weight gains. Those that gained as rapidly as the controls had relatively low attraction to blood, while those showing clear depression of gain tended to (but did not always) have enhanced attraction to blood. It is hypothesized that the depression of growth caused by inadequate protein nutrition predisposed the animals to enhanced attraction to blood, and that such a relationship may help to explain the widely reported link between dietary inadequacies and tail-biting.
Fraser, D., Bernon, D. E., & Ball, R. O. (1991). Enhanced attraction to blood by pigs with inadequate dietary protein supplementation. Canadian journal of animal science, 71(3), 611-619.