Conditioned aversion in sheep induced by Baccharis coridifolia

Abstract

In Southern Brazil, Uruguay, Argentina, and Paraguay, the invasive weed Baccharis coridifolia often poisons naïve animals. Farmers prevent B. coridifolia poisoning using several unconventional methods to reduce ingestion: (1) burning plant material under an animals’ nose, and having the animal inhale the resulting smoke; (2) rubbing the plant on the animals’ muzzle and mouth; and (3) gradually introducing animals into B. coridifolia-infested pastures. To determine if B. coridifolia would condition an aversion, and to test the efficiency of these three aversive methods, 18 adult sheep were used to induce an aversion to corn, a novel food. In Group IBc, four sheep ingested 0.25 g/kg bw of fresh B. coridifolia. In Group OMBc, four sheep were treated by rubbing the plant in the mouth. In Group IHBc, four sheep inhaled smoke produced by burning B. coridifolia. In Group LiCl, two sheep were treated by oral gavage with 175 mg/kg lithium chloride (LiCl). In Group IA, two sheep received alfalfa by oral gavage. In Group IHLm, two sheep inhaled the smoke produced by burning ryegrass. On days 1–5, 10, 30, 60 and 90, 100 g of corn were offered to the animals. All sheep that ingested B. coridifolia or were treated with LiCl and one that inhaled smoke produced by burning B. coridifolia developed an aversion to corn for the whole experimental period. After 1 year, sheep from Groups IBc, OMBc, LiCl, and IA were transferred to a pasture with B. coridifolia, and observed for plant consumption. Sheep from group IBc that were treated with B. coridifolia the previous year, did not graze the plant. Sheep from the other groups ingested the plant occasionally, had anorexia, and two showed signs of digestive stress and died. Results demonstrate that B. coridifolia is as efficient as LiCl in conditioning an aversion to a previously unknown food.