Validation and application of the voluntary water consumption test for the on-farm measurement of thirst in broiler chickens

Animal welfare monitoring schemes use poorly validated resource-based measures, such as the number of animals per drinker, for assessing thirst. We performed a novel animal-based test of thirst on 10 Belgian and 10 Brazilian broiler farms to (1) test its validity and sensitivity; (2) identify confounding variables; and (3) compare with the Welfare Quality® resource-based assessment of thirst. Per flock 4 groups of 5 chickens from 4 locations (2 near walls, 2 central) were enclosed and the amount of water consumed from an unfamiliar open drinker during 90 min was measured. After this first test, the groups of chickens remained enclosed for 6h before the test was repeated. Half the groups had access to water during this interval, the other groups had not. The mixed model indicated that water consumption in the first test was higher in Brazil than Belgium (P<0.001), but was not affected by location. The Welfare Quality® thirst scores were not correlated with the water consumption of Belgian (r=-0.10, P=0.55) or Brazilian (r=0.20, P=0.23) birds. In the 2nd test, the deprived chickens drank more than the control chickens, particularly in Brazil (country x treatment: P<0.001). Brazilian control birds consumed more water with decreasing relative humidity (P=0.020) and increasing temperature (P=0.025). ROC-curves showed that the test poorly detects a 6h-dehydration period in Belgium, but in Brazil 90% of the 6h dehydrated broilers would be detected if 15% false positives are allowed. We conclude that the voluntary water consumption test (1) does not correlate with the Welfare Quality® resource-based thirst score, (2) is sufficiently sensitive to discriminate between non-deprived and 6h deprived flocks in Brazil, but not in Belgium, and 3) ought to take into account indoor climatic variables and possibly genotype when interpreting the test outcome in terms of the thirst level experienced by broilers.