Growth rate, either through genetics or diet, mainly determines the outcome concerning broiler welfare

There is a trend towards broiler production systems with higher welfare requirements, which often use a combination of factors to improve broiler welfare. This makes it difficult to entangle whether improvements are due to housing conditions, diet, genetics or a combination of these factors. Therefore, it remains unknown to what extent differences in welfare can be attributed to breed (i.e., genetics), growth rate (i.e., diet) or the interaction between the two. We compared fast- (Ross 308, R), medium- (Ranger Classic, RC) and slower-growing broilers (Hubbard JA757, H) receiving diets differing in balanced protein (BP) content (i.e., 80, 90 and 100%). We identified effects on behaviour, responses to behavioural tests and litter quality at three target body weights (TBWs, 0.2, 1.2 and 2.4 kg), and welfare scores and litter DM content at TBW 2.6 kg. The experiment had a 3 × 3 factorial design with four replicates (pens) per treatment (a total of 36 pens). We hypothesised that reducing the growth rate will improve the welfare of all breeds and that breeds will not differ in welfare if they have a comparable growth rate. Indeed, reducing the growth rate improved hock burn scores in all breeds. R broilers also had improved cleanliness scores and RC broilers improved gait scores. Reducing the growth rate increased the number of R broilers approaching a human and novel object and showing running behaviour in the free-space test, while it reduced the number of RC broilers approaching a human and showing comfort behaviour and did not affect behaviour in H broilers. In addition, litter quality was improved for R broilers (wetness and DM), while reduced for H broilers (DM) with reduced growth rate, and no effects of growth rate on litter quality were found for RC broilers. Thus, reducing growth rate (i.e., reducing BP in diets) improves welfare in all breeds, but breeds did respond differently concerning behaviour and litter quality, with more beneficial effects for fast-growing broilers compared to medium- and slower-growing broilers. Hardly any differences in behaviour, welfare scores and litter quality were found between breeds when they had a comparable growth rate (R80 vs RC100 and RC80 vs H100), except for R80 having better scores for cleanliness and gait scores (only males) and higher litter DM content compared to RC100. These findings suggest that growth rate, either through genetics or diet, is mainly determining the outcome concerning broiler welfare.