Résumé
Grassland butterflies are on the decline throughout Europe. We tested an alternative rotational stocking (ARS) strategy based on theoretical and practical aspects of grassland ecology, designed to increase butterfly diversity while also meeting farmers production objectives. This management strategy implies taking animals away from one subplot of the rotation during the main flowering period. Its feasibility and benefits on butterfly diversity were tested by comparing ARS with continuous stocking (CS) in plots grazed by cattle at the same stocking rate: high in 2005 and 2006 then lenient in 2007 and 2008. At the high stocking rate, butterfly abundance (21.9 vs. 8.3, P < 0.01) and butterfly species richness (7.4 vs. 3.7, P < 0.001) were significantly higher in ARS than in CS plots, matching the increase in pasture flowering intensity and sward structural diversity. ARS was led according to the pre-planned schedule in 2005, but in 2006, the number of heifer grazing days in ARS was reduced by 19% because of unfavourable spring grass growth. At the lenient stocking rate, ARS was less beneficial for butterflies (abundance: 17.4 vs. 13.0, P < 0.10; species richness: 5.9 vs. 5.2, P = 0.35) but would present less risk for farmers in terms of providing livestock with sufficient forage. Alternative rotational stocking thus has potential to be integrated into grassland-based systems, but would require earlier grazing on the excluded subplot of the rotation in the event of unfavourable grass growth during spring.