Photosensitization in naïve sheep grazing signal grass (Brachiaria decumbens) under full sunlight or a silvopastoral system

Abstract

The work consisted of the evaluation of photosensitization in sheep grazing signal grass (Brachiaria decumbens) in a silvopastoral system or under full sunlight for 60 days in the summer. We hypothesized that shade provided by trees in the silvopastoral system prevents or alleviates the classic symptoms of photosensitization in grazing sheep and affects the concentration of the saponin protodioscin in signal grass. Twenty naïve crossbred sheep (Dorper x Santa Ines) were used in a randomized block design, with two blocks and five animals per block. Animals in both silvopastoral and full sun treatments grazed B. decumbens, and animals in both treatments presented with clinical symptoms, alterations in the liver, and increased serum levels of gamma-glutamyltransferase (GGT), aspartate aminotransferase (AST), and bilirubin over the duration of the experiment. Although the protodioscin content of B. decumbens in the silvopastoral system was lower than in the full sun treatment, it was sufficient to cause secondary photosensitization in the animals. The natural shading provided by the silvopastoral system does not prevent or ameliorate photosensitization symptoms in young naïve sheep grazing on B. decumbens.