Feeding behaviour of sheep on shrubs in response to contrasting herbaceous cover in rangelands dominated by Cytisus scoparius L.
Résumé
The foraging responses of ewes faced with a diversity of feed items and their effects on broom (Cytisus scoparius L.) consumption were examined. The experiment was conducted on a farm in the autumn with ewes (n = 33) grazing three small paddocks (0.44 ha on average, for at least 10 days each) located in broom shrubland. The effects of three different herbaceous covers on broom consumption were compared: 100% of paddock area previously grazed in summer; 50% of paddock area previously grazed in summer; and paddock area non-grazed during the year. The characteristics of herbaceous cover (availability and quality) and the ewes’ diet selection were encoded as bite categories. Flock activities were recorded through scan sampling. We used logistic regression to assess the relationship between feeding behaviour of sheep on herbaceous vegetation and on broom species, and calculated selectivity indices for this shrub. We showed that the presence of high-quality bite categories in the herbaceous cover affected the way ewes integrated broom into their diet. At the start of each paddock use period, ewes favoured high-quality larger and medium bites of the herbaceous cover. They gradually included larger bites of broom and reduced their bite size, but continued to seek out higher quality herbaceous plants, a pattern which suggested a stabilisation of their daily average digestibility and bite mass over time. A negative relation was observed between the percentage of ewes taking large and mediumbites on highly digestible plant parts and the percentage of ewes browsing broom. A maximum of 26% of the flock browsing broom was observed on any given day. Hence, ewes have a threshold for this target shrubby species that they do not exceed during any paddock utilisation period. This finding was interpreted as a mechanism to deal with post-ingestive consequences and complementary interactions between nutrients and toxins. When comparing broom selection between paddocks in autumn, we found an earlier and thus longer broom selection in areas with herbaceous cover that had not been grazed during the year (possibly because of a lower palatability). Our results provide new insights into ways to manipulate diet selection in order to stimulate the use of broom by ewes. Bite categories are proposed as functional feed indicators that facilitate prediction of the herbaceous cover state preliminary to initial broom integration in the sheep’s diet.