Anecdotal evidence from animal charities indicates that the number of overweight donkeys in the UK is increasing. Donkeys commonly have daily access to pasture therefore knowledge of grass intake is essential if feeding advice is to be relevant. The effects of herbage mass and length of grazing time on diet composition and dry matter intake (DMI) by mature donkeys were determined.
There were two measurement periods: period 1 during autumn when pasture was sparse (herbage mass 92 + 7g DM/m2) and period 2 during summer when pasture was more abundant (herbage mass 197 + 12g DM/m2). Twenty mature donkeys were selected and split into three grazing groups (8, 12 and 23 h daily grazing access). Barley straw was fed ad libitum and each donkey was given 150mg per day of an n-alkane marker Dotriacontane (C32) for the 12 d of each study period. Herbage mass significantly affected total DMI and diet composition. During summer DMI of donkeys in the 8 and 23h groups was significantly greater than during autumn (P<0.05). The proportion of grass in the diets of all donkeys was also greater in summer compared to autumn (P<0.001). Grazing time did not significantly influence total daily DMI during either season due to donkeys consuming more straw when grass intake was reduced. Restricting donkeys to 12h or less grazing per day in summer significantly (P<0.001) reduced their grass intake compared to that of donkeys with 23h access. When grazing sparse pastures (autumn) time allowed for grazing did not influence grass intake. The results show that time allowed for grazing per se was less important than the herbage mass available to the donkey in terms of grass DMI.