Associative effects on total tract digestibility in horses fed different ratios of grass hay and whole oats

The present experiment was performed according to a 4×4 Latin square design with diets composed of (on dry matter (DM) basis) the following ratios of grass hay to whole oats: 100:0 (O0), 80:20 (O20), 60:40 (O40) and 40:60 (O60). Four mature Standardbred geldings weighing 493 (S.D. 34) kg were used. The digestibility of DM, organic matter and energy (dE) increased in a curvilinear pattern with the increasing inclusion levels of oats. It is suggested that this was a reflection of the significantly lowered digestibility of nutrients and particularly of fibrous components for the O60 diet as compared with the other three rations, thus resulting in negative associative effects between grass hay and oats. On the basis of the present data, and excluding the results of the O60 diet, the digestibility of dE of oats was estimated to be 75%. As an expression of the associative effect at the O60 level, the dE of the total diet was estimated to be 58%, as compared with 63% when estimated on the oats value of 75% and the O0 diet, resulting in a depression of dE of the total diet by 8%, mainly affecting the utilization of fibrous components. Urinary energy losses decreased with increasing level of oats.