N-alkanes have been used as internal markers in digestibility trials with ruminants and non-ruminants for more than 20 years. In this study, two trials were conducted under different feeding regimes to (i) evaluate the release rate of n-alkanes of controlled-release capsules in the rumen of rumen-cannulated steers either grazing or restrained in metabolic stalls and (ii) estimate voluntary forage intake of the same steers in metabolic stalls. Six rumen-cannulated Nelore steers were allocated to individual metabolic stalls and were fed diets with varying forage to concentrate ratios (80:20, 60:40, and 40:60; respectively). Corn silage was the only forage source. In the grazing trial, the same steers were evaluated under three feeding managements (Brachiaria brizantha cv. Marandu unsupplemented or supplemented with either 0.3% or 0.6% of live weight of a concentrate). The release rate of the n-alkanes (mg d-1) was measured by multiplying the distance (mm d-1) the capsule plunger travelled after 3, 7, 10, 13, and 17 d of rumen infusion to the n-alkanes concentration of capsule tablets (mg mm-1). There was an effect of day of measurement (P < 0.05) on the release rate of animals restrained in metabolic stalls and grazing. However, no effect (P > 0.05) of feeding management or feeding management x day of measurement interaction was observed. Values averaged 6.9 and 14.8%, lower than proposed by the manufacturer for the C32, when animals were restrained in metabolic stalls and at grazing, respectively. Similarly, the values of C36 were 15.9 and 23.1% lower for those animals in metabolic stalls and grazing, respectively. The average release rate of C32 into the rumen was 372 and 341 mg d-1 for animals restrained in metabolic stalls and grazing, respectively. There was no difference in the release rate between feeding regimes (P > 0.05). The regression of the pooled data indicated an average release rate of 345 mg d-1. Estimated values of voluntary forage intake using the pair of C33:C32 n-alkanes using the animals restrained in metabolic stalls was not different from those effectively measured (P > 0.05). These findings indicated that n-alkanes capsules can be used to estimate forage intake of stall-fed animals, but concentration of n-alkanes in capsules and the release rate has to be measured before dosing animals to accurately predict intake.
bovine; fecal recovery; hydrocarbons; markers; dry matter intake