Use of plant hydrocarbons as markers to estimate voluntary intake and digestibility in beef steers

Calculating dry matter intake of grazing herbivores has been an issue in determining factors such as efficiency and digestibility of forages. Two experiments were conducted using alkanes to validate analytical methods with oven-dried and freeze-dried samples, compare and contrast measured and estimated intake, and to measure digestibility using a controlled-release capsule (CRC) or supplement. In the first trial, 7 beef steers consuming either alfalfa or switchgrass hay received either a CRC or supplement in a random crossover design. Each period lasted 20 days with fecal grab samples taken over the last 10 days and total fecal collection the last 5 days. In the second trial, 12 beef cattle were fed a blended switchgrass and alfalfa hay and fed a once-daily alkane supplement. Fecal grab samples were collected the last 5 days over the 14-day experiment. In the first experiment, steers fed switchgrass hay ate less and digested a smaller proportion of total diet (P < 0.02) than steers fed alfalfa. There was no difference (P < 0.35) between estimated intake and measured intake. Fecal concentrations of alkanes were not affected (P < 0.13) by drying method and no difference (P < 0.40) was seen between fecal grab samples and total fecal collection. Either dotriacontane (C32) or hexatriacontane (C36) can be used for estimating DMD. In the second experiment, there was no difference between measured and estimated intake (P < 0.35). There was no day effect between fecal concentrations and estimated intake. There was no difference (P < 0.65) between fecal alkane concentrations when duplicate samples were analyzed. Alkanes can be used to estimate intake in cattle. Fecal grab samples are sufficient to reduce labor and result in adequate estimated intake and daily feeding of alkanes can be used to replace the CRC.