Comparison of n-alkanes or chromium oxide methods for estimation of herbage intake by sheep

Herbage intake (HI) was estimated with two procedures: (a) with the n-alkanes method (HIal), which combines the use of odd-chain n-alkanes, naturally occurring in plants, and dosed even-chain n-alkanes, as internal and external markers, respectively; (b) with the ratio between faecal excretion, estimated with chromium sesquioxide (Cr2O3), and the herbage indigestibility, estimated in vitro (HICr). The two techniques were compared in an indoor experiment and under grazing conditions. In the indoor trial, 12 wethers were fed 40 g dry matter (DM) kg-1 BW0.75 day-1 of one of the following diets: herbage only (diet H); herbage plus maize (diet M; 26% of DM intake); herbage plus maize plus maize gluten meal (diet MG; 13% of DM intake each). Each wether was dosed once a day for 12 days with the external indicators impregnated on paper and inserted in gelatine capsules: n-dotriacontane, C32, 600 mg day-1; Cr2O3, 500 mg day-1. In the second experiment, six adult, lactating ewes grazing the pasture used for the indoor trial were divided into groups of three receiving 700 g day-1 of two different supplements: maize (diet M) or a 50:50 mix of maize and maize gluten meal (diet MG). The external indicators (Cr2O3 and C32) were administered for 8 consecutive days. The trial was repeated four times with the same procedure between 10th and the 40th day after lambing. Recoveries of Cr2O3, C32 and herbage n-alkanes (n-hentriacontane, C31, and n-tritriacontane, C33) were incomplete, with lower values for the natural n-alkanes than the dosed n-alkane (Cr2O3, 94.9 ± 5.5%; C31, 86.7 ± 8.7%; C32, 89.6 ± 8.5%; C33, 85.1 ± 9.9%). Therefore, HICr gave an overestimate of the true herbage intake of 5.5% and HIal an underestimate of 3.0%, with a significant discrepancy between methods (803 vs. 733 g organic matter (OM) day-1), not affected by diet. In the grazing trial, HICr was higher than HIal, but the difference between methods (1503 vs. 1273 g OM day-1) was higher than that obtained in the indoor experiment and was influenced by diet (380 g OM day-1 vs. 81 g OM day-1 with diet M and diet MG, respectively). A possible explanation of the different results obtained indoor or at pasture is discussed. © 1995.