Evaluating the statistical variation in estimating forage dry matter intake of grazing Brahman bulls using n-alkanes

The objectives of the current study were to determine the variation structure within a day and across days when determining dry matter intake (DMI) of Coastal bermudagrass (Cynodon dactylon (L.) Pers.) pasture using dotriacontane (C-32) as an external marker, to determine the optimal times for faecal collection for forage DMI estimation and to compare C-31 and C-33 as internal markers in estimating forage DMI in Brahman bulls. Sixteen Brahman bulls were allocated by weight to four pastures, and stocked at a moderate to low grazing pressure for 63 days from late June to the end of August. Three intake measurement periods (P1, P2, P3) were used; each period consisting of 10 days of twice daily C-32 (400 mg/day) administration. Faecal collections were taken during the last 5 days (07.00, 11.00, 15.00 and 19.00 h). The C-32 was individually hand fed using Calan gates, with maize gluten as a carrier for the alkane. Gas chromatography was used to determine n-alkanes in the forage and faecal samples. The concentration of C-31 was less than C-33 in the bermudagrass for all periods (P < 0.001), but the concentration of C-31 and C-33 in faeces was not significantly different (P > 005). The average concentration of C-32 in the forage was 5.1, 7.6 and 9.6 mg/kg dry matter (DM), for P1, P2 and P3, respectively, with an average of 7.5 mg/kg DM for all periods. During P1 and P2, the estimation of forage DMI using C-33 had a better fit (smaller-2 x log and Akaike's information criterion (AIC)) than using C-31 either with or without adjustments for C-32. The variation in estimated forage DMI decreased when forage C-32 was not included. The variances of forage DMI were similar using C-31 across days, but the Pearson correlations between days were low, which suggested that several days of collection were needed to estimate forage DMI accurately. Correlations between collection times within days were medium to high for all periods and varied from 0.65 to 0.97 for C-31 and from 0.26 to 0.96 for C-33. When all periods were analysed together, estimates of forage DMI either using C-31 or C-33 had low correlations between days of collection. Adjustment for C-32 did not improve the variance and (co)variance matrix. In summary, C-33/C-32 had the lowest variation in estimating forage DMI, and at least 5 days of faecal collection were needed to decrease the variability of estimating forage DMI. The optimum times for faecal collection were 07.00 and 19.00 h, and it was important to adjust for C-32 alkane concentration in estimating forage DMI in Brahman bulls grazing Coastal bermudagrass.