Estimation of intake and digestibility of silage by beef steers using a controlled release capsule of n-alkanes

Abstract

Twelve beef steers were fed a grass-legume silage either ad libitum or at 70% of ad libitum feeding. Intake and digestibility were measured directly or predicted using the alkane ratio technique. The C32 and C36 alkanes were delivered via a controlled release capsule (CRC) in the rumen. Level of feeding had no effect on digestibility (P > 0.05) but dry matter (DM) intake tended to be greater for steers fed ad libitum (P = 0.08). The release of alkanes from the CRC was erratic and 20% greater than expected. This influenced the exogenous to endogenous alkane ratios. Estimates of intake were therefore derived using both the expected and actual (determined from total collection of feces and their alkane analysis) exogenous alkane release rates. Estimated intakes based on the manufacturers’ release rate were underestimated and associated with high standard errors. For cattle fed restricted silage, estimated intakes obtained with C31:C32, C33:C32 and C33:C36 alkane pairs and using actual measured release rates of exogenous markers were associated with lower standard errors and averaged 87, 102 and 103% of actual values, respectively. For ad libitum-fed cattle the corresponding values were 81, 96 and 96% of actual values. There was no significant difference between actual and estimated DM intake when the C33:C32 and C36:C32 alkane pairs were used. The precision of intake estimates using manufacturers release rates for exogenous alkanes was poor, as evidenced by significant discrepancies (P < 0.05 for four of six estimates) between observed and predicted values averaging 2.26 kg or 30% of DM intake. However, when measured exogenous alkane release rates were used, standard errors were reduced and discrepancies for estimates using C33:C32 and C33:C326 were only 0.4 kg or 6% of DM intake. Fecal recovery of C27, C29, C31 and C33 alkanes was 73.4, 92.7, 85.6 and 100%, respectively. Both C29 and C33 gave good estimates of digestibility because of high recovery. Observed and estimated values were not significantly different. It is concluded that exogenous alkane release from CRCs was not satisfactorily consistent. Reliable estimates of intake could only be made if actual release rate was known. Endogenous alkanes can be used for determination of digestibility, provided fecal recovery is known. Key words: Beef, silage, alkane, intake, digestibility