Dosing and fecal sample collection effects on the estimates of intake using n-alkanes as markers

The use of indigestible markers continues to be among the best tools available for estimating dry matter intake (DMI) in situations where individual animal feeding cannot be feasibly measured. The constant animal handling required for dosing markers and subsequent fecal sample collection required are often limiting, especially when grazing or when animal handling facilities are not accessible. Identifying strategies for more efficient use of labor while not sacrificing the accuracy of DMI estimates are important for further application of these techniques. The objectives of this study were to: 1) Test potential diet and dosing effects of C32 n-alkane for intake estimates; 2) Compare intake estimates from 2 contrasting (i.e., intensive vs. less intensive) fecal sampling regimes; and 3) Determine whether intake estimates differ when 2 different n-alkane pairs (C31:C32 or C32:C33) are used. The study was laid out using a 2 × 2 × 2 factorial arrangement in a randomized complete block design. Treatments were 2 diets, 2 n-alkane marker dosing regimens, and 2 fecal sampling regimens. Diets were grass haylage (HLG) and a total mixed ration (TMR). Daily intakes were recorded using Calan gates. Alkane dosing was once (1×) or twice (2×) daily dosing of a bolus containing dotriacontane (C32) n-alkane. The 1× treatments received 2 capsules every morning, while the 2× treatments received 1 capsule in the morning and 1 in the afternoon, both receiving 933 mg of C32/d. Fecal samples were either taken 3 times daily over 4 d with times shifted each day (4 × 3), or twice daily at the same times each day (AMPM). The results indicate the use of C32:C33 n-alkane pairs were not accurate for predicting DMI (Lin's Concordance Correlation Coefficient [CCC] all < 0.5). The C31:C32 n-alkane pairs provided accurate estimates of DMI, having adequate agreement (CCC >0.6) between predicted and observed DMI estimates. Strategies for dosing either 1× or 2× daily were both accurate, within the confines of the experiment. In terms of fecal sampling, there was no discernable advantage for the intensive (4 × 3) sampling regimen compared to the twice daily (AMPM) sampling. In addition, intake estimates were similar with either the 1× or 2× dosing regimens. For studies conducted under similar conditions, it may be suitable to adopt 1× dosing regimen with twice-daily fecal sampling schedules.