Analysis Strategies for Calculating Intake for Cattle with Plant Waxes

Within grazing systems determining differences in efficiencies is difficult due to the complexity of measuring forage intakes. Plant waxes, particularly n-alkanes, have shown potential for estimating intakes. However, extracting and analyzing these compounds is costly. Minimizing the number of samples needed to reliably estimate intakes is key. We compared 3 strategies for using plant-wax contents of feces to estimate intakes: (i) from a single sample formed by pooling collections from 5 d, estimating intakes from the pool; (ii) from separate samples collected each of 5 d, averaging intake estimates; and, (iii) from a mathematical average of the 5 samples, estimating intakes from the average. Twenty-six heifers were fed a total mix ration (TMR; 69.8% corn silage, 30% alfalfa hay and 0.2% salt DM) with a Calan-gate system for 45 d. Heifers were dosed via supplement with C₃₂ alkane at 1.3 mg / kg BW / d the final 14 d. Fecal grab samples were collected the final 5 d. The C₃₁ and C₃₃ alkane contents of the TMR and of its components were obtained using gas chromatography, with the values of the components proportionally weighted (0.7 corn silage; 0.3 alfalfa hay) to emulate TMR. The C₃₁ and C₃₃ contents of the fecal samples also were obtained. Intakes were estimated from ratios of C₃₁ and C₃₃ to C₃₂ in the fecal samples and diets. Estimated intakes were more precise when based on pooled (R² 0.60 to 0.62) as compared to averaging (R² 0.47 to 0.51) strategies. This result reflected variability` in daily intakes over 5 d. When regressing predicted on measured intakes, regardless of strategy or definition of diet, the intercept did not differ from zero (P > 0.35) and the slope did not differ from one (P > 0.08). When emulating TMR as a 2-component mixture the slopes were higher (1.4 to 1.5 vs. 1.1 to 1.2) and estimated less precisely (SE 0.17 to 0.24 vs. 0.23 to 0.31) than with TMR. This reflected the low C₃₁ and C₃₃ contents of corn compared to alfalfa, confounding their delineation in the mixture. With increasing complexity, such as rangelands, delineating individual plants becomes even more challenging. Intake estimates were adequate regardless of the approach adopted. However, using a pooled fecal sample decreased the number of samples evaluated and increased precision, both advantages. Furthermore, with simple diets, intakes could be reliably estimated using plant waxes. USDA is an equal opportunity provider and employer.