The combination of the n-alkanes and C-13 techniques to estimate individual dry matter intakes of herbage and maize silage by grazing dairy cows

Two studies were conducted at Massey University in 1998. Expt 1 investigate d the combined use of n-alkanes and C-13 techniques for the estimation of i ndividual proportions and total intakes of herbage (H) and maize silage (MS ) by dairy cows. Expt 2 measured the variation in the amount of MS consumed by individual cows fed as a group. In Expt 1, six dry Holstein-Friesian co ws were kept indoors and fed a diet containing one of the following H:MS ra tios (dry matter basis): 100:0, 80.20, 60:40, 40:60, 20:80 or 10:90 for 25 days. Cows were dosed with a slow-release capsule containing 8 g of dotriac ontane (C-32) and 8 g of hexatriacontane (C-36). Intake estimates were base d on individual faecal samples collected twice daily during two 5-day perio ds. In Expt 2 (grazing trial), 12 early-lactation cows were selected from a commercial herd of 48 autumn-calving cows and blocked into pairs according to milk yield, lactation length and lactation number in a complete block r andomized design. Within each pair, cows were randomly assigned to two trea tments: supplemented, S (4 kg MS DM per cow after the morning milking in fe ed troughs) or not supplemented, NS. Another eight cows were randomly selec ted from the rest of the herd to increase the number of individual estimati ons of H and MS intakes. Cows grazed perennial ryegrass-white clover pastur e during the rest of the day. In Expt 1, H:MS ratios were not accurately pr edicted by the odd-chained n-alkanes, but there was a strong linear relatio nship between the concentration of C-13 in faeces and actual H:MS ratios in the diet. The C-13 method was therefore combined with the n-alkanes, resul ting in accurate estimations of H and MS intakes. In Expt 2, a large variat ion was observed among individual cows in their daily intakes of MS (range 0.94 to 5.09 kg DM per cow, coefficient of variation = 36 %), but this vari ation in MS intake was not associated with milk yield (P > 0.05). The resul ts indicate that the n-alkane and C-13 techniques can be successfully combi ned to estimate the intake of MS and H of grazing cows supplemented as a gr oup. Under the conditions of the present study, individual cows differ cons iderably in the amount of maize silage consumed per day, although the reaso ns for this are not clear.