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

Abstract

Two studies were conducted at Massey University in 1998. Expt 1 investigated the combined use of n-alkanes and 13C techniques for the estimation of individual 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 cows were kept indoors and fed a diet containing one of the following H[ratio ]MS ratios (dry matter basis)[ratio ]100[ratio ]0, 80[ratio ]20, 60[ratio ]40, 40[ratio ]60, 20[ratio ]80 or 10[ratio ]90 for 25 days. Cows were dosed with a slow-release capsule containing 8 g of dotriacontane (C32) and 8 g of hexatriacontane (C36). Intake estimates were based on individual faecal samples collected twice daily during two 5-day periods. 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 randomized design. Within each pair, cows were randomly assigned to two treatments: supplemented, S (4 kg MS DM per cow after the morning milking in feed troughs) or not supplemented, NS. Another eight cows were randomly selected from the rest of the herd to increase the number of individual estimations of H and MS intakes. Cows grazed perennial ryegrass-white clover pasture during the rest of the day. In Expt 1, H[ratio ]MS ratios were not accurately predicted by the odd-chained n-alkanes, but there was a strong linear relationship between the concentration of 13C in faeces and actual H: MS ratios in the diet. The 13C method was therefore combined with the n-alkanes, resulting in accurate estimations of H and MS intakes. In Expt 2, a large variation 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 variation in MS intake was not associated with milk yield (P > 0·05). The results indicate that the n-alkane and 13C techniques can be successfully combined to estimate the intake of MS and H of grazing cows supplemented as a group. Under the conditions of the present study, individual cows differ considerably in the amount of maize silage consumed per day, although the reasons for this are not clear.