COMPARISON OF N-ALKANES AND CHROMIUM-OXIDE METHODS FOR ESTIMATING HERBAGE INTAKE BY GRAZING DAIRY-COWS

The experiment was conducted on an alpine pasture composed predominant ly of gramineae (more than 62% of ground cover) with 18 lactating Brow n Swiss cows (average yield 21.0 kg FCM, SD 3.14), which received a su pplement of concentrates according to their milk production (range 1.5 -8.0 kg day(-1)), Herbage intake was estimated for all the animals wit h the chromium oxide method (twice-daily dosing), associated with the in vitro OM digestibility, and with the n-alkane method, using C-31 as the internal marker and C-32 as the external marker (once-daily dosin g). The sampling of the herbage by hand plucking and the faeces (four grab samples per day) was performed for 5 consecutive days. In order t o evaluate the variability of the markers excretion, the daily samples of faeces from six animals were analysed separately. The repeatabilit y within day of the faecal concentration of the n-alkanes was higher t han that for the chromium oxide (0.41, 0.48 and 0.55 for C-31, C-32 an d the C-31/C-32 ratio, respectively vs. 0.18 for the chromium oxide). The highest proportion of the variation in the estimated intake was as sociated with the differences between days for the n-alkane method (54 .6% of the total), while the samplings within days were more important for the chromium oxide technique (67.0% of the total). On average, he rbage intake estimated with the two methods coincided if the recovery of the chromium oxide was assumed to be 95.5%. The correlation between estimates was 0.62* and did not appear to be influenced by the level of concentrates in the diet. Considering the mode of executing the tw o methods under comparison, it was concluded that the n-alkane techniq ue is easier to perform and less laborious.