VARIABILITY OF N-ALKANE CONTENT IN A NATURAL PASTURE AND IN FECES OF GRAZING DAIRY-COWS

The experiment was conducted on a natural alpine pasture with six dairy cows grazing for two periods of 15 days each in contiguous sectors. Three animals only had access to the pasture (Treatment P) while the other three received 3 kg concentrates per day (Treatment PC). Each animal was dosed daily with a pellet containing an average of 1084 mg of C-32 on a paper support. During the last 6 days of each experimental period, four faecal grab samples per day were taken from each animal and later mixed to form individual daily samples. For 3 days in succession, the four samples from one animal per treatment were held separate. The botanical composition of the pasture was expressed as a percentage of the average area covered by the various families and species. The concentration of the n-alkanes in the pasture herbage was measured, for each grazing period, in samples collected from protected areas at the beginning and end of each grazing period. The n-alkanes were also determined in the concentrates and, for C-32, in the pellets. The pasture had a considerable variability both in terms of the number and distribution of the plant families and species and the content of the n-alkanes. The variability of the concentration of C-32 in the dosed pellets was very low. The concentration of n-alkanes in the faeces was influenced by the grazing zone while the diet factor had a significant interaction with the days of sampling. The diurnal variations in the faecal concentrations were lower than the variations between days. The comparison between the concentrations obtained from one, two, three or four daily samples showed that one or preferably two samples supplied the same information as four samples. The presence of concentrates in the diet reduced the diurnal variability of almost all the n-alkanes considered.