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 dair y cows grazing for two periods of 15 days each in contiguous sectors. Three animals only had access to the pasture (Treatment P) while the o ther three received 3 kg concentrates per day (Treatment PC). Each ani mal 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 p eriod, four faecal grab samples per day were taken from each animal an d later mixed to form individual daily samples. For 3 days in successi on, the four samples from one animal per treatment were held separate. The botanical composition of the pasture was expressed as a percentag e of the average area covered by the various families and species. The concentration of the n-alkanes in the pasture herbage was measured, f or each grazing period, in samples collected from protected areas at t he beginning and end of each grazing period. The n-alkanes were also d etermined in the concentrates and, for C-32, in the pellets. The pastu re had a considerable variability both in terms of the number and dist ribution of the plant families and species and the content of the n-al kanes. The variability of the concentration of C-32 in the dosed pelle ts was very low. The concentration of n-alkanes in the faeces was infl uenced by the grazing zone while the diet factor had a significant int eraction with the days of sampling. The diurnal variations in the faec al concentrations were lower than the variations between days. The com parison between the concentrations obtained from one, two, three or fo ur daily samples showed that one or preferably two samples supplied th e same information as four samples. The presence of concentrates in th e diet reduced the diurnal variability of almost all the n-alkanes con sidered.