Fecal N-alkanes variation in lactating dairy cows grazing a tropical pasture (cynodon nlemfensis vanderyst Var. Nlemfuensis)
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The objective of this study was to measure fecal n-alkane concentration variation and herbage intake in grazing lactating dairy cows fed with two sources of fat: conjugated linoleic acid (CLA) or Megalac (Control). Cows were dosed with n-alkanes using a controlled-release capsule. There was no difference in herbage dry matter intake between CLA and Control. In the first period, there were no differences in fecal concentration of both natural and synthetic n-alkanes from the capsules. For the second period, there was a difference in fecal concentration for time of collection for all natural, dosed, and alkanes ratios. In the third period, there was a difference for time of collection for natural n-alkanes and their respective ratios, with higher excretion values recorded in the afternoon. In addition, there was an effect of day of collection for all n-alkanes and ratios, with higher excretion values recorded on days 2 and 4. Overall, our results highlight the importance of strict control of grazing management and sward structure in studies where external markers are used. Monitoring herbage, sward and animal managements might have an influence on n-alkanes excretion, causing a change in the ratio of their fecal concentration. These modifications may result in erroneous estimates of intake.