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Fecal NIRS: a tool for predicting nitrogen content in feces and the content of digestible organic matter in diet of sheep grazing a natural pasture in southern Brazil

Marcelo Gindri ; Elissandra M. C. Zílio ; Mariana F. Rentas ; Ricardo Gonçalves Gindri ; Thais Regina Longo ; Andressa de Souza Guterres ; Claudio Antonio Pozo ; Gilberto Vilmar Kozloski
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The accurate estimative of the energy value of forages ingested by grazing ruminants is a
relevant challenge. The objective of this study was to develop models to predict the
nitrogen content in feces and the content of digestible organic matter (DOM) in forage
grazed by sheep in a natural pasture of southern Brazil using the near infrared reflectance
spectroscopy (NIRS) of feces samples. Data and samples were taken monthly throughout
June of 2014 to June of 2015,from six sheep grazing the natural pasture. The total feces
were collected and weighted in the last five days of each month, using appropriated
harnesses and bags. Fecal samples were taken, dried (55°C), ground (1 mm) and used to
quantify the dry matter (DM), organic matter (OM) and nitrogen (Nfecal) (% DM)
content, and the absorbance spectra in the near infrared range. The organic matter intake
(OMI) was estimated based on the amount of Nfecal excreted in feces using the model:
OMI (g/day) = 1.1 + (101.2 × fecal N (g/day)), which was developed based on data from
digestibility trials with sheep fed hay from a similar natural pasture. The DOM content of
forage was then calculated as: DOM (% of DM) = (1 – (fecal OM (g/day) / OMI)) × MO
in forage (% of DM). The N and DOM data were individually related to the fecal NIRS
spectra using the partial least squares regression method. For developing the models, 80%
of the total sample was used, and the remainder was used for external validation. The
coefficient of determination for the calibration and external validation were 0.90 and 0.88
for N, and 0.91 and 0.86 for DOM, respectively. The standard error of the calibration and
external validation were, respectively, 0.07 and 0.09 for N, and 1.64 and 2.14 for DOM,
respectively. The results indicate that the fecal NIRS can be used as a tool for predicting
Nfecal and DOM in sheep grazing natural pastures of southern Brazil.