Table 1.

Search strings employed for querying the selected databases.

Database name	Search string
Scopus	ABS ((“near infrared spectrometer” OR “near infrared spectroscopy” OR NIRS OR “handheld spectrometer” OR “portable spectrometer” OR “near-infrared”) AND (livestock OR animals OR “live animals” OR “livestock diet quality”) AND (fecal OR faecal OR food OR feed OR forage)). The search was conducted specifically within the abstract (ABS).
IEEE Xplore	((“near infrared spectrometer” OR “near infrared spectroscopy” OR NIRS OR “handheld spectrometer” OR “portable spectrometer” OR “near infrared” OR “near-infrared”) AND (livestock OR animals OR “live animals” OR “livestock diet quality”) AND (fecal OR faecal OR food OR feed OR forage)) (anywhere).
Science Direct	((“near infrared spectrometer” OR “near infrared spectroscopy” OR NIRS) AND (livestock OR animals OR “livestock diet quality”) AND (fecal OR forage OR feed)). The search was conducted within the title, abstract, and keywords.
Web of Science	X=((“near infrared spectrometer” OR “near infrared spectroscopy” OR NIRS OR “handheld spectrometer” OR “portable spectrometer” OR “near infrared” OR “near-infrared”) AND (livestock OR animals OR “live animals” OR “livestock diet quality”) AND (fecal OR faecal OR food OR feed OR forage)). The search string format for Web of Science is - AB(X) OR AK(X) OR TI(X). The search was conducted within the title (TI), abstract (AB), and author keywords (AK).
PubMed	((“near infrared spectrometer” OR “near infrared spectroscopy” OR NIRS OR “handheld spectrometer” OR “portable spectrometer” OR “near infrared” OR “near-infrared”) AND (livestock OR animals OR “live animals” OR “livestock diet quality”) AND (fecal OR faecal OR food OR feed OR forage)). The abstract and title were used as the search fields for conducting the search.