This catch-all phrase is used to describe any technique that measures the properties of something without actually touching that thing. For example, many of the satellites in orbit around the earth carry instruments that are continuously probing our surface and atmospheric environments. Electromagnetic waves of different frequencies are directed towards the planet and the materials (gases, liquids, solids) with which they interact reflect a unique wave signature back to the sensors. By comparing these unique signatures to the results of in situ (i.e. non-remote) measurements the instruments are calibrated to estimate conditions on and around the planet from hundreds of kilometers above. It really is rocket science! My primary research interest involves using such data to help understand how air pollution affects the health of large human populations.
What does this nerdy lecture have to do with horses? One of the methods by which to measure the nutritional content of hay is near infrared reflectance spectroscopy (NIRS). Core samples from multiple bales are mixed together and loaded into an instrument that (a) subjects them to electromagnetic waves and (b) measures the unique response signature of the hay. Results are then compared to those from hays that have also been tested via wet chemistry and the nutritional value of the forage is statistically inferred. It's not the most accurate method, but given my interest in remote sensing I think it's got the most geek cred. Yesterday the orchard grass hay I was hoping to buy went through the NIRS at Unifeed and was estimated to contain 15% water soluble carbohydrates (WSC, closely related to NSC). Too sweet for Tonka, so the search continues. As a stopgap measure my very helpful acquaintance at Unifeed has offered up some 8% WCS hay that her horses don't like -- people are so fantastic, aren't they?