You could sample the forage while it is going into the silo in an upright silo. The nutritive content will not change unless you ensile it when it is too wet or too dry. Under these two conditions, nutrient content will change. Make sure you get a representative sample.

If it is not an upright silo and is a bunker silo, you might wait until after the fermentation phase is complete to take a sample. The nutrient content will not change, but it will be easier to get a representative sample. After you open the face of the silo to feed, take a grab sample from the top, middle, and bottom at three locations across the face of the pit to get your sample. Then mix these grab samples to get the sample that you will send in for analysis.

If you are concerned about nitrates, you should sample after a full 28- to 30-day fermentation of the silage has been completed. The fermentation process will reduce nitrates by 33 to 50%.