Nitrate accumulates in soils during times of drought due to microbial action. Microbes create the nitrate as normal byproducts of their metabolism. In soils with normal moisture levels the nitrate dissolves into the water and moves at relatively low levels in to the plants to be used to create protein. In dry soils the nitrate can’t move into the plant roots so it accumulates in the soil. When the rain finally falls this accumulated nitrate all dissolves into the dater at once. The plants need time to recover so the nitrate Nitrate moving into plants stressed by drought moves upward into the plants slowly. This recover process can take 3-5 days in grass species including corn). Since the nitrates are higher in lower portions of the plant if you are going to harvest immediately for feed raise your cutting height. This is especially important with corn silage if you intend to green chop to feed while harvesting corn. Nitrates in silage reduce 50 to 60 % in silage but it can take 3-4 weeks for levels to drop. For extremely browned out grass species – like a lot of hay fields, pastures and lawns this summer the brown stalks will not take up the moisture – they are dead and the capillaries needed to move liquid up the plant have broken and will no longer function. This means that the forage that springs out of the stems at the soil surface – all the “new growth” bright green grass will be what contains the high nitrate levels. For pastured animals you should move animals off the recovering pastures and keep them confined into winter feeding areas and keep feeding them stored feed until the new grass has a chance to grow, reestablish and metabolize the extra nitrate into plant protein before the animals graze it again.
You can test for high levels by sending a sample to a forage testing lab. Nitrate N (NO3-N) levels <1,000 ppm on a dry matter basis are considered safe. Nitrate between 1,000 – 1,700 should be left to mature in the field or left to cure on a silage pad before feeding out. Nitrate levels > ~ 2,000 can be acutely toxic and potentially lethal to livestock. There will likely be a run on the labs since drought has been widespread throughout the Northeast region, so keeping the animals off pasture, or waiting to harvest for a week or so might be the best way to manage the situation.
For more in-depth information, here are a few links
https://extension.psu.edu/be-aware-of-the-risks-of-nitrate-accumulation-in-annual-forages-this-year
https://extension.psu.edu/what-to-do-with-droughty-corn-fields