Back in 2015, a dozen deer were found dead in South Hampton, NH, due to over-feeding during winter – an extreme case of how well-meaning attempts to help deer in winter can backfire.
Northern Woodlands magazine – which is, by the way, a great magazine; you should subscribe – has an article in the latest issue that explains the issue:
Deer are ruminants, which means they have a four-chambered stomach, like cows and sheep. Each chamber contains microorganisms to help with digestion. These microbes become tuned in to a winter diet of twigs and buds, nuts, any fruits and berries that persist, and whatever grasses they can find. A sudden change in diet – say to supplemental corn or rich hay – can wreak havoc on this system.
It’s a good article: Read the whole thing here. Here’s an interesting tidbit:
Because of this and other dangers of supplemental feeding, it is illegal to feed deer in Vermont. Maine and New Hampshire have not outlawed the practice; New Hampshire Fish and Game Deer Project Leader Dan Bergeron said several bills – supported by his department – seeking to ban deer-feeding have been introduced in the state legislature over the years, but none have passed into law.