Milorganite as a Deer Repellent

Milorganite is a nitrogen fertiliser that people spread to keep deer off things. It has been made from heat-dried Milwaukee biosolids since 1926, the name is a contraction of Milwaukee Organic Nitrogen, and nothing on the bag has ever mentioned deer.

Unusually for a folk remedy, this one has been tested properly. The University of Georgia ran both an ornamental trial and a food plot trial, and the results are more specific, and more limited, than the reputation.

What the Georgia trials found

The food plot study is the useful one, because it counted individual plants rather than eyeballing damage. Five properties in the north Georgia Piedmont, deer density estimated at 30 to 50 per square mile, paired half-acre plots of no-till soybeans, 240 lb per acre of Milorganite broadcast at emergence, 500 plants scored per plot.

Read the first row and it looks excellent. Read the last and four out of five treated plants have been browsed anyway.

That is the honest shape of it: Milorganite buys time. In four of the five soybean sites it delayed browsing from about a week to more than five weeks. It does not create a boundary the deer will not cross.

The protection decays, and reapplication is untested

The UGA authors suggest reapplying around day 14 would extend the effect. They also say plainly that they did not test it. That suggestion has since been repeated all over the internet as though it were a finding.

A later dose-response study in Texas tried three application rates on chrysanthemums. All three reduced damage, higher rates trended better, and the differences between the lowest rate and the higher ones were not statistically significant. More is not reliably better.

The same study attributed the protection to material landing on the plant itself rather than to odour dispersing through the surrounding air, which quietly undermines the way most people apply it.

Why the bag will never say "deer"

This is the part that explains the strange situation of a product with published efficacy data and no claim on the label.

Under federal pesticide law, a product becomes a pesticide the moment the seller claims, states or implies that it repels a pest. Not when it contains a pesticide. When the claim is made. Milorganite's own FAQ says it makes no claims regarding use as a deer repellent, and that sentence is doing legal work.

The cheap route is not open to them either. There is an exemption for minimum-risk pesticides, and the permitted active ingredient list includes putrescent whole egg solids, dried blood and garlic oil, which is why most consumer deer repellents can skip federal registration. Biosolids are not on that list. A deer claim on a Milorganite bag would require full registration, not the exemption.

So the silence on the packaging is not modesty and it is not a hint that the product secretly works. It is what the law requires of a fertiliser company that does not want to become a pesticide registrant.

The formulation changed after the research

Worth knowing before you copy an application rate off a forum.

Both UGA publications describe Milorganite as 6-2-0. The current guaranteed analysis is 6-4-0. The available phosphate has doubled.

Nitrogen is unchanged at 6 per cent, so the nitrogen arithmetic below still holds. But anyone applying the 2005 to 2007 ornamental rates today is putting down twice the phosphorus those researchers did, on beds that in most established gardens did not need phosphorus in the first place.

The nitrogen ceiling

Converted to the units on a fertiliser spreader, the trial rates separate sharply.

• The food plot rate that worked, 240 lb per acre, is about 0.33 lb of nitrogen per 1,000 sq ft. Agronomically trivial.
• The ornamental trial rate is about 1.4 lb N per 1,000 sq ft.
• The highest rate in the Texas dose-response work is about 6.1 lb N per 1,000 sq ft.

For scale, Maryland's lawn fertiliser law caps a single application at 0.9 lb N per 1,000 sq ft and the whole year at 2.7 for tall fescue. That top trial rate is roughly seven times the per-application cap and more than double the annual one.

Nobody is suggesting you fertilise a garden bed to a lawn statute. The point is that the rates which protected ornamentals best are in the range where you are making a real fertiliser decision, not a free one, and several states regulate that decision on turf. Check your own state before making this a routine.

There is a second-order problem too. Push soft, sappy growth with surplus nitrogen and you have made the planting more attractive to browse, not less.

The winter gap

This is the limitation that matters most and gets mentioned least.

Milorganite's nitrogen is released by soil microbes, and that release is gated by temperature and moisture. Microbial activity needs soil between roughly 55 and 85°F. Outside that window the material sits where it fell and waits.

Deer browse pressure peaks in late winter, when natural forage is gone. That is exactly when the soil is too cold for the product to do anything, and exactly when the damage that makes people search for a repellent actually happens.

Apply it in February and you have spread inert granules that will release their nitrogen into spring growth. For winter protection, this is not the tool. Fencing or trunk guards are.

It is the weakest class of repellent by design

Repellents are usually grouped by how they reach the animal: applied to the plant, or spread through an area.

In the National Wildlife Research Center's comparison of twenty commercial repellents on captive black-tailed deer over an eighteen-week winter test, contact repellents applied to the plant generally beat area repellents, and no product eliminated browsing. Eight of the nine most effective products over the first eleven weeks gave off sulphurous odours, with decaying animal proteins at the top.

The same body of work found little evidence that deer abandon an area treated with an area repellent, except where there is highly palatable food easily available somewhere else.

The UGA authors describe Milorganite explicitly as an area delivery repellent. It is, by construction, in the category that performed worst.

For contrast, Deer Away Big Game Repellent Powder, at 37 per cent putrescent whole egg solids, virtually eliminated browsing for four weeks and gave good protection for eight to twelve.

Using it sensibly

• Around beds rather than broadcast everywhere, and accept that you are delaying browse rather than preventing it.
• Reapply on a schedule you decide, knowing the day-14 suggestion was never tested.
• Watch the total nitrogen across a season, and check your state's rules if you are treating turf.
• Stop by late summer so you are not pushing tender growth into autumn.
• Do not rely on it between December and March. Nothing is releasing.

Where this leaves it

Worth using if you were fertilising anyway, have moderate pressure, and want to delay browsing on new planting through the growing season. There is real evidence for exactly that.

Not worth buying as a repellent. Egg-based sprays outperform it, work in the cold, and cost less per season. And where the planting genuinely matters, no scent product of any kind is the answer.

The trials were run on white-tailed deer in Georgia and Texas. Whether the same delay holds for mule deer or black-tailed deer on western browse has not been tested, and it would be a guess to say it does.