What are the brown fumes sometimes seen around a silage clamp?

This gas is formed when nitrates are broken down immediately after ensiling and is a mixture of nitrogen oxide gases, some of which are highly toxic. It has a yellow-brown colour and a bleach-like smell. It is heavier than air so tends to hang around near the base of the clamp.  Levels as high as 500 times the safe dose can be found.

With wet grass the gas dissolves in the moisture and within 2 to 3 days will have disappeared but with high dry matter grass there may not be enough moisture to trap it and it is free to be released. When dissolved in moisture it forms nitric acid, very high concentrations of which can turn silage bright yellow-orange with a pH of below 2. It can also dissolve in the moisture in lungs if breathed in.

This gas is a particular problem with enclosed clamps since open clamps allow the gas to disperse more quickly. With enclosed clamps make sure there is adequate ventilation and in all cases keep people and animals well away. It will usually disperse in a few days but can hang around for several weeks.