Applying liquid manure to stubble in the fall has several advantages. Crop growth isn't interfered with, fields are smooth, and there are usually enough dry days when the soil is firm. The downside is that much of the nitrogen will be lost to the air - up to 100 per cent of the ammonium fraction if the manure is applied in warm, dry conditions. There is also increased potential for runoff. Delaying application until cool weather, incorporating the manure, and avoiding application on sandy soils or soils with shallow groundwater will all help to reduce environmental risk.
Winter spreading of liquid manure is unacceptable at any time.
The risk of runoff is just too great. Pre-plant spreading of manure in the spring should give the maximum use of nutrients, but there are also some hazards to spring application. The effects of this compaction can last several years. Waiting until the ground is fit carries its own penalty, of course. For maximum yields, corn should be planted by May 10, but in central Ontario we get five field workdays before this date, two years out of three.