Up to one in seven farms fail to comply with Nitrate Vulnerable Zone rules, according to an Environment Agency report.
Indoor pig unit operators were particular offenders, with 33% found to have inadequate records of N spread on crops as manure or effluent.
This compares with 35% of dairy producers and 32% in the beef sector.
At stocking rates of 10 sows to the acre (25/ha), outdoor pig units are unlikely to risk breaching NVZ limits.
According to the National Pig Association, pig keepers should also keep their movement records up to date, including full destination addresses with postcodes and evidence that stock movements on and off holdings balance with livestock numbers on inspection day.
Movement records for the preceding six months may also come under scrutiny. These will need to match NVZ limits and tie in with Trading Standards and Rural Payment Agency figures.
Outdoor pig keepers will also need to avoid breaching the new boundary rules which came into effect in July under the Single Farm Payment system.
Outdoor pig land must have a 1m strip between the edge of the pig fields and ditch edges, or 2m from the centre of hedges.
These strips need to be maintained and kept in Good Agricultural and Environmental Condition, with failure risking the loss of Single Farm Payments.
Although the Environment Agency and DEFRA have been exercising a “light touch” during the early period of the new SFP regime, there are signs that inspection procedures will become tougher in future years.