Buildings housing livestock are the second largest source of ammonia (NH 3) emissions from UK agriculture. In the UK c. 42% of NH 3 emissions from buildings are from those in which cattle and pigs are bedded on straw. Additional straw may reduce NH 3 emission by reducing airflow across surfaces soiled by urine, and by immobilization of ammonium–N. The object of this study was to quantify the effects of increasing straw use on NH 3 emission from buildings housing cattle or pigs. The extra straw was applied in increments of up to 100%, either over the entire floor (broadcast) or targeted to the areas where most excreta were deposited. An increase of 33% straw, broadcast over the entire floor, reduced NH 3 emission from cattle by 50%, but greater addition did not give any further significant reduction. However, for pigs only the broadcast addition of 100% more straw reduced NH 3 emission and then by only c. 20%. Targeted use of an additional 33% straw reduced emission from cattle by 22%, compared with broadcasting the same quantity of straw, but further additions of targeted straw use did not increase abatement. When the straw was targeted in the pig buildings there was no benefit from using additional straw. Thus optimum results may be obtained from an addition of 33% extra straw, targeted to the soiled areas for cattle, but 100% extra straw broadcast over the whole floor for pigs.