The ruminant livestock sector in Northern Ireland (NI) is largely grassland based, with 96% of all agricultural land area classified as grassland (DAERA, 2018). Ruminant livestock traditionally graze outdoors from March/April until September/October, and are housed and offered predominantly grass silage based diets for the remainder of the year. However, in recent years there has been an increase in the number of NI farms where livestock, especially dairy cows, are either completely housed all year, or housed at night for extended periods throughout the year. This follows the trend observed within Great Britain (March et al., 2014). Given the small area of maize grown for silage in NI, grass silage looks set to remain the predominant conserved forage for the ruminant livestock sector, which is reflected in the fact that grass silage was produced on 37% (298,480 ha) of the total grassland area in 2017 (DAERA, 2018).Many factors affect grass silage composition and nutritive value, including sward composition, stage of maturity, weather conditions, soil type, harvest date, chop length, additive use, speed of silo filling and degree of compaction, type of cover, ammonia and fibre concentration, and feed-out rate post opening (Frame & Laidlaw, 2011).