Impact of soil type and fertilizer level on forage self sufficiency of Irish dairy farm

International audienceFor pasture-based dairy production systems, identifying the appropriate yearly stocking rate (SR; cows ha-1) based on the farm grass growth is the key strategic decision driving the overall farm business. This paper simulates the effects of varying stocking rates (2 to 3 cow ha-1, 0.25 unit changes), fertilizer nitrogen (N) application rates (0 to 300 kg N ha-1 50 kg ha-1 unit changes), soil type, (heavy and a free draining soil) through the PBHDM (Pasture-Based Herd Dynamic Milk) model in conjunction with the Moorepark St Gilles (MoSt) grass growth model. The model outputs show an average N response of 18.2 kg dry matter (DM) kg-1 N applied (range of 8.3-29.2 kg DM kg-1 N). Soil type did not affect grass growth or N response. More of the grass on the free draining soil was consumed as grazed grass (average +644 kg DM cow-1 and +1,611 kg DM ha-1 at 250 kg N ha-1 and SR 2.5 cow ha-1) due to increased grazing days, while cows on the heavy soils consumed more grass silage. Systems stocked at 2.5 cows ha-1 and applying 250 kg N ha-1 were self-sufficient for forage production, while a decrease of 50 kg of N fertilizer required a reduction of stocking rate of 0.20 cow ha-1 to maintain forage self-sufficiency