Minimizing the use of artificial fertilizer nitrogen (AFN) and optimizing conditions for biologically fixed nitrogen (BFN) in association with clover is a cost-effective strategy for lowering emissions from dairy farms. The objectives of the present study were to examine the productivity of clover-BFN-grassland for pasture and milk production, to quantify the BFN that underpinned this productivity, spring growth and the length of the growing season, nutritive value of pasture at different stages of the growing season and feed budgets. Data were collected from systems-scale experiments at Solohead Research Farm Co., Tipperary, Ireland (52°51′N, 08°21′W). There were 3 systems that had an average of 27 cows per system per year and an average stocking rate of 2.53 cows ha −1 . The intensive control (INT) included average annual AFN input of 265 kg ha −1 , the average clover content of herbage DM was 110 g kg −1 , and slurry was applied by splash plate. The BPN included average annual AFN input of 99 kg ha −1 , clover content averaged 230 g kg −1 and slurry was applied by trailing shoe. Best practices including AFN (BPO) received minimal (<5 kg ha −1 ) inputs of AFN, clover content averaged 280 g kg −1 and slurry was applied by trailing shoe. There were no differences in herbage DM production per ha and milk production per cow or per hectare between systems. Annual BFN ranged between 60 and 252 kg ha −1 depending on AFN input and clover herbage DM production, which were inversely related to each other. There were no differences between systems in spring herbage DM production, in the lengths of the grazing seasons and in each of the annual average quantities of pasture, silage, and concentrates consumed per cow. There was no evidence that annual herbage DM production was more variable from year to year on the clover-BFN-grassland (BPN and best practices with minimal AFN) compared with INT. Under current and likely future restrictions on AFN use within the European Union and elsewhere, optimizing clover-BFN-grassland receiving little or no AFN input can maintain herbage and milk production on pasture-based dairy farms.
Cashman et al. (Sun,) studied this question.