NUTRIENT LOSSES FROM TWO CONTRASTING DAIRY COW GRAZING SYSTEMS

A key concern in farming systems that seek to maximise the grass content of livestock diets is the management of grazing so as to avoid damage to soil structure during periods with high antecedent soil moisture conditions. The aim of this study was to investigate, under the current codes of good agricultural practice, the impact of two contrasting grazing systems on soil structure and nutrient export from grazed grasslands. The two grazing treatments were (1) WinterCalf treatment, involving winter-calving Holstein-Friesian dairy cows, which had a mean 'start of turnout' date of 28 March, and were offered 5.0kg concentrate/cow/day throughout a 214-day grazing season; and (2) SpringCalf treatment, involving spring-calving Jersey x Holstein-Friesian dairy cows, which had a 'start of turnout' date of 17 February, and were offered between 1.0 and 2.0kg concentrate/cow/day throughout a 260-day grazing season. Maximum reliance on grazed grass was a key objective of the latter system. Mean stocking rates during the grazing season were 5.1 and 4.3 cows per hectare with WinterCalf and SpringCalf systems, respectively. The experimental site was divided into four replicated blocks (A, B, C and D), with each block containing two grazing plots (425m(2) and 500m(2), for the WinterCalf and SpringCalf systems, respectively). In addition to the two grazed treatments, an ungrazed treatment (UG) was established within each of the WinterCalf and SpringCalf treatment plots by fencing off a 12m(2) exclusion sub-plots, which livestock could graze but not tread on. Changes in soil structure were recorded before and after grazing on all plots and with rainfall simulation to investigate the impact of soil structural changes on nutrient export to water. The results of the study demonstrated that over a complete grazing season, under the current code of good grazing practice, nutrient export from the SpringCalf treatment was not significantly greater than that from the WinterCalf treatment. However, results highlighted an increase in nutrient export from the SpringCalf treatment compared to the ungrazed treatment. These increases were as a result of a significant increase in overland flow volume from the SpringCalf treatment compared to the UG treatment. The time taken to initiate overland flow was a key factor in this relationship, with an increase from an average value of 536 seconds for the SpringCalf treatment to 1709 seconds for the Ungrazed treatment. Allowing time for soil to recover from compaction could be an effective mitigation strategy for reducing nutrient export from soil, particularly in areas where soil P is significantly above the agronomic optimum for grass production.