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  • Publication
    Evaluating the role of scale in the sustainability of nutrient management of pasture based dairy systems
    (University College Dublin. School of Agriculture and Food Science, 2020) ;
    Primary data was gathered from commercial grass-based dairy farms in the south and south east of Ireland. Nutrient use at the farm, field and paddock scale was characterised and evaluated simultaneously, to determine the sustainability of nitrogen (N), phosphorus (P) and potassium (K) management practices. Additionally, a spatially explicit precision nutrient advice strategy (including soil pH measurement, soil test for P (STP) and K (STK) concentrations), was implemented on the selected farms. The nutrient balance (BAL) i.e. nutrient source pressure, and use efficiency (UE) indicators, were evaluated across these farms and spatial scales from 2015 to 2017, while changes in soil fertility were monitored from 2015 to 2018. As well as this, herbage nutrient concentration was assessed through spring, summer and autumn of 2015 and 2016 at the paddock scale. Mean nitrogen use efficiency (NUE), and phosphorus use efficiency (PUE), were found to be significantly lower (p < 0.05) at farm scale (NUE: 28% and PUE: 52%), compared to results at field scale (NUE: 57% and PUE: 100%). Mean potassium use efficiency (KUE), was found not to be significantly different between these scales (KUE: 56% and 91%, respectively). With regard to nutrient source pressure, phosphorus balance (PBAL) was found to be significantly higher (p < 0.05) at the farm scale than at the field scale (9 and 6 kg P ha-1, respectively), while the differences in nitrogen balance (NBAL) (165 and 139 kg N ha-1, respectively) and potassium balance (KBAL) (23 and 39 kg K ha-1, respectively) were not found to be significant between scales. Furthermore, variability (denoted by CV: Coefficient of Variation) in UE was found to be substantially larger between individual fields and paddocks (CV, 67%, 74% and 127% for N, P, and K, respectively) than between individual farms (CV, 39%, 33% and 76% for N, P, and K, respectively). Variability in BAL, was also found to be substantially higher between individual fields and paddocks (CV, 77%, 294% and 179% for N, P, and K, respectively) than between individual farms (CV, 34%, 58% and 86% for N, P, and K, respectively). Indicators of soil fertility (soil test values and soil optimum indices distribution) and indicators of nutrient management (BAL and UE) were found to develop towards more agronomically optimum levels. For example, mean results for paddocks across these farms showed that low fertility paddocks increased their soil pH to 6.3 or medium/adequate soil levels, while the mean results for high fertility paddocks show that STP decreased to 6.7 mg l-1, and STK decreased to 142.9 mg l-1, closer in range to agronomic optimum soil levels. Mean field scale PBAL and KBAL surpluses decreased significantly between 2015 and 2016 also (from 7 to 4 kg P ha-1 and 52 to 34 kg K ha-1). Finally, it was shown that the paddock scale herbage nutrient concentration strongly relies upon soil nutrient supply and soil fertility management. STK level was the primary factor of herbage K and Calcium (¿2 = 0.26 and 0.11, respectively) while soil test Magnesium level was the primary factor of herbage Magnesium (¿2 = 0.28) and STP level was the primary factor of herbage P (¿2 = 0.35). Soil pH level was the primary factor of herbage Manganese and Calcium (both ¿2 = 0.15). The results presented in this thesis show that the scale at which nutrient use indicators are observed (e.g. farm or field/paddock), has a significant influence on the apparent nutrient efficiency and nutrient source pressure of specialised dairy farms. This has potentially important implications for agri-environmental policy makers, nutrient regulation and nutrient monitoring systems.