Abstract/Summary

Improving the sustainability of dairy farming in countries operating diverse dairy farming systems (i.e. Great Britain (GB)) requires information on phosphorus (P) management considered across multiple systems. Such information is currently limited in GB. Therefore, throughout this thesis the flow and management of P has been considered system-specifically. Furthermore, the current status of P balance on dairy farms needs to be determined to identify mitigation strategies to reduce P loss from dairy farms. In Experiment 1, questionnaire surveys of dairy farmers (n = 139) and feed advisers (n = 31) were conducted to provide new information on P feeding practices. The survey revealed most farmers (72%) did not know the P concentration in their lactating cow’s diet and did not commonly adopt precision P feeding practices, indicating P feeding in excess of the amount recommended optimum to support certain level of milk production. Regardless of system, farmers largely relied on a feed professional (70%), and these farmers were more likely to analyse forage P (P = 0.02), but farmers of pasture-based systems relied less on feed professionals (P < 0.05). Both farmers (73%) and feed advisers (68%) were unsatisfied with the amount of training on P management available. Therefore, feed advisers’ influence over P feeding should be better utilised, particularly in a housed system via training and other strategies need to be adopted to promote forage P analysis in pasture-based systems. In experiment 2, the farm-gate (FPB) and soil-surface P balance (SPB) and P use efficiency (PUE) were calculated for 29 dairy farms using the principles of the Annual Nutrient Cycling Assessment Tool, which allowed the capture of important differences in P flows between systems (i.e. P concentration in milk and manure). Additionally, the main determinants of P balance were investigated using regressions. The mean FPB and SPB of 9.58 kg/ha and 7.47 kg/ha, respectively, across all systems indicated opportunity to improve PUE. Blended pasture-based systems (classification 2 and 3) had higher PUE than the strict housed system (Classification 5; P < 0.05). The study findings confirmed that formulating dairy cow diets with a P concentration that closely matches dietary P requirement of dairy cows will reduce the amount of P import via concentrates, which will eventually improve PUE in housed systems. However, increasing the inclusion rate of home-grown feeds into a herd’s diet would improve PUE in pasture-based systems. Experiment 3 was the first to use data collected directly from farmers in FARMSCOPER to simulate environmental P loading and identify a cost-effective suite of mitigation methods for housed (n = 20) and pasture-based (n = 7) dairy farms. Across both systems, ‘current’ implementation of mitigation methods was simulated to have minimally reduced environmental P loading from a mean ‘baseline’ of 0.63 to 0.56 kg P/ha (11%). The environmental P loading in the ‘baseline’ and ‘current’ scenarios positively correlated with milk production on a kg and kg/ha basis (P ≤ 0.001 and P = 0.033, respectively). Therefore, the current study highlights the importance of mitigating environmental P loading from GB dairy farming especially considering the increasing prevalence of higher yielding herds and housed production systems. Simulated environmental P loading was reduced by ~50% and ~60% without incurring annual financial losses by implementing different existing mitigation methods for pasture-based and housed systems, respectively. Therefore, emphasis should be put on increasing the system-specific implementation of existing methods to mitigate environmental P loading (i.e. knowledge transfer). In conclusion, the current thesis provided much needed new information across diverse dairy farming systems in GB on 1) P management and flows, 2) the current status of PUE and 3) a suite of cost-effective mitigation methods to reduce environmental P loading. Collectively, this information will contribute towards developing system-specific strategies to improve the sustainability of GB dairy farming in regard to P use.