Managing the nitrogen status of subtropical dairy pastures for production, efficiency and profit

Pastures that contain winter-active annual ryegrass (ARG) in association with summer-active kikuyu are valuable for dairy production in subtropical regions. The two pasture phases have different challenges to increase production and profitability – for the kikuyu phase the management of soil fertility is challenging as it requires synchronization with soil, plant and animal demands for energy and protein. Unsure how best to manage the soil nitrogen (N) during the kikuyu phase and deterred by the risk of a poor pasture dry matter response to N fertilizer; farmers tend to under-fertilize the kikuyu phase which was hypothesized to limit the potential productivity of not only the kikuyu, but adversely impact the N nutrition of the subsequent ARG phase. For the first-time different farming systems for a subtropical location were comprehensively compared using the mechanistic model DairyMod. The farming systems compared consisted of N fertilizer to only the ARG phase, N fertilizer throughout the year to both phases, N fertilizer only at the start of the kikuyu growth cycle or increased feed-grain supplementation during the kikuyu phase. An overall summary of the results is that although applying N fertilizer to the pasture all year lifted pasture productivity, particularly of the kikuyu, much of the extra herbage grown could not utilized by grazing cows and needed to be cut. At the period of establishment of ARG pasture in mid -autumn, the new finding was that soil mineral N did not differ significantly between the different systems, and therefore using N fertilizer to maintain soil and pasture N within an optimum range was shown to be expensive and inefficient. The major novel finding of the study was that the farming system was most profitably and productively managed by addressing deficiencies metabolizable energy of kikuyu with supplementary grain feeding, rather than using fertilizer.