Targeted duration controlled grazing - the effects of timing of grazing on nitrate leaching and treading damage : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
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Date
2019
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Massey University
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Abstract
Duration controlled (DC) grazing is successfully employed for two main reasons: to reduce N leaching and to protect pastures and soils from treading damage in wet conditions. These two objectives are currently very important for the New Zealand (NZ) dairy industry and this will only continue to increase with changing environmental expectations and legislation. However, while there have been a number of studies showing the benefits of DC grazing, there has not been any detailed research into the means to modify or improve the management of DC grazing systems. Therefore, the overall objectives of this research were to further the understanding of the advantages of targeted cow standoff from pastures during the late-summer to early-winter period and its effect on nitrate (NO3-) leaching, and to identify the relationship between soil water deficit (SWD), grazing duration and treading damage (related to standoff in winter to early spring).
Two experiments were conducted on a fine textured Tokomaru silt loam soil at Massey University’s Dairy 4 Farm near Palmerston North, Manawatu, New Zealand. One experiment was conducted to investigate the interaction of grazing duration and SWD on soil damage and pasture production, while the other experiment compared NO3- leaching under a standard dairy grazing system and a grazing system that used targeted DC grazing (i.e. during late-summer to early-winter).
Irrespective of drainage season and grazing treatment, the greatest NO3- concentrations in drainage occurred in the first seven to eight drainage events, which equated to the initial 50 to 100 mm of drainage. The majority of N losses following this were in the form of total organic nitrogen (TON). The average reductions in NO3- and TN leaching following targeted DC grazing in the summer to early winter periods of 2015 and 2016 were 28% and 20% respectively. The uniform return of slurry contributed to the DC treatment maintaining similar pasture production to the standard grazing treatment. Compared with year-round DC grazing, targeted DC grazing could be relatively inexpensive and so is likely to be a good mitigation option for N leaching in many cases, particularly on free draining soils where treading damage is uncommon.
Treading damage can be easy to observe; however, it can be time consuming to measure and even more difficult to quantify in a spatially aware manner. Therefore, two new and contrasting methods of assessing treading damage were developed, namely, the visual scoring method and the pugometer. The advantage of the pugometer is that it can automatically capture spatial variability rapidly, which no current method is able to do.
Only minimal treading damage and no reduction in pasture production was associated with cows grazing pasture at SWD > 2 mm, and this SWD appears to be a critical value, which farmers on fine textured soils, like the Tokomaru silt loam, could use to schedule grazing to protect soils and pastures from pugging damage. However, due to the need to maintain pasture quality there will be numerous occasions when the pasture on farms practising DC grazing will need to be grazed at a SWD < 2 mm. A modelling exercise revealed that up to 60% of a farm with standoff facilities could be damaged in an average season. While the damage inflicted on pastures may look unsightly, this may have little influence on overall accumulated (annual) pasture production. However, when the SWD was less than 2 mm, there were short-term losses in pasture production of approximately 500 to 1000 kg DM/ha under the grazing regimes studied here (i.e. 4- or 8-hour grazing duration and single and repeat damage events). If there is a need to graze in wet conditions (at or near saturation), then short durations (up to 4 hours) are recommended. Targeted DC grazing management provides the opportunity to achieve improved environmental outcomes, and the effectiveness of targeted DC grazing as a multi-purpose tool is not undermined by the need to graze in wet conditions.
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Keywords
Dairy farming, Environmental aspects, Grazing, Management, Soils, Leaching, Nitrogen content, New Zealand