Response to Dr Edmeades’ letter: 'The effects of nitrification inhibitors on pasture production'

Mr Peter Carey

Land Research Services Ltd

Arts Workshop

PO Box 84

Lincoln University

Dear Sir,

Thank you for the opportunity to respond to Dr Edmeades' criticisms of our paper ‘Pasture dry matter responses to the use of a nitrification inhibitor: a national series of New Zealand farm trials’ (Carey et al. 2012). We offer the following as a rebuttal and hope this will allay any concerns about the integrity of the data and methodology used in the paper.

With respect, Dr Edmeades' criticism of rising plate meter measurements conflicts with the large body of published peer-reviewed scientific papers that prove that rising plate meters can be used successfully to measure pasture dry matter yield (e.g. Piggot 1989; Thomson et al. 1997; Morton et al. 2005; Gillingham et al. 2007, 2008; Litherland et al. 2008). A study by L'Huillier & Thomson (1988) showed that the rising plate meter reliably and accurately estimated herbage mass of ryegrass/white clover dairy pastures at Ruakura and Taranaki, and Piggot (1989) concluded it was also accurate in Northland. In addition, Lile et al. (2001) conducted a detailed evaluation of the use of rising plate meters on New Zealand dairy farms and concluded that the rising plate meter gives reliable information when using 50 readings per paddock taken in paddocks ranging from 1000 to 4000 kg DM/ha, and appropriate calibrations are used.

Dr Edmeades' criticism of rising plate meter measurements not only conflicts with the published literature but also implies that the results from all previously published peer-reviewed papers that used the rising plate meter are invalid. This cannot be an acceptable conclusion since it would imply that the excellent peer-reviewed published work by renowned scientists such as Piggot (1989), Thomson et al. (1997), Morton et al. (2005) and Gillingham et al. (2008) is now being questioned. Clearly, Dr Edmeades' criticism of the method of measurement used in the Carey et al. (2012) paper is untenable and should be rejected.

Dr Edmeades admits in his letter that his DCD trials were all conducted on either pumice or ash soils (in the North Island). However, the pasture data reported by Carey et al. (2012) includes data from 24 farm trials on sedimentary soils in the South Island. It is unscientific for Dr Edmeades to attempt to dismiss all of the Carey et al. (2012) data based on his own trials (on pumice and ash soil) when it appears that he has not provided any results whatsoever from sedimentary soils.

The pasture response frequency distribution presented by Dr Edmeades in his letter appears to be based on only 22 datasets, whereas the response frequency distribution presented in Carey et al. (2012) is based on over 135 datasets (Fig. 1). Figure 1 from Carey et al. (2012) shows that there was a very high frequency of response to DCD, with a median response of 18%.

Figure 1  DM frequency response graph for DCD treated over nil treatments for all paddocks (Carey et al. 2012).

Dr Edmeades cites a paper by Macdonald et al. (2010) as evidence. However, it should be noted that in the trials conducted by MacDonald et al. (2010) in Taranaki, the product tested was granular DCn, not fine particle suspension eco-n. In addition, it appears that the application time was outside the specifications for use of eco-n because the first application in 2008 was made in mid-June rather than in April or May (as recommended for eco-n). It is therefore likely that the application in mid-June would have been too late to be effective in reducing N losses from autumn grazing. This limitation of the study is acknowledged by MacDonald et al. (2010), who state that ‘it may be preferable to apply the DCn in early autumn to coincide with the period of greatest leaching risk’. In addition, the pasture measurements by MacDonald et al. (2010) were made by ‘visual assessment’ and not by the pasture cutting and weighing method recommended by Dr Edmeades in his letter.

The paper by Carey et al. (2012) includes data from a large on-farm trial in Taranaki that involved a comparison of 16 large grazed pasture areas on the Stratford Demonstration Farm. The results of the statistical analysis of this data showed that there was a statistically significant (P<0.001) increase in spring pasture growth of 26% on the eco-n treated areas compared with the control areas (Fig. 2). Whilst the paper reported a median increase of 18% in pasture production, we have no way of knowing whether the farm paddocks that compiled the dataset were better, worse or merely average in the way eco-n responded nor, similarly, can we predict performance in all areas and regions; however, on average, and based on a large number of observations, there was a significant benefit.

Figure 2  Stratford Demonstration Farm (May 10–January 11) 16 half-paddock trials shows a 26% increase (P <0.001) (Carey et al. 2012).

Finally, we would like to note that there have now been over 23 peer-reviewed scientific papers published by a range of science providers that prove that DCD is highly effective in reducing soil nitrification rates, reducing nitrate leaching losses and reducing nitrous oxide emissions from grazed pasture soils in New Zealand (Di & Cameron 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2011, 2012; Di et al. 2007, 2009a, 2009b, 2010a, 2010b, 2011, 2012; Moir et al., 2007, 2010, 2012; Menneer et al. 2008a,b; Smith et al. 2008; Monaghan et al. 2009; Shepherd et al. 2009; Sprosen et al. 2009; de Klein et al. 2011). These attributes alone make this nitrification inhibitor technology a valuable tool to help farmers meet the stricter regional council regulations that are being imposed on them to reduce N losses. The evidence shown in these peer-reviewed papers confirms that nitrification inhibitor technology can help New Zealand pastoral farming systems become more sustainable by reducing their environmental footprint whilst increasing efficiency. The paper by Carey et al. (2012) is another step in that direction.

Yours sincerely,

P Carey, S Jiang and A Roberts