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Much of the increased pasture accumulation on the SG plots compared with the DC plots was due to the greater N return from urine to this treatment, as a result of the cows spending longer grazing these plots. In addition, the single slurry application to the DC plots in 2008 was not sufficient to increase pasture accumulation on those plots on an annual basis. Increased K returns probably also had an increasing effect on pasture accumulation on the SG plots. The SG plots, at any one time, had a larger area covered in dung and urine patches, which showed evidence of growing disproportionately more grass compared to inter-dung and urine areas. This observation was consistent with previous studies, where N, and, in some cases K, were contributing directly to greater pasture growth responses under urine deposition (During and McNaught, 1961; Auerswald et al., 2010; Moir et al., 2011). Further discussion on the effect of K in slurry applications will be found in Chapter 7.

Nitrogen concentrations in herbage at all grazings (results not shown) were not significantly different (P>0.05) between treatments, but varied significantly (P<0.05) between years, and were within the range reported of ca. 2-5% for ryegrass/white clover pasture in a grazing preference study conducted in the Manawatu region of New Zealand (Cosgrove et al., 2002). The lower overall pasture accumulation for 2009/10 may also be attributed to a lower number of grazing days for that season (8 c.f. 9 for 2008/09 and 10 for 2010/11), and thus less urine and dung deposition, compounding to produce the lower pasture accumulation from less N being returned and thus available for plant uptake. This was evident in N concentrations in the herbage for 2009/10, where both treatments had lower N than in 2008/09 and 2010/11 (P<0.05), and also lower N leaching in 2010; all of these effects will be discussed in more detail in Chapter 4.

48 Chapter 3 – Pasture

Based on the time spent grazing, it was estimated that, on average, 77% of the N ingested by SG treatment cows was returned to the paddock in urine and dung combined. This value is similar to the value given by Haynes and Williams (1993), of 80% for dung and urine combined, but greater than the 52% return calculated using the model of Salazar et al. (2010). It was estimated that in 2008/09, 2009/10 and 2010/11 ca. 318, 297 and 339 kg N/ha/yr, respectively, were deposited in excreta on the SG plots (Table 3.4). In comparison, an average of only 47% of the N ingested by DC treatment cows was returned to the plots in urine and dung. Therefore, in 2008/09, 2009/10 and 2010/11 ca. 385, 161 and 300 kg N/ha/yr were returned on average to the DC plots in excreta plus slurry applications, respectively. These estimates suggest that on average, 36 kg N/ha/yr less was returned to the DC treatment.

Studies in Europe have also included concepts around DC grazing, not only to monitor cow intakes and grazing residuals (‘restricted access time’) (Kennedy et al., 2009), but also to reduce N leaching (Oudshoorn et al., 2008). For example, Oudshoorn et al. (2008) used ‘time-limited’ grazing to study urination frequency and spatial distribution of urine. They compared the effects of grazing times of 4, 6.5 and 9 hours per day on urine frequency, and found that frequency did not change with increasing time spent grazing. Therefore urine deposition throughout the day was unaffected by whether cows were being grazed or stood off. This was also consistent with the present study, where the reduction in time spent grazing on the DC treatment was related to the reduction in NO3- leached, which had a

direct relationship with the number of urine patches deposited to the plots (Chapter 4). However in this study, the rate of defecation (dung pats/cow/hour) was slightly lower at night grazings than during the day (0.49 c.f. 0.59 dung pats/cow/hour for night and day grazings in 2008/09, respectively) (Table 3.5). This was mainly due to cows spending more time lying down during night grazings (Draganova et al., 2010).

There was a significant, albeit small and short-lived, increase in pasture production on the DC plots following the first slurry application in December 2008 (Figure 3.4). This response may have been small because the slurry that was applied was lower in mineral N (16.6%) than typical slurry from a cow house (ca. 40%, (Houlbrooke et al., 2011)). There was also a relatively high proportion of feed matter in the slurry being applied, and therefore a high carbon content (Clough and Kelliher, 2005), which had the potential to encourage immobilisation of mineral N by transformation into organic forms. In addition, the increased carbon content of the slurry may have led to more N loss by denitrification

Chapter 3 – Pasture 49

(Barton and Schipper, 2001), particularly in the warm wet spring conditions of 2009 that followed.

In 2010/11 the slurry was applied in smaller amounts, with a higher mineral N content (Table 3.4). This mineral N content, with an average of 39.7% of total N, was more comparable to that reported by Houlbrooke et al. (2011) for slurry analysed from cow houses (38.7% and 43.3% for HerdHome® and wintering barn slurry, respectively). Slurry application had a positive effect on pasture growth in this year, and this was particularly evident when comparing daily average accumulation rates between treatments (Figure 3.4). Daily pasture growth rates improved on the DC plots immediately following slurry application in 2008 and the first 3 applications in 2010/2011. The increased response in growth rates only lasted for one grazing rotation, however. Furthermore, to be effective, the more frequent applications of slurry must occur when soils are moist and temperatures permit pasture growth to actively respond to the addition of nutrients. This was also evident in an earlier study at a nearby site, where the effect of dairy farm effluent application on pasture growth was measured (Bolan et al., 2004). In that study, increasing rates of N via effluent increased pasture response, although it was not possible to distinguish between water and nutrient effects. In the present study, it was evident that single slurry applications did not have long-lasting effects on pasture growth, as demonstrated by the decline in accumulation relative to SG plots following the last application in April 2011 to the end of measurements in September 2011 (Figure 3.4), and the lack of a larger response to the first application which added N at such a high rate.