Pasture Production under Short and Long Centre Pivots

Transcription

1 Pasture Production under Short and Long Centre Pivots Project Report prepared by Aqualinc Research Limited in conjunction with The AgriBusiness Group as part of SFF C07/004 Adapting to a Drier Environment by Improving Irrigation Practices June 2010

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3 Summary Pasture Production Under Short and Long Centre-Pivots This study was conducted during the irrigation season as part of the Sustainable Farming Fund s Adapting to a Drier Environment project (grant no C07/004). It was intended to demonstrate how high water application intensities under long centre-pivots affect pasture production. To do this, pasture growth measurements were collected 2-3 days prior to grazing at various locations along the length of two trial centre-pivots over the course of the irrigation season. Trial Farm 1 clearly showed the expected trend. Pasture production decreased along the length of the centre-pivot. Production at 910 m from the centre of the centre-pivot averaged approximately 500 kg DM/ha less (per grazing) than production at 250 m from the centre of the centre-pivot. Trial Farm 2 did not show a clear trend. Pasture production increased and decreased along the length of the centre-pivot. It is suspected that other factors (i.e. spatially variable soil fertility or water holding capacity) affected the results for this farm. The results of this study suggest that high application intensity can contribute to a reduction in pasture growth. Because high application intensities occur at the end of long centre-pivots, a very large area is potentially affected by reduced production. This has the potential to seriously impact on-farm profitability. Further work needs to be done to quantify the potential impacts of high application intensities under long centre-pivots, such as: how much water and nutrients are lost due to runoff and drainage what are the potential environmental impacts financial analysis, including capital and operating costs, and identifying situations in which reduced production is most pronounced. Background Long centre-pivots (those > 600 metres in length) produce extremely high water application intensities under the outer spans. Because these outer spans have to move so fast to keep up with the rotation of the inner spans, the water at the ends has to come out faster (i.e. higher application intensity). This has the potential to cause ponding, runoff, and loss of production. Figure 1 shows the calculated application intensity and measured application depth under the centre-pivot at Trial Farm 1.

4 2 Figure 1: Application intensity increases with centre-pivot length, despite an even application depth. Data is from Trial Farm 1. It has been shown that high application intensities do not water the root zone of plants as well as a more gentle application. High application intensities cause uneven infiltration into the root zone (top image in Figure 2). Low application intensities (gentler rain) allow for more even infiltration (bottom image in Figure 2). Figure 2: Measured wetting fronts at different application intensities (Clothier and Heiler, 1983). The soil plot described by the top image received water at a intensity of mm/hr. The soil plot described by the bottom image received water at an intensity of 4.1 mm/hr. Both plots received 14.2 mm total application. Under centre-pivots, most of the land area is covered by the outer spans, which are producing the highest application intensities. For very long centre-pivots, this can potentially result in large negative effects on production.

5 Description of the Study Two North Canterbury dairy farms were selected to demonstrate the relationship between irrigation application intensity and production: Trial Farm 1 This dairy farm in North Canterbury takes water from the Waimakariri Irrigation scheme and applies it through an 880 metre-long centre-pivot. There is a corner arm and end-gun on the centre-pivot which adds additional wetted length. Trial Farm 2 This dairy farm in the Amuri Basin takes water from the Waiau Irrigation scheme and applies it though a 691 metre-long centre-pivot. There is a corner arm and end-gun on the centre-pivot which adds additional wetted length. Pasture cover measurements were taken from December 2009 through April Pasture cover was measured along six transects at different distances along each centre-pivot using a plate meter. Measurements were taken 3-4 days prior to each grazing. It was assumed that a residual pasture cover of 1,500 kg DM/ha was left after each grazing. Soil moisture was also periodically monitored at several locations under the centre-pivot at Trial Farm 2. This was done using neutron probe technology. 3 Results Consistency Checks Prior to the Study Field measurements were made to confirm that the actual layout and operation of the centrepivots was consistent with the design of the study. The performance of the Trial Farm 1 centre-pivot was checked during the irrigation season. A CU of 0.83 was measured, indicating relatively even distribution along the length of the machine. However, as shown in Table 1, the average applied depth under some of the individual spans seems to vary significantly. This was taken into consideration when analysing the pasture growth data. The average applied depth under the Trial Farm 2 centre-pivot was found to be relatively consistent across all pasture sampling locations (see Table 2). Because the average applied depth is relatively consistent along the length of the pivot, and the paddocks are managed similarly, it may be assumed that any differences in production observed are likely to be due to the instantaneous application intensity. Span lengths of the Trial Farm 2 centre-pivot were found to be different than originally reported by the system supplier. The measured lengths are thus: Main Spans: Main Spans + Corner: Total Wetted Circle: 691 m total radius 775 m total radius 800 m total radius

6 Results Trial Farm 1 Pasture measurements from Trial Farm 1 showed the expected trend. Pasture production decreased along the length of the centre-pivot. Table 1 and Figure 3 summarise the field measurements from Trial Farm 1. Table 1: Pasture growth results Trial Farm 1. Span Distance from Pivot Centre (m) Measured Application Depth (mm) Estimated Application Intensity (a) (mm/hr) 4 Average Pasture Cover Prior to Grazing (kg DM / ha) , , , ,171 Corner ,083 Overhang (b) 88 1,767 (a) Calculated at the middle of each span, based on the total flow rate entering the centre-pivot. (b) Not measured because arm was over the fence line. Figure 3: Pasture cover measurements Trial Farm 1. Pasture cover was consistently lower at greater distances from the centre of the centre-pivot. Production at 910 m from the centre of the centre-pivot averaged approximately 500 kg DM/ha less (per grazing) than production at 250 m from the centre of the centre-pivot. This occurred despite a higher measured application depth under the corner arm. The reduced production is expected to be due to the high application intensity. Pasture cover was even lower under the end gun than under the corner arm. This is expected to be due to a high application intensity combined with poor application uniformity.

7 Results Trial Farm 2 Pasture measurements from Trail Farm 2 did not show a consistent trend. Pasture cover under this centre-pivot varied considerably along its length. This suggests that there are other factors that needed to be considered in addition to the irrigation application intensity. Table 2 and Figure 4 summarise the field measurements from Trial Farm 2. Table 2: Pasture growth results Trial Farm 2. Span Distance from Pivot Centre (m) Measured Application Depth (mm) Estimated Application Intensity (a) (mm/hr) 5 Average Pasture Cover Prior to Grazing (kg DM / ha) , , , ,480 Corner Gun (b) (b) 60 2, ,233 (a) Calculated at the middle of each span, based on the total flow rate entering the centre-pivot. (b) Not measured because corner was folded in Figure 4: Pasture cover measurements Trial Farm 2. Most of the pasture measurement events show a pasture cover at the end of the centre-pivot that is very similar to pasture cover nearer in, with large variations in between. This information does not suggest that pasture growth was negatively affected by application intensity. Peaks in pasture cover were consistently experienced at approximately 600 metres from the pivot-centre. This suggests that some other unknown variable was affecting production at this location. Further work would need to be done to determine the cause of this peak. No clear or relevant trends were observed after a preliminary analysis of soil moisture data.

8 Discussion Results from Trial Farm 1 showed the expected trend in pasture production, while Trial Farm 2 did not. This indicates that high application intensities under long centre-pivots can result in a negative effect on production and profitability, but perhaps not in all cases. There are likely to be other factors involved. Further work needs to be done to quantify the potential impacts of high application intensities under long centre-pivots, such as: how much water and nutrients are lost due to runoff and drainage what are the potential environmental impacts financial analysis, including capital and operating costs, and identifying situations in which reduced production is most pronounced. Further work should include more rigorous monitoring of all inputs and outputs effecting pasture growth. 6 What this Could Mean for Farm Profitability Because the end spans of a centre-pivot cover a much larger area than the inner spans, any reduced production is projected over a large area. The result is that longer centre-pivots have the potential to suffer from more total production loss. Figure 5 and Figure 6 describe expected pasture growth under each 100 metre band of a 1,000 metre-long centre-pivot, based on the results obtained from Trial Farm 1. Figure 6 indicates that the total amount of pasture expected to be grown in the metre band of a centre-pivot is approximately 31,000 kg DM. This is slightly more than the amount of pasture expected to be grown in the 900-1,000 metre band, even though the area of the 900-1,000 metre band is approximately 50% larger. The severity of this effect is expected to depend on the soil type, system capacity, centre-pivot length, and spray width of the sprinklers on each farm. The pasture growth trend observed on Trial Farm 1 may not seem very significant at first. But, if it applies to most long centre-pivots, it could have large implications for production across Canterbury.

9 7 Figure 5: Areas covered by concentric rings, each 100m larger in radius. Bands farther from the centre of the centre-pivot include a larger area than those closer in. Figure 6: Expected per-grazing production under each band of the centre-pivot in Figure 5. The total kg DM expected to be grown in the m band is approximately the same as the total kg DM expected in the 900-1,000 m band, even though the area of the 900-1,000 m band is approximately 50% larger.

10 The analysis in Table 3 shows the expected average production under centre-pivots of various lengths, based on the results obtained from Trial Farm 1. 8 Table 3: Estimated per-grazing pasture production under centre-pivots of various lengths. Pivot Length (m) Area (ha) Average Pasture Cover Prior to Grazing (kg DM/ha) Average Pasture Grown Between Grazings (kg DM/ha) Total Pasture Grown Between Grazings (kg DM) ,591 1,091 30, ,554 1,054 52, ,507 1,007 79, , , , , , , , , , ,001 Notes: Assumes 1,500 kg DM/ha residual after each grazing. All pasture growth values are based on measured pasture data from Trial Farm 1. From Table 3, it is expected that a 700 metre-long centre-pivot could grow approximately 139,000 kg DM between each grazing. Two 700 metre-long centre-pivot could grow approximately 278,000 kg DM between each grazing. A 1,000 metre-long centre-pivot is expected to grow approximately 232,000 kg DM between each grazing. This indicates that two 700 metre-long centre-pivots could grow 20% more pasture than one 1,000 metre-long centre-pivot, while covering approximately the same land area. Preliminary Calculation: Is it worth it to put in smaller pivots? Preliminary budget calculations have shown that smaller centre-pivots have the potential to be more cost effective than larger centre-pivots (see Table 4). This is due, at least in part, to the improved production observed under smaller centre-pivots. The preliminary budget in Table 4 includes some capital and operating costs and estimates of production, based on the results from Trail Farm 1. It does not include all costs, i.e., the cost of land. Further, the production values used will not apply in all situations they will vary by season, soil type, climate, and several farm-scale inputs. This budget is only indicative, to show that there is potential for smaller centre-pivots to be more cost effective in the longterm. More detailed analysis of this topic is required.

11 9 Table 4: Estimated costs and production gains under centre-pivots of various lengths. Item 1,000 m Centre-Pivot 700 m Centre-Pivot 500 m Centre-Pivot Pivot / Production Details Number of pivots Area (ha) Average cover at grazing (kg DM/ha) 2,250 2,400 2,500 Pasture eaten, 10 grazings (kg DM/ha) 7,500 9,000 10,000 Value of pasture eaten ($/ha/irrig. season) $2,750 $3,300 $3,667 Capital Costs Centre-pivot ($) $500,000 $700,000 $1,000,000 Mainline ($) $100,000 $90,000 $76,800 Pump station ($) $80,000 $95,000 $110,000 Miscellaneous ($) $5,000 $10,000 $20,000 Total capital cost ($) $685,000 $895,000 $1,206,800 Total capital cost ($/ha) $2,180 $2,907 $3,841 Operating Costs Electricity ($/yr) $94,000 $90,000 $86,000 R&M ($/yr) $3,000 $4,000 $6,000 Total operating costs ($/yr) $97,000 $94,000 $92,000 Total operating costs ($/ha/yr) $309 $305 $293 TOTALS Value of pasture eaten ($/ha/irrig. season) $2,750 $3,300 $3,667 Annualised capital (10yr, 10%) ($/ha/yr) -$335 -$446 -$590 Total operating costs ($/ha/yr) -$309 -$305 -$293 Net value ($/ha/yr) $2,100 $2,550 $2,785 Assumptions: Residual pasture cover after each grazing = 1,500 kg DM / ha 15 kg DM = 1 kg MS Milk price = $5.50 / kg MS Electricity cost = 15c / kwh Acknowledgements Thanks to Dave Lucock (The AgriBusiness Group) for pasture measurements and Geoff Dunham (The AgriBusiness Group) for pasture and grazing information collected for this study.