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Reading on disc pasture meters

DATE: 2021-04-20

AUTHOR: John L. Godlee

Note that this information has since been expanded upon and is available as a .pdf here[1].

1: /files/dpm_reading/dpm.pdf

Apparatus dimensions summary

┌─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┐
│ Publica │  Long   │  Long   │ Sleeve  │ Sleeve  │ Disc di │ Disc th │ Total  │
│  tion   │   rod   │ rod ext │  pipe   │ pipe ex │ ameter  │ ickness │ weight │
│         │ length  │ ernal d │ length  │ ternal  │  (cm)   │  (mm)   │   of   │
│         │  (cm)   │ iameter │  (cm)   │ diamete │         │         │ free-  │
│         │         │  (mm)   │         │ r (mm)  │         │         │ moving │
│         │         │         │         │         │         │         │ parts  │
│         │         │         │         │         │         │         │  (kg)  │
╞═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╡
│ Bransby │         │         │         │         │         │         │        │
│ and     │ 180     │ 22      │ 120     │ 27      │ 45.800  │ 1.500   │ 1.500  │
│ Tainton │         │         │         │         │         │         │        │
│ (1977)  │         │         │         │         │         │         │        │
├─────────┼─────────┼─────────┼─────────┼─────────┼─────────┼─────────┼────────┤
│ Dorgelo │         │         │         │         │         │         │        │
│ h et    │ 180     │ NA      │ NA      │ NA      │ 45.800  │ NA      │ NA     │
│ al.     │         │         │         │         │         │         │        │
│ (1985)  │         │         │         │         │         │         │        │
├─────────┼─────────┼─────────┼─────────┼─────────┼─────────┼─────────┼────────┤
│ Zambati │         │         │         │         │         │         │        │
│ s et    │ 180     │ 22      │ 97      │ 27      │ 45.800  │ 1.500   │ 1.500  │
│ al.     │         │         │         │         │         │         │        │
│ (2006)  │         │         │         │         │         │         │        │
├─────────┼─────────┼─────────┼─────────┼─────────┼─────────┼─────────┼────────┤
│         │         │         │         │         │ 46*46   │         │        │
│ Rayburn │ 100     │         │         │         │ cm =    │         │        │
│ et al.  │ (metre  │ NA      │ NA      │ NA      │ 0.21    │ 5.600   │ NA     │
│ (1998)  │ stick)  │         │         │         │ m^2     │         │        │
│         │         │         │         │         │ square  │         │        │
└─────────┴─────────┴─────────┴─────────┴─────────┴─────────┴─────────┴────────┘

Bransby and Tainton 1977 - The disc pasture meter : Possible applications in grazing management

Diagram of DPM from Bransby and Tainton (1977)

They call it a "single stem disc meter".

180 cm aluminium rod with an external diameter of 22 mm.

120 cm long aluminium sleeve with external diameter of 27 mm. Internal diameter should allow it to move freely on the central rod.

Aluminium disc 1.5 mm thick, 45.8 cm diameter attached to the base plate on the sleeve by 4 bolts.

Small weights should be attached to the base plate so that the sleeve together with the baseplate and disc has a mass of 1.5 kg. The disc should be released from a standard height of 60 cm above ground level. The sleeve can also be gently placed on the grass surface and measured when held at constant height for 15 seconds.

The "sampling cylinder" should be able to be placed over the disc and should have the same diameter. The meter should then be removed so that the material inside the cylinder can be sampled. Material rooted outside the cylinder should still be sampled up to the edge of the cylinder.

Dorgeloh et al. 1985 - Calibrating a disc pasture meter to estimate above-ground standing biomass in Mixed Bushveld, South Africa

45.8 cm diameter base plate over a 180 cm long aluminium rod.

They took 9 measurements with the disc pasture meter per square of grass clipping and averaged them. To the nearest mm.

Hardy and Mentis 1985 - The relative efficiency of three methods of estimating herbage mass in veld

All samples were clipped using hand shears. Fifty disc meter readings were taken in each plot to estimate the mean disc height per (1 Ha) plot.

Didn't follow the grass sampling technique of Bransby and Tainton (1977) because they were working on long grass and it was feared that they would accidentally include material in the sampling cylinder.

Instead, they laid out a 2 x 2 m quadrat and determined mean settling height of 9 DPM readings inside the quadrat. The grass of the whole quadrat was then clipped.

Zambatis et al. 2006 - Re-evaluation of the disc pasture meter calibration for the Kruger National Park, South Africa

During fieldwork in Kruger, it was found that the DPM frequently settles above the bulk of the grass leaf mass, at times resulting in inflated DPM settling heights. This was particularly noticeable in long grass, maybe because these grasses have highly lignified culms.

The DPM was originally developed in New Zealand (Phillips and Clarke, 1971).

The mass of the disc and rod should definitely be known as it affects calibration, settling height.

Their measurements:

97 cm long tube of 2.7 cm external diameter attached to a 17.5 cm diameter 0.6 cm thick base-plate, with a disc of 45.8 cm diameter and 1.5 mm thickness. Total mass of descending parts is 1.5 kg. Long aluminium rod of 180 cm, 2.2 cm external diameter.

Clipping should be at a height not more than 3 cm above the ground.

Bransby and Tainton (1977) say that the relationship between DPM height and grass mass is linear.

Used a broken stick regression to show the breakpoint at which the relationship between DPM height and mean crop mass changes, getting more flat at height DPM heights.

More links

Phytosphere Gear - Falling plate meter

More images

Top of falling plate meter from Phytosphere Gear

DPM in action in Mongolia

DPM in action in USA with sampling cylinder

Information on common pipe sizes in the UK

┌─────────────────────┬─────────────────┬─────────────────┐
│        Pipe         │ Ext. diam. (mm) │ Int. diam. (mm) │
╞═════════════════════╪═════════════════╪═════════════════╡
│ Rainwater downcomer │ 68              │ 63.800          │
├─────────────────────┼─────────────────┼─────────────────┤
│ 1 1/2" waste pipe   │ 41.400          │ 36.600          │
├─────────────────────┼─────────────────┼─────────────────┤
│ 1 1/4" waste pipe   │ 35              │ 30              │
├─────────────────────┼─────────────────┼─────────────────┤
│ Large overflow pipe │ 25              │ 21              │
├─────────────────────┼─────────────────┼─────────────────┤
│ Small overflow pipe │ 20              │ 16              │
└─────────────────────┴─────────────────┴─────────────────┘

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