Average Annual Pasture Growth Rates

Average annual pasture growth rates on Tasmanian dairy farms have been estimated to range between 7 to 21 tonnes of dry matter per hectare (t DM/ha) using DairyMod computer simulations for high fertility soils on well managed farms. Average pasture growth of around 7 t DM/ha were calculated for lower rainfall districts such as Cressy and Bushy Park while growth rates of up to 21 t DM/ha were estimated for pastures in higher rainfall regions that have irrigation. The average pasture growth for a number of Tasmanian locations calculated using computer simulations based on the last 40 years of climate data are shown in the table below.

Average Annual Pasture Growth Rates

Region Average t DM/ha/yr  
  Rainfed Pasture Irrigated Pasture
Burnie 11.3 20.3
Bushy Park 6.8 13.7
Cape Grim 11.3 19.7
Cressy 7.3 16.9
Currie 10.3 17.8
Deloraine 11.0 20.4
Mawbanna 15.2 19.7
Mole Creek 12.7 20.6
Redpa 14.2 19.4
Ringarooma 14.1 20.3
Scottsdale 12.5 19.7
Sheffield 12.7 21.0
Smithton 14.0 20.1
Wynyard 12.1 20.2

Source: Tasmanian Regional Pasture Growth Rates Tool; tasdairyprojects website


Pasture Utilisation

Not all the pasture that is grown is able to be utilised by dairy cattle; there is always some wastage. The Tasmanian dairy benchmarking program has been estimating pasture utilisation for participants for a number of years based on milk production, livestock numbers and the quantities of purchased feed used on the farm. The pasture utilisation for a number of Tasmanian dairy regions based on the results of dairy benchmarking participants is summarised in the table below. The pasture utilisation figures from the benchmarking participants suggest that around 60% of the growth estimated by the computerised simulations is actually utilised.

Pasture Utilisation

Region Pasture Utilisation, t DM/ha/yr  
  Rainfed Irrigated
North East 6.5 10.9
Deloraine/Longford/Cressy/Tamar 7.4 11.1
South 5.2 10.1
Devonport/Sheffield/Ulverstone 8.4 12.1
Wynyard/Yolla/Burnie/Riana 8.2 12.7
Circular Head 9.7 12.5
King Island 7.1 8.9

Source: Tasmanian dairy benchmarking analysis


Irrigation

Farmlet trials at the Tasmanian Dairy Research Facility demonstrated that irrigation can increase pasture utilisation by approximately 1 t DM/ha/year but there is seasonal variation around this level of response. A recent computer simulation for an irrigation proposal at Gawler (near Ulverstone) indicated pasture utilisation would increase by 0.6 – 1.3 t DM/ha/yr. The low utilisation was estimated for pasture with low soil fertility and utilisation of more than 1 t DM/ha/yr was estimated for pastures with adequate phosphorus and potassium levels and with more than 200 kg nitrogen applied per hectare.

Irrigation water requirements depend on soil type, climate, soil fertility and pasture species. The estimated annual irrigation requirements for well managed perennial ryegrass dairy pastures are shown in the table below. Irrigation of well managed dairy pasture with high soil fertility is likely to increase pasture utilisation by 3-4 t DM/ha/yr and this will typically require around 4 ML/ha/yr of water to be applied.

Irrigation

Region Irrigation Requirement, ML/ha/yr
Burnie 4.6
Bushy Park 4.8
Cape Grim 3.9
Cressy 5.3
Currie 4.0
Deloraine 4.7
Lilydale 4.5
Mawbanna 2.6
Mole Creek 4.1
Redpa 2.7
Ringarooma 3.5
Scottsdale 4.1
Sheffield 4.2
Smithton 3.0
Wynyard 3.9

Source: Tasmanian Regional Pasture Growth Rates tool; tasdairyprojects website


Monthly Growth Rates

Growth rates measured on five farms assisted and monitored to demonstrate best practice (companion farms) recorded pasture utilisation figures of 9,100 to 13,500 kilograms of dry matter per hectare. Irrigation was used on several of these farms and 100 - 280 kilograms on nitrogen per hectare was applied.

Monthly Growth Rates

  Central North West   Central North   North East  
Month Irrigated (kgDM/ha/day) Dryland (kgDM/ha/day) Irrigated (kgDM/ha/day) Dryland (kgDM/ha/day) Irrigated (kgDM/ha/day) Dryland (kgDM/ha/day
July   10 10 10 10 10
August   25 25 25 20 25
September   45 45 45 40 45
October   55 65 55 55 55
November   65 65 45 60 45
December   40 55 20 55 25
January   30 50 15 40 15
February   20 40 15 50 30
March   25 35 20 35 20
April   20 20 15 20 15
May   20 20 20 20 20
June   15 15 15 15 15
Annual Total   11,300 13,500 9,100 12,800 9,600

Source: DPIWE, Companion Farm Project 2002-2005


Average Dryland Pasture Growth Rates

Computer modelling carried out by Dr Richard Rawnsley (TIAR) using climate and soil data highlights the differences in pasture growth rates that occur between regions and between months. (1966-2005)

Average Dryland Pasture Growth Rates

Source: Dr.R.Rawnsley, TIAR, calculated using DairyMod (1966-2005)