WATER NEEDS OF PASTURES USED IN DAIRY PRODUCTION.

Ideally, pasture management should emphasize production of optimum forage yield and quality, without compromising the environment. Accurate irrigation scheduling, therefore, plays an important role in the success of a dairy enterprise by affecting forage yield and quality, irrigation input and energy usage, and environmental pollution. For years farmers arbitrarily have used an irrigation guideline of 25 mm of irrigation water per week for most temperate grasses and legumes, regardless of season or region. Obviously, evaporative demand differs between locations and over time for a specific location, and as crop canopy cover varies, a rigid guideline of 25 mm per week will lead to over or under irrigation in different seasons. The authors cited below, acknowledged the need to determine irrigation requirements of lucerne and common grass/legumes mixtures used for dairying and other purposes. This can be done by developing site specific guidelines or charts that indicate when and how much to irrigate. The irrigation requirements developed should of course be flexible by deducting measured rainfall since the last irrigation event. The objectives of their research, therefore, were to determine water requirements of lucerne and most used grass/legume mixtures through testing and evaluation of available models, thereby developing generic guidelines for efficient irrigation management.  

Whilst there are a number of model options to be selected, the authors used the DairyMod model and the Soil Water Balance (SWB) model in their study. The DairyMod model is a daily time-step model with modules for pasture growth in response to climate, pasture utilization by grazing animals, animal physiology, growth, metabolism and lactation, water dynamics including transpiration, soil evaporation, runoff, infiltration and deep drainage, soil organic matter, and nitrogen dynamics, including leaching and gaseous losses through volatilization and leaching. There are flexible options for pasture management, irrigation, fertilizer application, stock management and supplementary feeding strategies. The SWB model can estimate real-time crop water requirements (day-to-day water use during the growing season) and recommend the irrigation amount and date, based on the current crop water usage and set user preferences. 

Monthly general irrigation intervals were developed for a deep, well drained and fertilized, medium textured soil for most common high producing areas. General irrigation intervals were developed by irrigating the lucerne and common grass/legume pastures when 25 mm soil water was depleted so that 25 mm will be replenished (similar to the rigid recommendation, but scheduling the timing according to the long-term water requirement). Key production areas for lucerne and mixed pastures were selected for the purpose of this study to illustrate the difference in water requirements of different pastures in response to site specific climatic conditions and pasture management. 

The results provided the following general guidelines to irrigate various pastures. In comparison to the rigid guideline, most requirements during autumn, winter and spring were lower and higher in summer.

Table: Estimated water requirements of various pasture species and mixtures thereof (full

canopy)

Pastures                                                                              Autumn      Winter     Spring      Summer

                                                                                                                  (mm per week)

Subtropical pastures – ON STATION:

Kikuyu                                                                                    17.0            12.0 #      20.0           30.0

Temperate pastures – ON STATION:

Tall fescue                                                                             16.0            14.0         17.5           30.0 *

Lucerne                                                                                  21.0            15.0         15.5           30.5 *

White clover                                                                         23.0            18.0         19.5           29.0 *

Subtropical grass – Temperate legume mixed pasture – ON STATION:

Kikuyu/Lucerne                                                                   20.0            15.0          19.0           34.0

Temperate grass – Temperate legume mixed pasture – ON STATION:

Tall fescue/Lucerne                                                            18.5             15.5         17.0           28.0

Tall fescue/White clover                                                    19.0            16.5         18.0           30.0

Temperate pastures – ON FARM:

Lucerne (hay crop)                                                              28.0             12.5        32.0           44.0

Subtropical grass – Temperate grass mixed pasture – ON FARM:

Kikuyu/Perennial ryegrass (Grazing)                               14.6               9.5         21.0           29.5

* High evaporative loss (Dormant season)

# Risk of increased drainage (Dormant season)

  

It was concluded that the DairyMod and SWB models can be used by farmers or consultants to determine their irrigation requirements with relatively few and simple inputs. Therefore, irrigators can follow different strategies for making a decision on when and how much to irrigate depending on particular situations. In the absence of irrigation scheduling tools, site specific irrigation guidelines can be calculated using the models, and would be better than a rigid guideline of 25 mm a week. It needs to be stressed, however, that irrigation scheduling with the aid of real time modelling or measurements is the best way of calculating irrigation guidelines using a model. The models are available on the web and can be downloaded free of charge. 

Reference: 

W. Truter, O. Sehoole, M. Murphy, M. Fessehazion, J. Annandale, C. Jarmain, M. Dlamini & C. Everson, 2016. IRRIGATION GUIDELINES FOR MIXED PASTURES AND LUCERNE. WRC Report No. TT 697/16, December 2016.