Somehow milksolids (MS) production from irrigated pasture exceeds what we’d expect if we simply took into account the extra drymatter (DM) production achieved by applying water.
Modelled productivity is greater than “extra grass grown times normal feed conversion efficiency”.
The answer to the question comes down to a combination of several factors that lead to increased pasture quality and total energy availability plus management opportunities.
Plant available water and plant growth
The first factor is a basic agronomic one, but one that may not be obvious because the effect on plants of a moisture deficit is not always visual. Before a pasture plant exhibits stress, moisture deficits will influence pasture yield. By having irrigation in the periods that pasture would otherwise be entering into and exiting an observable stress period, its growth will be higher. This means well-managed irrigation systems will grow more grass over an extended period.
When soil moisture is 60-100% plant available water (PAW), plant growth is at maximum. As it drops below 60% plant growth rate drops at a faster and faster rate. In the example in Figure 1, pasture growth is reduced after nine days and stops after 15 days at an evapo-transpiration rate of 4mm/day.
Table 2: Results from the Ruataniwha case study 2013
The cash surplus shown in Table 2 is before any pre-conversion debt and milk revenue is based on $6.50/kg milksolids inclusive of dividends on a fully shared up property. For this property 75% of the farm is being irrigated, but the pasture harvested described is for the entire farm.
Future needs
Looking ahead lower North Island irrigated dairy farms should be able to exceed 20t DM/ha grown as pasture with a metabolisable energy at or exceeding 12 MJME/kg DM.
However, this might reveal another limitation already identified in Australia.
Is pasture the most efficient target for irrigation water? Could an integrated cropping regime and-or plants like lucerne actually show even greater potential? If we could quantify the higher levels of pasture or crop yields, and then model these, we would find the potential for milk production under irrigation is yet to be fully realised.
Chris Lewis is a Partner of Baker & Associates (Wairarapa) Ltd based in Masterton and a member of NZIPIM. He was 2014 Farmax Dairy Farm Consultant of the Year.
