Growing maize to conserve as silage for dairy cows has become increasingly popular in northern Victoria and southern NSW over the past decade or more.
This crop has high input costs of seed, fertiliser, herbicides and water, as well as requiring a high level of management skill.
Most dairy farmers employ the services of a local agronomist to guide their decision making when growing maize as small errors in management can lead to large losses in potential yield or quality.
More recently, a small number of dairy farmers have been trying their hand at growing grain sorghum varieties as an alternative to maize.
Grain sorghum has a reputation of being easier and cheaper to grow and more forgiving of adverse growing conditions while still producing a silage product comparable to maize.
The trade-off is in the lower yield and slightly lower quality of grain sorghum silage.
Recent research at Gatton in Queensland has demonstrated that grain sorghum can actually produce higher yields than maize when cut twice during the season.
This is achievable in Queensland because of the warmer climate and longer growing season, but is unlikely to be achievable in northern Victoria.
Last summer Murray Dairy partnered with the Irrigated Cropping Council to conduct a research trial in Kerang to see how grain sorghum compares with maize under a range of irrigation strategies.
The aims were:
● to compare grain sorghum yields with maize yields;
● to determine the differences in the quality of each species; and
● to test if grain sorghum is more resilient under water stress conditions.
The results
The trial compared a red sorghum (Sentinel), a white sorghum (Liberty), a forage sorghum (Megasweet) and two maize varieties (PAC 440 and PAC 606).
Three irrigation treatments were imposed on all species to simulate a high irrigation level (11 Ml/ha) that might favour a high yielding maize crop, a medium irrigation level (8 Ml/ha) that might favour a sorghum crop, and a low irrigation level (6 Ml/ha) that would potentially stress both crops.
Yield data for each forage can be seen in Table 1. The maize varieties out-yielded both grain sorghum varieties. PAC 440 out-yielded the forage sorghum (Megasweet) however there was no significant difference between the Megasweet and the PAC 606 maize, although the maize quality was higher. The red grain sorghum (sentinel) was the lowest yielding of all forages, however bird attack on the seed heads late in the trial may have accounted for much of that yield loss.
The yield data for each irrigation treatment can be seen in Table 2. This is the average yield for all the forages combined. There was no significant yield difference between the high and medium irrigation treatments. However, the forages in both the high and medium irrigation treatments out-yielded those grown in the low irrigation treatment.
The 2020-21 summer was mild compared to average years, with only one brief period of temperatures over 40°C that coincided with the irrigating of the low water treatment. A different result may have been seen in the low irrigation treatments if a combination of high temperatures and water stress had occurred.
Soil moisture meters were installed at 20cm and 40cm depth in a Liberty white sorghum plot and a Pac 440 plot in each irrigation treatment. These moisture meters indicated that the maize was able to extract more water and to a greater depth in the soil than the white sorghum in all irrigation treatments. The respectable yields in the low irrigation treatments suggest that the plants were able to draw moisture from lower in the soil than was measured with the moisture meters.
Some key quality indicators for the maize and silage plots can be seen in Figures 1, 2 and 3. The metabolisable energy and starch was higher in the Liberty and maize than in either the Sentinel or Megasweet sorghum, while low neutral detergent fibre was an indicator of higher quality in the maize.
Final observations
The trial was unable to distinguish if the maize or the sorghum was more resilient to varying irrigation levels, as both forages were able to adapt enough to the different water treatments to prevent a statistically valid difference being observed.
Maize remains the crop with the highest yield and quality potential for this region, and the most efficient crop per megalitre of water.
Grain sorghum still has a place in a dairy system for those who are new to growing crops, want to reduce the financial risk and to test their management skills while still producing a product of high enough quality to incorporate into a dairy ration for either milk production or body condition gain.
Further work will be done on maize and grain sorghum in the 2021-22 summer as part of the Fodder for the Future project.