In recent years, growing grain sorghum for silage has become a popular on-farm strategy across northern Australia, with an estimated 8-10 per cent of farms in Queensland now growing and feeding it to the lactating dairy herd.
So what's influencing so many farmers to make such a change to a forage base that was once based on corn silage?
While all sorghums are a low-cost, drought-tolerant crop that can be grown well in both dryland and irrigated systems, grain sorghum varieties are typically a shorter plant, have a higher grain-to-forage ratio and require a lot less water to grow than corn crops.
Acknowledging that dry matter yield will be lower than most corn crops, if grain sorghum is planted in spring and two cuts of forage are harvested from the one sowing, then the yields, the quality and cost really show the potential this crop has as a primary forage source in partial mixed ration and total mixed ration dairy systems.
To get the most out of a grain sorghum crop, there are three key components that need to be considered:
White grain sorghums are far superior in starch digestibility than the red varieties.
A study comparing the starch digestibility of four red and four white varieties of grain sorghum silage found that despite red sorghums containing higher initial starch concentrations, white varieties have a much higher starch digestibility and digestion rate.
Table 1 shows that if the cow consumes a white sorghum silage and it remains in its rumen for seven hours, it would potentially digest 36 grams more starch per kilogram of dry matter than if it were eating a red sorghum variety.
Further research has also shown that if red or white grain sorghum silage is ensiled for at least six months, the digestibility of the starch in the grain is increased by 10.4pc compared with only being ensiled for a three-month period.
Aim to plant in spring and cut the white grain sorghum crop twice.
Growing white grain sorghum as a silage crop is a cost-effective alternative to corn, and when planted early in the season, the crop could nearly double the potential dry matter yield harvested (as two cuts) under irrigated conditions.
Table 2 shows that when growing and harvesting the sorghum for two cuts, there is an increase of $7/t DM in cost from additional fertiliser, water and a second harvesting cost.
When compared with corn silage, the final cost in the pit is lower, with both silages containing a similar nutrient quality and forage structure if harvested at similar dry matter contents (see Figure 1).
When there is plenty of feed stored, consider harvesting grain sorghum crops for headlage to offset the use of purchased grain.
In a season when there will be plenty of stored feed reserves, there is an opportunity, particularly with grain sorghum crops, to store starch in the form of headlage, which is shown to be a more economical option than purchasing grain.
Table 3 shows that headlage can be about half the cost of barley grain when costed for starch, and while there can be a close cost comparison between white sorghum silage and headlage (if high starch concentration is achieved for the silage), consider the feed intake differences.
For example, for a cow to consume the equivalent starch as 1kg of barley grain, it must consume 2kg as fed of headlage or 5.4kg as fed of silage, therefore consuming more fibre if it is coming from a forage source (see Figure 2).
Using headlage is a safer way to feed higher amounts of starch because it has a fibre source within it.
So, how does feeding white sorghum stack up?
Farmers across south-east Queensland are achieving high yields of sorghum silage of 10-16 t DM/ha for the first cut and 8-11 t DM/ha for the second cut. In a feeding demonstration at the Gatton Research Dairy monitoring both a PMR and TMR herd, the DAF dairy team was able to feed high amounts (between 48-53pc DM of the total diet) of white sorghum silage (33pc DM starch) and headlage (58pc DM starch) to the point where the diet was nutritionally balanced, and no grain was required to meet starch requirements.
The PMR achieved 24 L/cow/day at a diet cost of $3.89 and the TMR herd achieved 26 L/cow/day at a diet cost of $5.27.
This resulted in an average margin over feed cost (MOFC) of $10.51 for the PMR herd and $10.33 for the TMR herd, averaging 42pc above Queensland's industry dairy herd average for that year.
With drier weather patterns and increasing irrigation costs, white grain sorghum is a forage source worth considering.
It can reduce the costs and risks of growing corn, potentially reduce the use of purchased grain and provide resilience to the farm system.
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