Summer forage crops can produce a large bulk of green feed over summer and early autumn, particularly if irrigated, but this feed is often low in energy and protein. Some varieties can be toxic at certain growth stages. They are generally grown to be grazed but some can be conserved as hay or silage.
Maize is almost always conserved as silage to ensure that the cobs and stems are chopped and thoroughly mixed. Grain sorghum may be an alternative to maize for silage where conditions are less than ideal for maize.
Types of summer forage crop:
The types of summer forage crops, their main characteristics and some varieties of each are shown below.
Forage crop toxicity:
Forage Sorghums, Sweet Sorghums and Sudan Grasses contain compounds which are converted to prussic acid when eaten by stock. For these compounds, levels are highest in
• Plants under six weeks old
• Plants stressed from lack of moisture or from frost
• Young regrowth
Conserving suspect crops as hay generally does not reduce the risk from toxicity.
To reduce risk when grazing these crops, do not graze them until they are at least 50 cm tall, do not put hungry stock onto suspect crops and provide licks containing salt and at least 10% sulphur.
Management:
Irrigated crops
These crops generally establish better in cultivated soil than where they are direct drilled. Seeding into moist soil will promote quick germination but seeding dry then irrigating can also be successful. None of these crops like being waterlogged for any length of time. If salinity is a problem, Japanese millet is the preferred crop.
Irrigated grazing crops should be seeded with a nitrogen/phosphorus fertiliser and should receive 40 - 50 kg/ha of nitrogen after each grazing. Irrigated crops are expensive to establish and grow. Seed (25 kg/ha @ $4.50/kg), fertiliser (300 kg/ha nitrogen/phosphorus fertiliser @ $350/t plus 200 kg/ha urea @ $400/t) and water ($150/ha) costs add up to about $450/ha, plus labour costs for fertilising and watering, assuming the crop is grazed.
Conserving the crop as hay/silage or cutting and feeding it adds to the cost. Flood irrigated crops will need between 5 and 8 TCM/ha irrigation water, depending on when they are planted, soil type and the weather over summer (TCM = thousand cubic metres).
The Sudan Grasses, Forage Sorghums and their crosses should be grazed when they are between 0.8 and 1.0 m tall. Grazing earlier than this could see problems with toxicity; crop quality declines rapidly once plants get much above this height.
The Sweet Sorghums can be allowed to grow for longer to provide a bulk of feed for autumn grazing. The millets should be grazed when they get to about 0.3 m tall. If they are allowed to grow much taller than this, quality drops off rapidly.
All crops should be strip grazed to reduce wastage.
Grain sorghums cut for silage will regrow but this growth will be toxic until plants reach a height of about 1 metre. It is probably more useful to spray the crop out and plant pasture. Planting pasture early gives it a chance to establish before winter slows growth.
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