Summer Fodder Crops

Sorghum

KEY POINTS

  • Summer fodder crops are a viable feed source on wet areas and in wet summers.
  • Soil temperatures must reach 14 to 19°C for germination, depending on species.
  • There are some possible health problems with sheep grazing sorghum.

Author: Tim Wiley, Department of Agriculture, Jurien Bay

Soil types/niches

Summer fodder crops will grow:

  • On most soils provided there is a reasonable level of nutrients and soil moisture.
  • In areas where winter crops have failed from waterlogging.
  • In permanently wet areas.
  • After good summer rains.

Soil pH

The suitable range for pH of the soil is between 4.5 and 7.

Salt tolerance

The tolerance to salt in the soil is moderate to poor depending on the species of the crop.

Cost of establishment

It costs about 80 to 150 dollars per hectare to grow fodder crops.

Types of summer fodder crops

Sorghum

Sweet Sorghum

Sorghums are summer growing grasses with a reasonable drought tolerance. There are forage and grain varieties of sorghum. There is also a range of hybrids derived from crossing sorghum with sudan grass. However, only the forage types of sorghum and their hybrids should be used for grazing, as they are more productive and of better quality.

Sorghums usually produce a greater bulk of summer feed than other summer fodder crops. Sorghums contain prussic acid that can be toxic to stock. Therefore, newer varieties have been bred with lower levels of prussic acid.

Sorghums are usually biennial. This means that they become dormant over winter and regrow in the second summer, after which they usually die out. Varieties that show promise in Western Australia include ‘Jumbo’ and ‘Superdan’.

Sorghums will not germinate well until soil temperatures reach 18°C at 9.00 a.m. Some germination occurs at lower temperatures, but the resulting seedlings are weak and do not perform well.

Sweet sorghums

Sweet sorghums have higher sugar levels than the other sorghums and this makes them more suitable for silage. They also have coarser stems than the other sorghums and are, therefore, less suitable for grazing by sheep.

Millet

Temperate millet

As a fodder crop, millets are suited to one summer season. In some cases millet has regenerated from self-sown seed. The seed is much less temperature sensitive than sorghums and will germinate at a 9.00 a.m. soil temperature of only 14°C. (See further discussion about soil temperatures and germination under the heading ‘Other issues’ in this section). Millets do not contain prussic acid but are usually not as productive as sorghums or pennisetums. Varieties showing promise in Western Australia include ‘Shirohie’ and ‘Japanese’.

Forage pennisetums

Forage pennisetums were previously known as pearl millets. Forage pennisetums generally have higher protein and digestibility than sorghums. The pennisetums do not contain prussic acid. They also have finer stems and are the preferred summer forage for sheep.

Pennisetums are frost sensitive and can be killed if sown too early. Pennisetums require some rain after seedlings emerge to promote the development of the secondary root system. They will germinate at 16°C. A variety showing promise in Western Australia is ‘Nutrifeed’.

Maize

Maize is extremely productive, but is probably only viable in Western Australia with irrigation.

Production potential

Table 1. Average growth rates and condition scores of Merino and crossbred lambs on forage sorghum

Dry matter production of summer fodder crops is dependent on the moisture supply. Yields of two to 10 tonnes dry matter per hectare could be expected depending on moisture supply and nitrogen application. Because they usually rely on some summer rain, their production in Western Australia tends to be unreliable except on wet areas.

When considering the benefits of summer fodder crops it should be remembered that they need to be fitted in with other management components for the whole farm. Where crops have failed from waterlogging, summer crops are a useful option as they dry the site out and also control weeds. Perennial pastures are a better option on permanently wet sites as there is a once only cost of establishment.

Table 2. The nutritional value of sorghum before and after grazing

Case study – Sorghum over summer

Fiona Jones, Department of Agriculture, Katanning

The performance of crossbred and Merino lambs on Betta Graze forage sorghum was monitored at Beaufort River.  A good stand of sorghum was established which provided a valuable source of feed over the summer period, particularly with the high summer rainfall causing a rapid decline in dry feed quality.

The sorghum was seeded in October in 75 centimetre rows and urea was spread at 100 kilograms per hectare before the January rainfall. Application of urea can increase the risk of prussic acid poisoning on sorghum and needs to be monitored closely. The lambs were stocked at 43 DSE per hectare for 15 days with sulphur blocks to reduce the risk of prussic acid poisoning.

The average liveweight gain for the Merino and crossbred lambs was 146 grams per head per day with no supplementary feed provided during this period. (See Table 1.) The crude protein of the sorghum was high at the start of grazing (Table 2), when the plant had plenty of fresh leaf material. By the end of the grazing period, the crude protein of the sorghum was still more than adequate but the metabolisable energy was lower due to a higher proportion of fibrous stem and less leaf material.

The sorghum was spelled for three and a half weeks between the first and second grazings. The plan was to graze the sorghum three or four times during the summer. Used in this way, sorghum provides a way to get lambs to a reasonable weight over summer before finishing them in a feedlot.

Feed quality

The quality of summer fodder crops will vary with soil moisture, soil fertility and grazing management. Protein can vary from less than 10 to 25 per cent. Digestibility is normally in the range of 55 to 70 per cent.Protein and digestibility both decline with increasing age of a stand over the summer and increasing height of regrowth. Nitrogen fertiliser will improve the protein level but not the digestibility of summer fodder crops.

While summer fodder crops can provide feed of reasonable quality over summer, supplements with lupins may be required for maximum growth rates of animals. Generally, more lupins will be required as the stand ages. Summer fodder crops are generally low in both sodium (salt) and sulphur. Balanced mineral licks containing these nutrients should be made available to stock.

Grazing management

Grazing of fodder crops should be delayed until the plants are well established. For sorghums, do not put stock in until the crop is about one metre high. Sweet sorghum can be grazed when the crop is 1.5 metres high and millet and pennisetums when the crop is 0.5 metre. After this, the best results will be with rotational grazing, although set stocking is still possible.

Do not graze the stand too low. The sheep should be removed once the height of the forage is down to 15 centimetres. Regular grazing helps delay flowering and this maintains the quality and growth rate of the fodder for a longer period. If the fodder is left ungrazed, it is more likely to suffer moisture stress. If sorghum plants are allowed to mature their protein level and digestibility will decline rapidly.

Grazing problems

Zea Mays

Prussic acid poisoning

Animals can be affected by prussic acid poisoning when grazing sorghums. Prussic acid is converted to hydrogen cyanide in the gut of the animal. The cyanide reduces the uptake of oxygen into the blood and symptoms of the poisoning include muscle tremors, staggers, deep and rapid breathing, frothing at the mouth and gasping for breath. Death can occur in extreme cases. In sheep, the poisoning can be treated by drenching animals with 14 grams of sodium thiosulphate in 500 millilitres of water.

Factors that increase prussic acid levels are excess nitrogen, inadequate phosphorus and moisture stress. The risk of prussic acid poisoning can be reduced by ensuring that:

  • sheep are not hungry when first put on to sorghum;
  • there is other feed in the paddock;
  • the sorghum is at least 0.8 metre tall;
  • the plants are not grazed when they are (moisture) stressed and;
  • a mineral lick containing sulphur is available. Nitrate poisoning Nitrate poisoning can occur in sheep grazed on a wide range of grass fodders. Fortunately it is not a common problem. However it will occur when there are very high levels of soil or fertiliser nitrogen. The poisoning causes severe gasping, convulsions and death. It can be treated by drenching with vinegar.

Other issues

Millet seedlings

Sow at two to five centimetres depth where there has been good weed control. Sow about five kilograms per hectare of seed at wide row spacings of 0.5 to one metre. Close row spacing can lead to very poor results.

Use rates of fertiliser similar to those used for wheat grown on the same soil type. However, be careful where nitrogen and potash are sown with the seed as these fertilisers can kill emerging seedlings. It is safest to drill the fertiliser before seeding or with alternate runs of the seeder. Use sulphate of ammonia if top dressing as urea is rapidly lost to the atmosphere when spread in summer.

Landcare and rotational benefits

The degree to which summer crops help to dry out wet areas for the following season is not yet clear. Spraying out weeds in spring will reduce their seed bank for the following year. This may well provide a means of preparing for a winter crop, especially where weeds have become resistant to chemicals. However this will also reduce the legume seed bank that is important for subsequent pasture regeneration.

Further reading

Fry, J. (Compiled by) (1997). A reference manual for farmers involved in Woolpro productivity evaluations.

Segment 5 – Tactics for different summers, pp 15-18. Department of Agriculture.

Stuart, P.N. (1993). The forage book. Pacific Seeds, Toowoomba, Queensland.