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Protein content in pasture

All animals require protein in their diet. Grazing livestock depend on pasture to supply their requirements. If the pasture doesn’t contain enough protein, then growth and production will suffer. A low protein content in pasture usually equates with a low energy content, but this is not necessarily so in formulated rations, which can have a high starch content but a low protein content.

The protein content of pasture species changes greatly with time of year and species. Legumes, of course, are high in protein, being able to draw on atmospheric nitrogen via the symbiotic Rhizobium bacteria in their roots. Grasses tend to have a lower protein content. Young pasture early in the growing season has a higher protein content than old pasture. A mixed pasture early in the season has a typical digestible crude protein content of 25% (on a dry matter basis). By flowering, this has dropped to 10%.

How much protein is enough?

Protein requirements vary with the time of year and the reproductive cycle of the animals – higher in later pregnancy and during lactation, for example. Early growth (of good quality pasture) typically contains enough protein to support lactating animals, but the same pasture when mature can fall well short of supplying enough protein to support the same animals approaching calving or lambing.

Mature cattle require 6% to 8% crude protein in the diet to maintain weight. Therefore, growing animals, lactating animals and pregnant animals need much more.

These ranges are a guide only and depend on breed, genetics, environmental stress and other feed components (water quality, mineral nutrition, energy levels).

Working out the protein content

The easiest way to measure protein content of pasture forage is to measure the total N content. All proteins contain N: without N, protein cannot be made. Bulk protein contains more or less a consistent 16% N. Therefore, multiplying the total N content by 6.25 (100%/16%) will give a good estimate of the protein content.

More accurate assessments take into account the digestibility of the N. For example, tannins present in the feed can bind to protein and make it indigestible.

For analysis, collect samples of several species from several places in your paddocks. Focus on those species that stock are most likely to eat. Remember to identify where you collected the samples, because this will have a direct influence on where you will need to make improvements.

Improving the protein content

The conventional way to improve the protein content of pasture is to encourage legumes (keeping in mind that too much legumes can lead to bloat). Because good legume growth depends on adequate sulphur and manganese in the soil (to support the growth of the bacteria), a soil test to establish these is useful. You can sow improved legumes or improve the P, K, S and Mo nutrition to encourage the legumes already present.

Management practices that increase young growth will increase the protein content also; for example, sowing annual grasses and legumes each year or fertilising early in the season to promote the perennial species. Harvesting of pasture to make hay or silage before letting animals on it lets you store good-quality fodder for later in the season when the quality drops.

Supplementing the nitrogen intake

Sometimes it is necessary to achieve an immediate increase in N intake by stock. The usual way to do this is with supplemental urea. Because the bacteria in the rumen of ruminants, such as cattle, sheep and goats, break down protein to simple N compounds, the actual form of N taken in is not critical. So urea, with a 46% N content, provides a good and cheap source of N to supplement poor-quality pasture or hay. Typical rates range from 25 to 50 g per head of cattle a day, and 8 g for sheep. Beware that too much urea initially can kill animals, so the urea must be introduced gradually, and never add more than 1% by weight of urea to fodder.

Alternatively, if you are making hay from poor pasture, you can spray the pasture with a urea solution. Simple calculation:

Increase the total N in the standing crop by 1%

Estimate standing crop at 10 dry t/ha

1% × 10 000 kg/ha = 100 kg/ha of N required

Urea is 46% N; therefore, apply 100/46 × 100 = 217 kg urea/ha

This is best applied as a foliar spray dissolved in water at 2 g/L maximum to avoid foliar burning. Do not apply in heat wave conditions.

Further information

http://www.dpi.nsw.gov.au/agriculture

http://www.dpi.qld.gov.au/

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