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Growing forage brassicas can help give your stock a quick growing highly nutritional feed stuff over the Summer/ early Autumn period when most of the permanent pasture species have gone reproductive and lost quality. Brassicas and leafy turnips are a high protein, high energy, low NDF feed source that is available about the same time as spring lambs are starting to be weaned. It can also be used to increase growth rate of store cattle that are waiting to be finished or to keep dairy heifers on their growth curve.

But after I have sown my crop, how do I make the best use of the money that I have invested in the crop?

When you have a look through all the research of live weight growth rates on summer crops, we see a wide range, 60- 320 g of LWG per day in lambs and 0.7 kg to 1.2 kg LWG in cattle. Why such as range? Stocking rate, crop type and stage of maturity, year? They all have an impact on the performance of the animals, but the nature of the plant itself has a big effect on the animals that are grazing it.

When we are putting weight on young animals like lambs and weaner cattle, we have a protein requirement of 16 % CP total diet. At this level, the animal’s requirements to build muscle is being met without any adverse effects from excessive protein. Many summer crops especially brassica have a total plant crude protein level way above the animal’s requirements. “So what” I may hear you ask or even “the more the better”, well not in the case of protein. When the animal starts to digest the crude protein from the plant, nitrogen in its different forms are released into the rumen, where the rumen bugs consume it as food. If there is too much or there is not enough bugs in the rumen, the nitrogen levels will increase to the point where it is unsafe for the animal. The animal’s safety mechanism in enacted and the nitrogen is sucked through the rumen wall into the blood stream where it is sent to the liver to be converted into urea. Some is then recycled through the animal’s saliva and the rest is excreted through the urine back out onto the pasture or crop. Changing the form of nitrogen into urea in the animal’s liver comes at a huge energy cost. This energy could have been used for live weight gain.

If we looked at a 36 kg lamb it has the potential to eat 1.8 kg DM of crop and put on 370 g of LW, so why don’t all lambs grow at this high rate. Well we use an industry calculation of what the energy cost is to recycle the extra protein to end up with a more realistic growth rate. Protein levels in the leaves of brassicas have been tested at 32 % CP + 0.18 MJ per % of CP over 16x intake

0.18 MJ x 16 x 1.8 = 5.1 MJ per head per day.

So now there is only enough energy for our lamb to put on 200 g LW per day and this is a figure more farmers see on their farms. So, what can be done on a farm level to minimise the lost production?

  • Grazing management can play a big part in getting the animal to eat the leaf and the stem together. Forage tests have seen that the stem of the plant has a vastly reduced protein content than the leaf. If we can get the animals to eat both together the total protein content is much closer to the animals requirement and we minimise the over allocation of nitrogen on the animal and thus reduce the amount of protein recycling the animal has to do.

  • Strip grazing or block grazing paddocks with temporary electric fencing will put pressure on the animals to eat more of the whole plant in the one sitting

  • If we don’t have the ability to increase the grazing pressure, we need to supplement the animals with hay/ straw that is highly palatable and they will freely eat it, over the crop. This can be very hard to find as animals will prefer to eat green leafy material over dry stemmy straw most days.

  • If this is the case and any of your animals refuse to eat the fibre source, then providing them with a salt regulated lick that has added bentonite that will slow the rate of passage down allowing the animals more time to digest the crop before it passes through them. TPM Boost Go is such a product, with its high salt content the animals will actively consume it and added bentonite will help slow the rate of digestion down, added minerals will also help to balance the diet for maximum growth potential for your animals

  • Feeding the animals an energy source like cereal grains whilst grazing the brassica crop can also help balance up the high protein from the brassica

To get the best out of your summer brassica crops call one of our TRAC Experts to tailor an individual plan.


Our Consultants


Mark Facy

0427 243 320

Owen Rees

0429 437 823

Mikaela Baker

0457 243 319


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Utilsing Brassica_Summer 2020-21
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