Sheep nutritional requirements when grazing stubble

Crop stubble can provide a valuable source of nutrition to sheep in summer and autumn. Grazing stubble also has the benefit of reducing high stubble loads. The feed quality varies significantly and supplements are often required. Animal health is a priority. Paddocks should always have sufficient ground cover to prevent soil loss from erosion.

Grazing value of stubble

An adult dry sheep of 50 kilograms in condition score 2 has a minimum daily requirement of:

• 7 megajoules (MJ) of metabolisable energy
• 6% crude protein.

The energy and protein content of common feeds for sheep can vary considerably.

Energy

The most important thing to consider in an animal’s diet is the energy content. It is measured as megajoules (MJ) of metabolisable energy (ME). Most energy comes from carbohydrates in the diet.

Protein

Protein is the second most important criterion. It is measured as crude protein (CP) as a percentage.

Digestibility

Digestibility refers to how much of the feed an animal can use for its nutritional needs and is related to the energy content. Sheep feed should have a minimum of 55–65% digestibility. This means for every kilogram of feed consumed, 550 &emdash; 650 grams are used by the animal and the rest is excreted.

Grain has higher feed value as energy, protein, and digestibility than straw as shown in Table 1.

Animals will eat the most digestible parts first (i.e. grain and green shoots) and leave the least digestible parts last (i.e. straw and trash).

Roughage

A sheep’s diet must also include at least 10 – 30% roughage. Roughage helps sheep produce saliva. This helps to lubricate food and provides a buffer to the acid produced when digesting food.

Stubble is likely to be very high in fibre, so supplementation would not be required. Supplementing with roughage is usually only required on high grain diets.

If the diet is too high in fibre, animals will not be able to eat enough to get their energy and protein requirements.

The maximum dry matter a sheep (or cow) can take in is 2 – 3% of their body weight per day. However, if the fibre content is high, it will be less.

Feed value of stubble

The value of feed in stubble comes from the amount of residual grain, plus green plant biomass. This includes shot grain and summer weeds. However, modern grain farming practices have resulted in less grain being left behind and better summer weed control.

The feed value of stubble varies considerably between crop types and plant parts (Table 1). This feed value should always be compared back to the sheep’s feed requirements (e.g. a 50 kg adult dry sheep in Condition Score 2, requires a minimum of 7 MJ metabolisable energy per day and 6% crude protein for maintenance). Various classes and sizes of sheep will have different energy and protein requirements.

Variation in stubble value

The feed value of stubble varies considerably between crop types and plant parts (Table 1). This feed value should always be compared back to the sheep’s minimum daily feed requirements (e.g. a 50 kg adult dry sheep requires 7 MJ metabolisable energy per day and 6% crude protein).

DMD = dry matter digestibility, as percentage of dry matter; ME = metabolisable energy, as megajoules per kg dry matter; CP = crude protein as a percentage of dry matter. Source: Grain & Graze website.

The value in stubble for feed also varies between seasons. While rainfall decreases digestibility, it also encourages the germination of seeds to provide a green feed source. Spring rains can also promote the development of late tillers which remain after harvest.

Dry seasons and heat damage mean residual grain has less energy, but the stubble can be more nutritious. Windy weather during grain filling can leave more residual grain.

Measuring the value of stubble

The feed value of stubble can be measured with a feed test. This is the only way to determine energy and protein content. It can be difficult to sample and measure accurately due to variability across paddocks and being able to get a sample of what animals are eating.

Monitoring a sheep’s liveweight and/or body condition score is a good way of measuring how well the stubble is meeting their needs. While this can be labour intensive, measuring a sample from the mob can provide a good indication and electronic ear tags can make it easier to monitor the same sheep.

Timing of grazing

The best time to start grazing stubble is immediately after harvest as the quality decreases with time.

As a general guide, stock should be removed after 6 weeks. This, however, depends on a number of factors, including the season, paddock size, and stocking density.

Stock should be removed once grain and green shoots fall below 40 kg per hectare (or 4 g per square metre).

At least 50% ground cover must be maintained to minimise the risk of wind erosion of the soil and 70% ground cover will minimise the risk of water erosion.

Stock should also be removed immediately if animals are not maintaining weight unless supplements are provided.

A stock containment area (SCA) can be used to protect soil and pasture resources during adverse seasons. Find out more at Feeding Livestock.

Licks and supplements

Stubble that lacks adequate energy and/or protein can be supplemented by pellets or grain.

Urea

A urea supplement may be used where stubble is low in protein. It provides a source of nitrogen for the microbes in the rumen to make protein.

It is usually cheaper and more efficient to supplement with lupins or other high protein grains than urea licks, which can be toxic.

It was previously thought that supplementation with urea could help to break down the structural carbohydrates (lignin) in the stubble by increasing the number of microbes in the rumen. Recent research has shown that urea supplementation has little to no impact on lignin breakdown.

Cereal grains are low in sodium and calcium so sheep grazed on these stubbles for long periods may have an inadequate intake of these essential minerals.

Calcium

Calcium is especially important for young growing animals and pregnant or lactating ewes.

Many forms of calcium supplementation exist. It is easiest to put out limestone for sheep to consume ad libitum. Adding salt to calcium encourages uptake. Limestone with the addition of salt solves the calcium and sodium requirement.

Salt

Salt encourages sheep to drink more water, helping with the prevention of bladder stones and reducing the incidence of heliotrope toxicity.

Trace elements: copper and selenium

Stubble can have low levels of trace elements compared with pasture and farmers need to be aware of this to prevent livestock deaths on stubble.

While trace element deficiencies, such as copper and selenium, can be deficient in some areas, it is recommended to only provide a supplement if symptoms are visible and/or a deficiency has been diagnosed.

Supplementation of some trace elements can cause toxicity if not required, incurring an unnecessary expense.

Copper deficiency can cause swayback disease in lambs. However, it is more common to see copper toxicity in sheep rather than copper deficiency. Cattle have a higher requirement for copper than sheep, so a cattle ration can lead to copper toxicity if fed to sheep.

Selenium is important for normal muscle function, growth, wool production, and reproduction. Selenium levels can be determined by a blood test or post-mortem liver samples. Some vaccines and drenches contain selenium, farmers should be wary of delivering excessive amounts particularly to young sheep which can result in toxicity and even death.

Vitamins

A lack of green feed can result in vitamin deficiency. Stubble can have low levels of vitamins compared with pasture and farmers need to be aware of this to prevent livestock deaths on stubble.

A vitamin A, D and E drench (or injection) will last 3 months. Sheep are not likely to overdose on vitamins. Management of thiamine (Vitamin B1) deficiency is detailed below.

Vitamin A is required for eyesight. Vitamin D can be obtained from the diet or sunlight exposure and is important for calcium absorption across the gut. Vitamin E is important for muscles.

Health issues with grazing stubble

When grazing sheep on stubble, it’s important to be aware of the following potential health issues.

Water belly (Urinary Calculi)

Mineral imbalances and the low moisture content of stubbles mean that sheep, especially wethers, can develop bladder stones. This may result in water belly if the stones get caught in the urethra where they can cause partial or complete blockage and ultimately death if the bladder ruptures. Water belly can be prevented by providing salt in the diet which encourages the sheep to drink more. It can also be prevented by addressing the mineral imbalances in the diet, particularly calcium and phosphorus.

Grain poisoning and toxic summer weeds

Sheep are at risk of grain poisoning when consuming high starch diets (such as cereal grains) without an adaptation period. Therefore, stubbles with high cereal grain content need to be grazed carefully. Sheep should have full stomachs before being introduced to a stubble paddock, or they can be gradually introduced to the stubble by initially limiting the time they are allowed to graze or by grazing the stubble in cells.

Every season has its own toxic plant risks. Most issues with toxic weeds follow summer rain, especially seen in sheep grazing stubble.

Thiamine deficiency/polioencephalomalacia

Stubble tends to be low in thiamine (Vitamin B1). In addition, stubble diets encourage the growth of thiaminase-producing bacteria in the rumen, further depleting available thiamine and leading to thiamine deficiency. Thiamine is essential for brain functioning. Sheep with thiamine deficiency become dopey and will stargaze. They may look blind.

Some sheep will respond to thiamine injections, but others do not recover.

Nitrate or nitrite poisoning

Some stubble types have high levels of nitrate, which can lead to nitrate and nitrite poisoning.

High nitrate can irritate the stomach wall and intestine, leading to stomach cramping, scours and diarrhoea.

Nitrite poisoning is worse and can be caused by high levels of nitrate entering the bloodstream. Nitrite poisoning results in a compound forming with haemoglobin and preventing oxygen being carried in the blood.

Most animals are found dead. Others may be seen having difficulty breathing, gasping for air, and staggering prior to collapsing, convulsing and ultimately dying.

A coffee-brown discolouration in the mouth due to the change in colour of the blood is a sign of nitrite poisoning. However, this is only evident for a short period of time as the blood will return to its normal colour.

Lupinosis

Lupinosis is caused by the toxin from a fungus living on lupin stubble.

The toxin causes massive liver damage when ingested by a sheep. The animal gradually becomes unwell as the liver failure progresses. The eyes and mouth become yellow (jaundice).

Worms

Crop stubble should be clean from worm infestation if the paddock has not been grazed for some time.

The first summer drench may coincide with sheep going onto stubble and this would help to ensure they stay clean (as long as an effective drench is used).

Worm burdens need to be monitored and managed on stubble as on pastures, particularly if sheep are eating short green feed and weeds after summer rain.

The only way to accurately measure the worm status of your sheep is to conduct a worm egg count. Sheep should be contained to obtain fresh faecal samples. These tests can be undertaken through a veterinary clinic or stock agent. The cost is low relative to the cost in lost production and the cost of drenches.

Drenching programs should always be approached strategically.

Acknowledgements

Authors: Felicity Pritchard. Technical editors: Cathy Bunter, Jo Cameron and Jane Court.