Toxic weed seeds

Weed seeds are constantly found in coarse grains used in pig feed. Some weed seeds are highly poisonous and toxic, and can cause severe illness and death. Others are non-toxic but can interfere with digestion or severely lower nutrient intake, reducing growth. Many factors affect their toxicity level, including season, where they are grown, whether the seeds are ground and how they are stored.

This page describes some weed seeds that are common in Australian grain. It provides the common and scientific names, notes on toxicity and safe feeding levels where possible. The weed seed species are described according to comparative toxicity. Weights of 100 seeds enable the estimation of contamination rates in grain. Tolerable concentrations in pig feeds derived from experimentation are compared with those currently permitted by Queensland's Agricultural Standards Regulations (QASR) for stock food.

Harmful at low inclusion levels

Castor oil plant (Ricinus communis)

The large seeds of the castor oil plant contain a potent poison called ricin, a toxic protein (toxalbumin). Symptoms of poisoning in pigs resemble botulism except that faeces often contain mucus and are tinged with blood. Depression and paralysis are common. A lethal dose can be as little as two seeds per pig (100 seeds = 22 g). Seeds are more toxic when ground. Under QASR, this plant is prohibited in stock food.

Mexican poppy (Argemone ochroleuca and A. mexicana)

The small, dark-brown to black seeds are like miniature peppercorns, and contain a group of isoquinoline alkaloids and derivatives: protopine, berberine, chelerythrine and sanguinarine. The seed is extremely toxic to humans and poultry. Seed at 2-6 per cent in the feed of a 20 kg pig causes severe food rejection by the second day. Symptoms of poisoning are lethargy; oedema of the skin, lungs and lower limbs; reddening of the skin; oily yellow diarrhoea; and possible haemorrhage.

Finisher pigs (>50 kg) may tolerate contamination rates up to 1 per cent for short times but for younger pigs, where maximum feed intake is desirable, concentrations should not exceed 0.2 per cent. Exposing crushed seed to sunlight markedly reduces its toxicity. QASR allow a contamination rate of 20 seeds/kg grain (0.004 per cent, as 100 seeds = 0.2 g). However, the regulations have to consider other factors, such as greater toxicity to poultry. This level is very conservative for pigs.

Potato weed (Heliotropium europaeum)

This plant, also called 'common heliotrope' is an annual weed that infests the wheat-producing areas of southern Australia. It produces toxic pyrrolizidine alkaloids that damage the liver. In 1993, a serious poisoning outbreak occurred in South Australia when 1000-4000 pigs died over three months because wheat contaminated with potato weed seed was incorporated into diets at about 0.2-1 per cent. Lower levels slowly damage the liver and severely affect growth without obvious signs. Poisoning outbreaks tend to occur every few years when wet summers favour weed growth and delay wheat harvest. Although a safe feeding level has not been established, a maximum of 0.01 per cent might be an interim guideline (about 100 seeds/kg, as 100 seeds = 0.13 g). QASR currently has no standard for this seed because it does not grow in northern Australia.

Sesbania pea (Sesbania cannabina)

This small brown to dark-green rod-shaped seed is common in grain sorghum. Its toxic principles are thought to be a combination of a gum in the seed coat and an amino acid (canavanine) in the seed. Sesbania makes pig diets highly unpalatable. A maximum safe feeding level of 0.25 per cent (w/w), equal to 300 seeds/kg, is recommended (100 seeds = 0.8 g). QASR allow only 200 seeds/kg of grain (0.17 per cent).

Thornapple (Datura ferox, D. stramonium)

These flattened, kidney-shaped seeds contain toxic tropane alkaloids (hyoscyamine, scopolamine). The degree of toxicity varies between seed batches. Crushed seed is much more toxic than whole seed. In pigs, symptoms of intense thirst, dilated pupils, flushing of the skin and irritability are typical of poisoning. The maximum recommended feeding level is 0.05 per cent because as little as 0.1 per cent in feed causes rejection. QASR allow only 5 seeds/kg (about 0.005 per cent) in stock feed (100 seeds = 0.64 g D. stramonium or 1.56 g D. ferox).

Jute (Corchorus ollitorius)

These small, grey seeds cause severe food rejection, vomiting and scouring in pigs. Reduced feed intake and growth rate are related to jute seed level in the feed and total food rejection occurs at 0.5 per cent in feed. The maximum safe feeding level is 0.01 per cent (100 seeds = 0.6 g). QASR allow 10 seeds/kg (0.006 per cent).

Harmful at low to moderate inclusion levels

Bellvine (Ipomoea plebeia)

Three seeds are usually contained in a round capsule that is grey to black in colour and shaped like a mandarin segment. Frequently a contaminant of sorghum and maize, bellvine seed is difficult to grade out. The seeds contain indole alkaloids and lysergic acid amide. Preliminary studies show that 10 per cent inclusion in the diet causes severe enteritis and food refusal. A safe feeding level of 1 per cent or less is presently recommended (100 seeds = 2.1 g). QASR allow 500 seeds/kg (1 per cent).

Buckwheat (Polygonum convolvulus)

Also known as climbing or black bindweed, buckwheat seed is small, three-sided, four-pointed and hard. Field evidence suggests that the sharp points cause gut irritation but no toxic factors have been found. Protein content is high quality while digestibility is low, probably due to the high fibre and/or tannin content. It is safe to feed to pigs; however, for each 1 per cent inclusion, the growth rate is depressed by 0.4 per cent (100 seeds = 0.57 g). QASR allow 1500 seeds/kg.

Noogoora and Bathurst burrs (Xanthium pungens and X. spinosum)

Noogoora and Bathurst burr seeds contain a potent glycoside, which can cause severe gastroenteritis, depression and convulsions in pigs. About 20 per cent (w/w) of ground burr in the feed will cause death. The spines on whole burrs are likely to cause physical injury. QASR allow 2 burrs/kg.

Paterson's curse (Echium plantagineum)

Also known as Salvation Jane, the seeds of this plant contain various pyrrolizidine alkaloids (echimidine, echiumine) in quantities that depend on location and season. The toxins cause liver damage leading to photosensitisation and finally kidney failure. There are no confirmed field cases of poisoning in pigs in Australia. Safe feeding levels have not yet been accurately determined but liver damage is evident with 2 per cent seed in the diet. More than 6 per cent of Paterson's curse seed in the diet results in a decline in growth rate and feed conversion (100 seeds = 0.37 g). QASR allow 100 seeds/kg (0.04 per cent).

Rattlepods (Croatalaria species)

Rattlepod seeds contain various pyrrolizidine alkaloids that cause liver and kidney damage at a level greater than 0.5 per cent in the diet. Safe feeding levels have not been determined. These seeds are prohibited materials with respect to stock foods under QASR.

Harmless weed seed contaminants

Many other weed seed species that occur in grain appear to be safe to feed even at high levels such as 25 per cent. However, they probably cause a decline in performance due to nutrient dilution (high-fibre content) or poor digestibility. The following weeds appear to be in this category:

  • African turnip (Sisymbrium thellungii) (QASR allow 20,000 seeds/kg; 0.8%)
  • barnyard grass (Echinochloa spp.)
  • barley grass (Hordeum leporium)
  • black oats (Avena fatua )
  • caltrop (Tribulus terrestris)
  • phalaris grass (Phalaris spp.)
  • wild cotton (Hibiscus trionum)
  • wild radish (Raphanus raphanistrum)
  • mint weed (Salvia reflexa).

Summary of growth depression

This table also shows the maximum inclusion rate used in experiments (limited either by toxicity or practicality, in the case of non-toxic seed) and the weight of 100 seeds.

Table 1. The percentage decline in growth rate of pigs fed 1% of a range of weed seeds
Common name of weed Scientific name Growth decline (%)* Maximum fed in experiment (%) Weight
100 seeds (g)
African turnip Sisymbrium thellungii 0 9 0.04
Caltrop Tribulus terrestris 0 10.5 4.0
Wild cotton Hibiscus trionum 0 9 0.27
Bellvine Ipomoea plebeia 0.1 8 2.1
Saltbush Atriplex muelleri 0.3 12 0.33
Buckwheat Polygonum convolvulus 0.4 25 0.57
Turnip weed Rapistrum rugosum 1.3 21 0.5
Paterson's curse Echium plantagineum 1.4 16 0.37
Wild turnip Brassica tournefortii 3 9 0.14
Mexican poppy Argemone ochroleuca 25 2 0.20
Sesbania Sesbania cannabina 10 1.5 0.80
Thornapple Datura stramonium/ferox 20 0.8 0.64/1.56

* for each 1 per cent inclusion of weed seed

Further information