I appear to have been appointed the Swami of Swine as, following on from the email about onions, I received an email asking why we feed mangels to pigs as “they’re poisonous!”
No, as I pointed out on the River Cottage forum a few weeks back, mangels are no more poisonous than fodder beet, sugar beet, leaf beet or beetroot—they’re all closely related vegetables.
Members of the Amaranthaceae (particularly the chenopodioids sugar beet, mangel, Swiss chard, spinach etc), Geraniaceae (geraniums etc) and Polygonaceae (rhubarb, sorrel, buckwheat etc) plant families can contain large amounts of oxalic acid, which can be toxic if the plants contain more than 10% of their dry weight as oxalic acid and if an animal eats too much of it too rapidly.
In general, the highest concentration of oxalic acid is found in the leaves and the least in the roots.
With some plants, oxalic acid content increases as the plant matures, in others it decreases.
Mangels and sugar beet have a high oxalic acid content during the early stages of growth, but the level decreases as the plants mature.
Next, the internal flora of a pig’s large intestine can adapt to break down oxalates, so the trick is to start them with small amounts of mature roots of mangels and other beets.
After a while, the amount of beets being fed can rise and include small amounts of leaves, although I wouldn’t go higher than a diet of 20% mature beets (and much less immature ones or leaves).
Also, feeding large amounts of immature roots and leaves to hungry, underfed pigs is likely to result in oxalate poisoning.
The result is hypocalcaemia, which occures when the oxalates react with calcium in the body, depositing calcium oxalate in tissues throughout the body, particularly the kidneys. In other words, the pigs get kidney stones.
Acute oxalate poisoning (ie a hungry, undernourished animal eating a large amount of oxalate rich fodder in a short space of time) results in breathing difficulties, staggering, weakness, collapse, coma and death.
As well as oxalate poisoning, there’s also a chance of nitrite poisoning.
Beets (including mangels, fodder beet and sugar beet) plus kale, swedes, turnips and rape can all accumulate large amounts of nitrates in some circumstances.
Nitrates are not particularly toxic in themselves, but bacteria in the gut converts them to the much more toxic nitrites.
Nitrites react with haemoglobin to form methaemoglobin, which is incapable of transporting oxygen. Nitrite poisoning results in oxygen deprivation, weakness, low blood pressure, and death by asphyxia.
The main factors associated with excess nitrate take-up are heavy application of nitrogenous fertilisers, shade, drought and heavy herbicide use. Avoid too much of those and your beets and brassicas should be fine for livestock (and people).
Having pointed out all the potential negative consequences of feeding beets to pigs, I’d now suggest people relax and not worry unduly.
The main thing is to be moderate in feeding beets of any sort to pigs (and indeed other livestock and people), to be aware of the factors that increase the risks, and to manage those risks to keep them to an acceptable level.
We not only feed mangels (and fodder beet, turnips, swedes and kale to our pigs), but we also eat them ourselves and mangels make a particularly fine ale.
We wouldn’t be doing that if they were toxic and poisonous.
The main advice I’ve come across in relation to human consumption of beets and their leaves is that they should be avoided by people with a propensity to kidney stones and eaten in moderation by everyone else.
Much the same as pigs, in fact.