Last update: April 5, 2012 09:02:24 AM E-mail Print


Food Value Of Some Karoo Shrubs

EE Buttner



THIS shrub is known under various names such as Swart Karoo and Skaapbos. Its scientific name is Phymaspermum parvifolium. In some areas it is also known as Bitterkaroo. The plant is found in many parts of the Karoo, mostly on flat ground and hardly ever on hills. While the protein content is never very high, it varies from about 7 per cent. to about 22 per cent., depending upon the season and prevailing climatic conditions. During most of the year, however, the protein content varies from about 11 per cent., to about 15 per cent., which is still quite satisfactory. The figures given are, of course, calculated on the dry matter, that is, the freshly taken sample dried in a drying apparatus until it has become absolutely bone-dry.

Farmers may often wonder why chemical analyses are not given calculated on the sample as found in nature, as the animal eats it, for it seems rather impractical to give figures based only on the bone-dry sample. The reason for calculating all values on bone-dry samples is that the moisture content of the plants varies so much that it would be unfair and confusing to compare the nutrient values of different plants of any given species, or of one species with another, because the animal eating the plant takes little notice of the moisture content but mostly of the protein, carbohydrate, mineral and vitamin content. In short, the animal is only interested in the dry matter content, which is the actual nutrient portion.

When we study the protein content of any feed we must also bear in mind that there are great differences between one protein and another. In fact, the word protein can be very misleading at times because its so-called biological value may vary considerably and it is the biological value that determines how much protein the animal will eat or require, especially when still in the growing stage. The protein requirements also depend upon the acid-base relationship of the diet taken as a whole. It is thus not sufficient to compare the mere protein content of foods for their protein value or nutrient value. This explains why often a food with less protein is superior to one with more protein. The acid base relationship of a diet depends upon its mineral or ash content to a great extent. The greater the proportion of acid elements the food as a whole contains, the higher will the protein content have to be, because then part of the protein has to be broken down uneconomically to form the necessary ammonia to excrete the acid elements with the urine as ammonium salts.

Unfortunately the fibre content of the Vaal Karoo is rather higher than that of the other Karoo shrubs. This is so for most of the year, and yet the plant is known to be very fattening for sheep. It may be that the particular type of fibre in this plant is more digestible or nutritive for some reason or other than is the fibre of many other plants. Perhaps its fibre is especially attractive to certain types of digestive organisms allowing them to thrive and multiply to a very great degree. The fattening properties of Vaal Karoo cannot be due only to its relatively high carbohydrate content because the shrub known as Beesvygie (Mesem. hamatum) contains even more carbohydrate and yet the latter is known to cause loss of body weight in sheep grazing on it extensively. But in the case of the Beesvygie it may be the high oxalic acid content that is the cause of the trouble.

The calcium content of the shrub is fairly high during practically the whole year, while the phosphorous content is also relatively high during the whole year, varying but little from season to season. Probably this is one of the main advantages of this plant, because Karoo sheep require fairly high amounts of daily phosphorus to offset the high calcium intake from both the food and the water. The high potassium intake also increases the phosphorus requirements, because animals living in the Karoo regions live on an excessively alkaline diet, with a deficiency of acid elements. While this tends to make the protein eaten more available to the body, it tends to upset the phosphorus metabolism and increases the danger of calculus (gravel in the bladder, or kidney stones).

This is why wheaten bran is one of the best supplements for the Karoo areas. Bran is both rich in phosphorus and acid-forming elements, to say nothing of protein. Another advantage of bran as a supplement is that it is a natural food and not a synthetic product. Bran is also rich in vitamins, as well as minerals and trace elements generally. About the only thing it is poor in is sodium, obtained, however, from common salt or brak water rich in sodium chloride. Bran is very concentrated so that a little goes a long way.

Another advantage of Vaal Karoo is its low magnesium content. This allows the animal to utilise its phosphorus intake more economically, because magnesium excess renders phosphorus unavailable to a certain extent. Unfortunately the shrub cannot stand overgrazing, so that controlled grazing would produce a greater amount of feed per year. It is also known that Vaal Karoo produces a greater increase in body fat than flesh and thus the wool tends to become more greasy, which, of course, protects the fibre from the ravages of sun and weather, especially the tip of the fleece.

Vaal Karoo is one of the dominating plants generally found on what we look upon as good sheep farms, preferring non-brak soil and thus showing us which is a good soil as far as base-forming minerals are concerned. Another characteristic of Vaal Karoo is its strange moisture content during several months of the year. The shrubs seem to dry out more than do the other Karoo plants, and in dry weather the water content may fall as low as 33 per cent. of the fresh sample as found growing on the veld and tested before withering. The plant is thus well adapted to dry conditions, for in other plants not so well adapted the protein content drops considerably in dry weather, while Vaal Karoo maintains a fairly good protein content. As regards palatability the shrub is fairly well grazed by sheep and is looked upon as a hardy arid tough plant in dry seasons. The plant also seems to be well digested by sheep judging by the little residue left in the droppings.



Merino Breeders Journal 24(3)