A study of the mineral content and feeding value of natural pastures in the Union of South Africa (Final report)

Abstract

(1) The chemical composition of grass pastures differs in many respects considerably from that of bush pastures. Especially is this the case in connection with the changes to which the composition of pastures are subject with changing meteorological conditions from January to December. (2) Bush pastures are on the whole richer in all the constituents determined than are grass pastures. Another distinguishing feature in the two veld types is the difference in chemical composition between winter and summer pasturage: whereas the former type shows only minor fluctuations in the course of the year, the difference in the percentage contents of certain constituents during winter and summer are considerable in the case of grass pastures. (3) Qualitative differences in the pastures of specific regions within any one of the two main veld types are also encountered. (4) Expressed as percentages of the dry matter the phosphorus content of grass pastures is 0.12-0.17 during summer and 0.05-0.07 during winter. In the pastures composed mainly of bushes this constituent varies between 0.11 and 0.20 per cent. Crude protein values of the grass pastures fall from 7.0-9.0 per cent. in summer to 3.3-4.0 per cent. in winter, while the percentage of this constituent in those pastures composed mainly or wholly of bushes varies between 7.0 and 10.0 in the course of the year. (5) Calcium and magnesium values show only minor seasonal variations. Differences in the percentage contents of these constituents are, however, appreciable in the pastures of one region as against those of another. Expressed as an annual mean the variation in the calcium and magnesium contents of grass pastures in different regions are 0.25-0.48 per cent. and 0.12-0.21 per cent., respectively. Similarly, the calcium and magnesium averages for bush pastures are 0.60-1.05 per cent. and 0.21-0.37 per cent., respectively, depending on the region. (6) Maximum potassium and chlorine values occurring during summer in the grass pastures are 1.32-2.31 per cent. and 0.32-0.59 per cent., respectively. During winter the former constituent varies between 0.29 and 0.63 per cent. and the latter between 0.06 and 0.22 per cent. in the grass pastures of different regions. Sodium average figures fall from a maximum of 0.08 per cent. to a minimum of only 0.01 per cent. in some grass pastures while in others these values vary between 0.10 and 0.25 per cent. in the course of the year. (7) The percentage potassium remains comparatively high throughout the year in most bush pastures. Values seldom are below 1.00 per cent., while most figures fluctuate between 1.40 and 2.00 per cent. Sodium and chlorine averages are exceptionally high in some of the pastures composed mainly of bushes. Thus, for the greater part of the year figures for sodium are 1.00-1.86 per cent., and chlorine values exceed 0.70 per cent. (8) Crude fibre averages are lowest in pasture at the younger stages of growth, the mean annual variation is 34.6-37.2 per cent. for grass pastures and 32.0-35.0 per cent. for bush pastures. (9) Calcium-phosphorus ratios ranging from 2.00:1 to 11.60:1 occur in the dry matter of South African pastures. (10) Judged by the estimated requirements of cattle and sheep for growth all South African natural pastures, composed mainly or wholly of grasses are deficient in phosphorus, crude protein and, in certain areas, sodium for a period ranging from five to nine months of the year, depending on the area. There are indications that in certain of these regions the pasture may be deficient in phosphorus throughout the year. Furthermore, on the basis of the average values for phosphorus these grass pastures contain at no time of the year sufficient of this nutrient to provide in the requirement for an additional function (e.g., gestation or lactation) of the animal, superimposed on growth. (11) Provided sufficient food is available an intake of phosphorus, crude protein or sodium below the optimum requirements for growth will seldom occur on pastures composed mainly of bushes. (12) The requirements of calcium, magnesium and potassium for growth and moderate milk production will at all times be met by the amounts of these constituents present in South African pastures. Especially is the danger of a potassium and magnesium deficiency extremely remote. (13) The pastures of the Union are at no time deficient in chlorine. A cow producing two gallons of milk may, however, not ingest sufficient of this constituent on the winter pastures of some of the grassland regions.