Research and Training Strategies for Goat Production Systems in ...

Vegetation structure
Herbage intake of grazing animals is affected by nutritional factors like digestibility, but also
by non-nutritional factors of the sward which could influence grazing behaviour such as the
amount and distribution of herbage, sward structure, leaf:stem ratios, etc. (Hodgson, 1981).
Hence the grazing height and leaf:stem ratios were determined in order to quantitatively
describe the structure of the vegetation available to livestock.
Grazing height
This was determined by measuring the height of the grazing sward (average height of leaf
material) during botanical composition surveys.
Leaf:stem ratios
Samples of grass species were collected, dried at 60°C and the leaf and stem fractions
separated by hand. The two fractions were then weighed and expressed as a percentage of the
total dry mass.
Standing biomass
The amount of DM available to grazing animals is of critical importance for animal production
(Engels, 1983). The pasture disc meter developed by Bransby and Tainton (1977) was used
and a regression equation developed by Trollope (1983) to convert disc readings to kg ha -1 .
One hundred measurements per site in different seasons were done to determine the standing
biomass.
Range productivity
Standing biomass (as measured above) expresses the amount of forage at a given time in a
rangeland and is not a measure of the production of grazable material through a growing
season. Net primary production (herbaceous yield) was measured continuously through the
growing season as described by McNaughton, Milchunas and Frank (1996). This method
requires that moveable enclosures be employed with a movement frequency timed to reflect
both the intensity of herbivory and plant growth rate, measuring plant biomass and moving
enclosures accordingly. The biomass in caged and uncaged, grazed plots are estimated and
net primary productivity is then calculated as follows:
Consumption by grazers = (Caged biomass - uncaged biomass) at the end of each period.
The estimates of consumption of all the periods are added to the final harvest of residual
vegetation in the grazed area and corrected for initial standing biomass. This method does
not measure total productivity since senescence and decay are not accounted for (which
will be more important in mature swards). It does, however, measure the net production of
grazable material in a season.
Measurements were conducted throughout the growing season, starting approximately
September and ending in May (after first frost). Twenty movable enclosure plots of 1 m 2 each
were placed in each research site (10 per sample site) together with permanent enclosures of 4
m 2 which were harvested only at the end of the growing season to account for site differences
in potential production.
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