Chapter 1: EXTRACTION RATES: CEREAL AND GRAIN LEGUMES

The first step in estimating the availability of crop residues for animal feed is to determine the crops grown and the proportions of the land area that are devoted to the predominant crops. Information on cropping patterns on a national and/or regional basis can be obtained from FAO, Ministries of Agriculture, National and International Crop Research Institutes and the scientific literature.

Selected surveys at the farm level for major cropping patterns are required to assess the current use of the crop residues either as animal feed or for alternative purposes (eg: fuel and construction) and the potential quality of the residue when it becomes available for animals. For example, maize is widely cultivated in the tropics but is often inter-cropped, and allowed to dry completely before harvesting, so the residue (stover) is of low feeding value when it becomes available for grazing.

To estimate the quantity of residue produced it is possible to use the harvest index of a crop (ratio of grain: vegetative matter)(dry matter basis) so that grain yield becomes an indirect measure of the vegetative DM yield per crop. The harvest index figures may be obtained from crop research institutes dealing with the specified crops. When such data cannot be obtained, or a more direct measure is desired, grain and vegetative DM yields may be determined in situ. When direct measurements are used, care must be taken in crop residue sampling so as to give the proportions and moisture content of plant part (leaves, stems etc.).

Relationships between grain and vegetative DM were determined under experimental and farmer conditions in central Nigeria (Table 1.1). Although the measurements were taken from various single crop and intercrop enterprises, the harvest index of the crops remained constant for the different cropping patterns (ICRISAT 1981, 1982).

However, factors such as low rainfall, planting date and densities can affect grain and DM relationships. It is therefore necessary to be cautious when using grain:vegetative DM ratios derived during years of normal rainfall distributions and timely plantings.

(i) Number of 50 m2 plots from which all grain and residue washarvested; 11 sorghum and 13 millet plots were 100 m2 infarmers' fields

Table 1.2

Mean values () and standard deviation (s) for percentage composition(i) of sorghum, millet, maize and groundnut residue in Central Nigeria (Powell 1985)

Component(i)	Sorghum		Millet		Maize		Groundnut
		s		s		s		s
Panicles	1	.5	2	.5	-	-	-	-
Upper leaves	9	3	9	3	14	2	-	-
Lower leaves	13	1	12	3	20	3	-	-
Upper stalk	24	5	29	3	18	2	-	-
Lower stalk	53	7	48	3	48	3	-	-
All leaves							38	3
Entire stem							35	5
Roots							27	4

(i) 8 to 9 plots (50m2) from which the residue was fractionatedinto morphological components. For groundnut, 10 plants wereselected at random from each plot and fractionated

(ii) As percentage of total stover dry matter

Because plant parts vary in their relative nutrient content and digestibility, residue components may be divided into upper and lower plant parts (Table 1.2). Such a separation may be desirable for predicting the relative contribution of each plant part in the total crop residue diet.