Agriculture Reference
In-Depth Information
The R-test for Heterogeneity
The R-test is used to detect
off-range
heterogeneity within a seed lot by using test results such as purity,
germination, or noxious (other seed) weed seed contamination.
In order to calculate the R-value, it is necessary to know (as shown in the ISTA rules):
No =
number of containers in lot.
N =
number of independent container-samples.
∑
N
X
Mean of all X-values determined for the lot in respect to the
adopted attribute.
X=
The range found as maximum difference between independent
container-samples of the lot in respect to the adopted attribute.
R=X
max
-X
min
n
=
number of seeds tested from each container-samples (1000 for purity, 100 for germination,
and 10,000 for other seed counts, see 3.30.
X =
test results of the adopted attribute in a container-sample.
Σ
=
symbol for sum of all values.
Then the R-value will be:
This R-value is compared with those in Tables 13.20, 13.21, or 13.22 for non-chaffy seeds, depending on
which attribute was tested (i.e., purity, germination, or noxious weed seed). For chaffy seeds, review Tables
2E, Part 2, 2F, Part 2, and 2G, Part 2, in ISTA rules, 2010; these tables are not contained in this topic. For
example, assume a R-value on the basis of 15 independently drawn samples for a seed lot with an average
germination of 90% was calculated as 20%. Table 13.21 shows a tolerated R-range of 17 (assuming non-
chaffy seeds); thus this seed lot is considered to be excessively heterogeneous.
Attaining Homogeneity in Large Turf Seed Lots
The United States turf seed industry has become very proicient in attaining homogeneity within seed lots
comprised of only one species or a blend of two or more species. First, their customers demand homoge-
neity of product and second, this is attainable by use of large state-of-the-art rotating cylindrical blenders
equipped with langes as found in cement mixers. Finally, by using computer-generated feeder-blenders
from different sources, homogenous blends of different seed lots can be prepared with great accuracy and
precision. However, as of 2010, the recommended OECD seed lot size is still limited to 10,000 kg (22,000
lbs) because of ISTA restrictions.
As explained earlier, there are good reasons for the seed size restrictions on seed lots. Excessive in-
range heterogeneity often exists in large seed lots, especially from natural production units (e.g., ields).
Such lots are seldom if ever blended by state-of-the-art equipment, but are simply conditioned, bagged, and
marketed. Thus, the inherent heterogeneity commonly occurs among the containers (bags) in the seed lot.
