Seed dormancy is common in some species and ‘primary’ dormancy (sometimes called post-harvest dormancy) is usually associated with either the physical structures that surround the seed (lemma, palea and glumes) preventing moisture reaching the seed, or chemical inhibition that declines with time, or mechanisms within the embryo.
As a result, seed samples can have a high proportion of viable seeds, but have low germination. Primary dormancy is a natural mechanism that enables seed to survive for extended periods in the soil and germinate at different times.
Katambora Rhodes grass, Bambatsi panic, Swann forest bluegrass, Premier digit grass and kikuyu generally do not display any seed dormancy. Purple pigeon grass, green panic, setaria and signal grass are species that have primary dormancy.
For example, one-year-old purple pigeon grass seed may have germination as low as 15%, but two-year-old seed can have a germination of 70%.
Similarly, setaria may require 4–5 months storage to reach acceptable levels of germination, while green and Gatton panics can require 8–10 months and signal grass 10–12 months of storage to achieve maximum germination.
‘Secondary’ dormancy can also occur and may be caused by several uncontrolled factors, such as high temperatures during seed harvesting or storage. Since the mechanisms that induce this type of dormancy are unknown, so too are the processes required to break secondary dormancy.
Some certificates of analysis may give results for a tetrazolium test. This test determines if a seed has a viable embryo and is an important test for seeds with high levels of dormancy. It also includes seeds that are dormant, immature and damaged and so should not be included in the germination percentage.
If the certificate of seed analysis for species with primary dormancy indicates a high proportion of fresh seed then determine whether the time from harvest to sowing is sufficient for the seed to reach an acceptable level of germination.
If there is insufficient time between harvest and sowing, then either source alternative seed with a higher percentage of normal seedlings to sow this season, or consider buying the seed and storing it in dry, well-ventilated conditions until the following season.
This is particularly important for producers in Western Australia as some of the most commonly sown grasses (e.g. panic grass) have primary seed dormancy. Since the majority of panic grass seed is harvested in northern Queensland in January–April there is insufficient time for primary dormancy to break down and the germination to reach acceptable levels for sowing in Western Australia in August–September of the same year.
Examples of two seed lots with markedly different quality:
The following examples for Premier digit seed from commercial seed samples demonstrate the effect of seed quality on the potential sowing rate of seeds that may germinate and the resulting seed cost.
To sow the seed in Example 2 at a sowing rate of 2 kg/ha, this seed would have the equivalent of 0.066 x 2 = 0.13 kg/ha of PLS. To sow at the same rate as the average PLS you would need to sow over six times (2 x 0.40/0.13) as much seed, which would cost about $250/ha. For more information refer to FFI CRC Tropical Perennial Grass Guide Number 4 Tropical Perennial Grasses – Sowing rates.
Most tropical perennial grass seed is harvested and sold ‘in the floret’. However, tropical grass seedheads tend to ripen unevenly and at harvest some of the florets may be empty or contain immature seed. A high proportion of empty florets can be indicated by a high value on the certificate of seed analysis for ‘inert material’.
Similarly, a high proportion of seeds other than the cultivar that you are buying (including weed seeds) will be indicated by a high value for ‘other seeds’. This also applies to coated seed, as the tested seed sample may include all of the harvested material (florets with seed, empty florets, straw and other seeds can all be part of coated seed).
All seed is sold and sown by total weight. Seed coating improves the flow of light, fluffy seeds when sowing, and is good for checking sowing depth, but does not affect its purity and germination. Sowing rates need to be increased for coated seed. For example, a 2:1 seed coating (2 kg of seed coating for every 1 kg of ‘seed’ material by weight) triples the cost per hectare of sowing seed.
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