Field Notes April 19,2010; allelopathy, barnyardgrass, signalgrass, jointvetch

Parasitic wasp and aphids

  At left center is a tiny wasp.  I had a visitor riding with me last week and he saw the thing without magnification.  I had to use my hand lens.  As a size reference the two brown, somewhat spherical objects are aphids.  The wasp is one that parasitizes the aphids by laying its eggs in the aphid.  The brownish color of the aphids indicates they have already been parasitized. 

Jointvetch seedling

At left is another review photograph.  The seedling shown here has two cotyledons (seed leaves) and two true leaves. Both true leaves are compound thus it is Jointvetch. If the the first true leaf had been simple it would have been Hemp Sesbania. Remember simple leaf, Sesbania. 

Ligule of Broadleaf Signalgrass

Barnyardgrass has no ligule

Hairy sheath of Broadleaf Signalgrass

Barnyardgrass is smooth

Above is a set of four photographs.  The two on the left side of the page are of broadleaf Signalgrass while those on the right are of Barnyardgrass.  Both were about 3 leaf seedlings.  If you follow the leads in the Schematic Diagram for Seedling Weed Identification in Rice you will note the presence or absence of a ligule will separate these two weeds.  If Fall Panicum was included in the photographs it would have to be separated from Broadleaf Signalgrass on the presence or absence of pubescence.  The ligules of these last two can be very similar, but the hairs on the lower surface of the leaves of Signalgrass distinguish it from Fall Panicum.

Ryegrass in rice field

Ryegrass is allelopathic toward rice.  The word describes the ability of one plant to suppress the growth of another.  In this instance the allelopathic plant is ryegrass and the plant being suppressed is rice.  In no-till studies years ago Dr. Pat Bollich discovered a great deal of diversity between cover crops and their allelopathic effects.    Ryegrass is strongly allelopathic to rice.  As long as it is green and for a few weeks following its death rice growth will be suppressed. 

If I had been asked to do stand counts in the field shown in the three photographs below I would have said the stand was too thick.   In the second photo lanes created by burning the stubble from last year reveal two things: first, burning stubble does not produce enough heat to really control weeds; second the stand is very uneven. 

The real surprise is that this rice was not planted – at least not this year.  Last year a non-Clearfield hybrid was planted in this field.   Enough rice shattered from the second crop to produce this crop.  Clearly a lot of seed made it to the soil and the dormancy of hybrids helped the seed to survive. 

Volunteer rice seedlingsField view of volunteer riceStand of volunteer rice

Field view of volunteer rice

Volunteer rice seedlings

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