1. Alfalfa Pest Updates Heath Sanders Area Agronomy Specialist
Oklahoma State University
March 21, 2018

2. Weed Infestations Limits yield potential Competition for resources
Water
Nutrients
Sunlight
Quality
Tonnage Issues

3. Proper Weed I.D. is Critical
Know and prioritize your weeds
Broadleaf or grass
What products can I use to control them?
Correct timing
Are they effective?
Know the cost

4. Common Weeds Cutleaf Evening Primrose Henbit Prostrate spurge
Chickweed
Pigweeds
Winter grasses
Yellow woodsorrel
Mustards
Bermudagrass

5. Why Weeds? Little weed competition 30 alfalfa stems/sq.ft.
Stands begin to thin
Little weed competition
30 alfalfa stems/sq.ft.
Reduced forage yield
20 alfalfa stems/sq.ft.
Weeds may take over
10 alfalfa stems/sq.ft.

6. Weed Control in Alfalfa
Established or Seedling
Weed competition
Type and severity
Quality
Tonnage
Protein
Market or Objective
Can you allow for a few weeds?

7. Alfalfa Yield Reduction
Weed competition in new stands of alfalfa often exceed 1,000lbs/ac
Severe infestations can cause stand failure
Thinning stands result in loss of ½ to 1lb/ac of alfalfa for every lb of weeds produced
30 stems of alfalfa /sq. ft. over entire field is defined as a full stand

8. Winter Annual Weed Competition
Density
Yield Loss
(#/ft2)
(% 1st Cutting)
Henbit
10-20
None
>20
Slight
Sheperdspurse
1-4
5-10
>4
10-50
Tansy mustard
1-2
10-30
3-6
30-90
>6
50-100
Winter grasses
5-20
10-60
20-100

9. Herbicides for Seedling Stands
Appl.
Herbicide
Method
Balan
PPI 8 5 9
Buctril 2 EC
POST 6
Butyrac 200
Eptam 7-E 7
Cyclone Max
Dorm.
Kerb 50W
Poast
Pursuit 2 AS
Raptor
Select 2EC
Annual Grasses
Henbit
Mustards
Chickweed

10. Herbicides for Established Stands
Appl.
Herbicide
Method
Butyrac 200
POST 6 8
Cyclone Max
Dorm/BC 9
Poast
Pursuit 7
Raptor
Roundup
Select 2EC
Sencor
Dorm.
Sinbar
Velpar
Annual Grasses
Henbit
Chickweed
Lambquat.
Pigweeds

11. Roundup Ready Alfalfa Good Potential for Weed Control Expensive System
No residual control
Expensive System
Seed Cost
Weed Resistance Concerns
Apply a herbicide with residual during dormancy
Then make in season application of Round-up

12. Roundup Ultra Max 40 fl oz/A 4 WAP
Roundup Ready Alfalfa
Roundup Ultra Max 40 fl oz/A 4 WAP
Untreated

13. Roundup Ultra Max 40 fl oz/A 4 WAP
Roundup Ready Alfalfa
Pursuit fl oz/A 7 WAP
Roundup Ultra Max 40 fl oz/A 4 WAP

14. Cowpea Aphids Relatively easy to scout Color Female
Smoky-gray nymphs
Black- Adults
Female
Reproduce Asexually
Various pressure levels
100/stem in some fields
Fall Seeded
1-2/Plant can thin a stand under drought conditions

15. Alfalfa Weevil
Weevil Populations remain low 3-5/30 stems with all being first instar
Coincides with low egg counts observed earlier in the year

16. Treatment Considerations
Based on number of aphids per stem
Fall Seeded up to 5 per stem, Influence by environment
Established Alfalfa, that has 10” of growth up to 50 per stem
Insecticide treatment
Lorsban Continues to be preferred choice
1 pt/ac show excellent activity
Pyrethroid tank mix for weevils may be an option for some growers.
Will we have to make a second application for weevils?? Not if numbers remain light

17. Thanks!