HCD Newsletter for April 2007

(Plant Stands, Fertilization, Gophers and Prairie Dogs, Weeds)


We’ll, here it is – the beginning of another irrigation season for the Hammond Conservancy District. Seems like yesterday when I was harvesting the last few chiles and tomatoes from my garden and putting my grass pasture to bed with a final irrigation.

Thanks to a decent amount of precipitation (4.2 inches) since the end of the 2006 irrigation season (October 15), a good balance should be available in the soil moisture bank of fields that are now bare, contain stubble from last year’s planting, or are planted to warm-season perennials such as some trees, buffalograss, blue grama grass, etc.   In alfalfa fields, cool season pastures, or in fields where weeds such as mustards, cheatgrass, or filaree have been allowed to grow however, much of the stored soil moisture has already been transpired through the actively growing crop. A deep (3 to 4 inch deep) irrigation now (April 20), should bring the soil profile moisture back up to near field capacity in these fields.

In fields that are to be planted in May or June, future weed problems can be suppressed by using light irrigations and rototillings or herbicide sprayings. Light irrigations are applied to germinate weed seeds, which, after emergence, are killed by disking, rototilling, or spraying with glyphosate (i.e. Roundup) or a similar product prior to planting. Avoid excessive rototillings however, to prevent deterioration of soil structure.  

Several impediments to crop production were noted during irrigation audits conducted on the HCD during 2006. Four problems that particularly stood out were: 1) poor plant stands, 2) poor soil fertility, 3) gopher damage, and 4) weeds. This newsletter is designed to provide information that can be helpful in improving these problems.

Plant Stands

Many alfalfa stands appeared to be quite old and (just like some of us guys who’ve lost some hair with age) were becoming pretty thin. The usual recommendation is to replant (or interseed) your alfalfa stand when there are less than 5 plants per square foot. Because older alfalfa plants appear to have an allelopathic (inhibitory) effect on the germination of alfalfa seeds, however, it’s best to interseed with grass. Also, because of this allelopathic effect, a new seeding of alfalfa should not immediately follow the plow down of an old alfalfa stand. Rather, one or two years of a grass crop should be grown between plow down of an old alfalfa stand and planting of new alfalfa. During the grass crop, use a broadleaf herbicide (i.e. 2-4-D) to control broadleaf weeds, including alfalfa. For more information see: http://www.sanjuanweeds.com/FactSheets/AlfalfaProdFS.pdf

Soil Fertility

Soil sample analyses from alfalfa and mixed alfalfa/grass fields indicated deficits in phosphorus (P) or nitrogen (N) or both. Generally, pure, established alfalfa stands do not require N fertilization since atmospheric nitrogen is fixed (converted to a form that can be used by plants) by microorganisms that form root nodules in alfalfa roots. Newly-seeded alfalfa should be fertilized with a low rate of N (i.e. 25 to 50 lbs N/acre), however, since it takes some time for root nodulation and N fixation to begin.

Alfalfa is a heavy user of P in the form of phosphate (P2O5) and all analyzed soils tested low to very low for P.  Phosphorus fertilizer recommendations ranged from 150 to 200 lbs of P2O5 per acre.

The two most common P fertilizers available locally are mono-ammonium phosphate (11-52-0) and treble super phosphate (0-45-0). Product application rates to satisfy a 150 lb/acre P2O5 rate would be about 290 and 330 lbs/acre with these two products, since they are 52% and 45% P2O5, respectively. In an alfalfa/grass mix, or in newly-seeded alfalfa, it would be more beneficial to use 11-52-0 to provide some N for the grass (or to new alfalfa prior to nodulation). Because P is somewhat immobile (does not readily move down into the soil with irrigation), the effects of P fertilization may not be seen for some time. Additionally, because of Ps immobility, it should be plowed or disked into the soil (and into the root zone) prior to planting new crops.   

In a solid stand of grass, or in an alfalfa/grass mix where more than 50% of the stand is grass, N fertilization is also recommended. The two most commonly available N products locally appear to be urea (46-0-0) and ammonium sulfate (21-0-0). Because of N’s solubility and leaching potential, it’s wise to apply N in small increments over the entire growing season, particularly if heavy irrigations are applied.  For example, in a solid grass stand requiring 120 lbs of N per season, fertilization may be more effective if the N is applied at a rate of 40 lbs/acre (87 lbs 46-0-0 or 190 lbs 21-0-0) in April, June, and August than to apply the full 120 lbs at the beginning of the season.

Fertilizer recommendations for alfalfa, grass pastures, corn and other crops are available from the Cooperative Extension Office in Aztec (334-9496) or can be found on the following websites:





Gopher and Prairie Dog Control

Gophers and prairie dogs can wreak havoc in all crops grown on the HCD. Gophers reduce crop production by feeding directly on the roots of crops, particularly alfalfa. In addition, gopher mounds can cover above-ground plant growth, interfere with harvesting, and present physical hazards to both man and livestock. Prairie dogs commonly build their large burrows close to agricultural fields so they can conveniently feed on newly-emerged crops. At NMSU’s Agricultural Science Center (ASC), they seem to be particularly fond of newly emerged soybeans, pinto beans, and corn.

Controlling these rodents is not easy. Trapping and poisoning can be effective but innocent or beneficial critters (such as ferrets or burrowing owls) can also be killed. Shooting can be used to help control prairie dogs but this method requires much time and patience. There is some evidence that the use of anhydrous ammonia (a gaseous nitrogen fertilizer) can suffocate these below-ground varmints but, as with all the techniques discussed so far, it can be quite dangerous. A new product that creates a concussion-causing explosion through the burrows of these rodents seemed to be effective when used on the ASC. More information on this propane-powered device can be found at:

http://www.gophernator.com/ or


Other, non-lethal methods are available and can be effective, although some may be expensive. These include large vacuums, live traps, and screening. It seems prairie dogs like to able to see long distances and will usually not enter into an area that is screened by fences, plastic mesh, bushes, trees, etc.  At the ASC for example, a screen, built around small plots using a plastic mesh erosion-control fabric seemed to deter the animals from entering the area.

For a more complete discussion of non-lethal, prairie dog control methods, see:


It’s questionable whether repellents (such as coyote urine or sonic devices) are effective. For more information see:



One way to reduce gopher populations is to plant grasses for a few years rather than alfalfa or plant grasses around new alfalfa plantings. The grasses do not provide sufficient root mass to feed a large gopher population so they move away from the grass area. During a pasture grass study at the ASC, it appeared gophers avoided plots of tall fescue.

Numerous web-based publications from various universities are available that discuss prairie dog and gopher control:

http://cahe.nmsu.edu/pubs/_l/L-201.pdf (Prairie Dogs in NM)


(Prairie Dogs - Colorado)


(Gopher Control – Montana)


(Gopher Control – California)


(Gopher Control – Colorado)


(Gopher Control – Nebraska)


(Gopher Control – New Mexico)


Weeds have always been a major scourge to the farmer and as a consequence, many effective herbicides have been developed to control these yield and quality-reducing pests. Improper handling and use of these products however, can create worse problems than those they’re designed to solve.

It must be understood that herbicides are classified into several different categories and that selection and timing are of utmost importance.

Pre-plant herbicides, for example, are applied prior to planting a new crop.  Glyphosate (i.e. Roundup) can be used as a pre-plant herbicide to kill emerged weeds prior to seeding. Other products that inhibit seed germination of selected weeds but do not harm the crop seed (i.e. benefin, EPTC, trifluralin in alfalfa for example) must be incorporated into the soil before seeding. It’s essential that the correct product and rate be used for the crop being planted to prevent damage to the desired seed.  

Pre-emergent herbicides are also designed to prevent weed seed germination and are usually applied to dormant perennial crops such as alfalfa or pasture grass in late fall, winter, or very early spring to prevent emergence of annuals such as mustards, cheatgrass, sand bur, pigweed, Russian thistle, etc. Some examples for alfalfa are diuron (Karmex) and hexazinon (Velpar).  

Post-emergent herbicides are used to kill weeds that are already emerged and these may be designed to kill either broadleaf weeds (2-4-D), grassy weeds (clethodim or Select) or both (glyphosate or Roundup). Certainly, you would not want to spray a broadleaf weed killer on your emerged alfalfa or a grassy weed killer on your turf or pasture.

From this brief discussion, you can see that weed control in a mixed alfalfa/grass pasture can be quite difficult as the herbicides available for use are quite limited. Specific information on weeds and weed control in alfalfa can be found through the links below:

Weed Control in Alfalfa – San Juan County Cooperative Extension Service


Worst Weeds of the West:


Weed Program – New Mexico State University


Invasive Weeds of San Juan County, NM


Other San Juan Coop. Ext. Svc. Publications

Establishing a Pasture, Varieties and Fertilizer, etc.


Alfalfa Weed Control in Oklahoma


Roundup Ready Alfalfa


The most important thing to remember about using chemical methods for controlling weeds is to:





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