Preemergence Herbicides in Alfalfa

Bill McCloskey

University of Arizona

With the cooler temperatures of fall and optimum planting times for alfalfa approaching, winter weeds will soon be germinating in established alfalfa fields. It is human nature to think that winter weeds germinate later in the year because that is when they are large and visible along roads and in fields. In reality, they germinate earlier than we often realize. Now is a good time to be thinking about preemergence herbicide options for established alfalfa fields. Some winter weeds such as common groundsel (Senecio vulgaris) are toxic while others impart off-flavors to milk when fed to dairy cows.

Apply preemergence herbicides to alfalfa fields shortly after cutting and removal of the forage. This minimizes the amount of preemergence herbicide bound to leaf and stem surfaces and the loss of weed control this causes. Some preemergence herbicides such as Chateau, Velpar AlfaMax Gold, diuron, hexazinone and metribuzin have contact-burndown activity if mixed with an adjuvant such as a surfactant or a crop oil concentrate. To avoid alfalfa damage do not mix them with these types of adjuvants. Imazethapyr (Pursuit) has both preemergence and postemergence activity and is selective in alfalfa. Other preemergence herbicides such as norflurazon (Solicam), EPTC (Eptam), Prowl H2O, and trifluralin granules do not have significant foliar activity. Velpar AlfaMax Gold, diuron, hexazinone and metribuzin are mobile enough in soil that in warm conditions alfalfa can absorb enough herbicide into the roots with soil water to cause injury (see labels). Chateau, Prowl H2O and trifluralin granules have very limited mobility in soil, just enough to control weeds. Observe the pre-harvest intervals on herbicide labels to avoid illegal residues in the harvested forage. There will always be weeds that escape preemergence herbicide applications so you may need to spray a postemergence herbicide later in the fall. Be sure to consider potential crop rotations and plant-back intervals when using soil active herbicides.

Additional Reading:

Tickes, B. and M.J. Ottman. 2008. Alfalfa Weed Control in the Low Deserts of Arizona. Accessed September 26, 2017.

Alfalfa Stand Replacement

Michael J. Ottman
University of Arizona

The decision to replace alfalfa stands is based on stand establishment costs, hay price, and yield of the alfalfa stand in question and other factors.  As an example, replacing an alfalfa stand with yields 2 ton/acre less than a healthy stand will pay for itself in 3 years if the cost of alfalfa establishment is $900/acre, hay price is $150/acre, and harvesting costs are assigned on a per acre basis.  If harvesting costs are assigned on a per ton basis, then a 3 ton/acre rather than a 2 ton/acre stand decline is required in the scenario described above.  The economic benefit of replacing an alfalfa stand increases as the cost of establishment decreases, the hay price increases, and the yield reduction due to the poor stand increases.

Replace an alfalfa stand if plant density is less than 4-6 plants per square foot.  The plants or crowns  should be counted soon after cutting to be able to distinguish individual crowns.  Actually, the density of stems rather than plants is a better indicator of the adequacy of a stand because some plants may have few stems and not contribute much to yield.  The stems should be counted when about 6 to 10 inches tall.  An alfalfa stand should be replaced if the stem density is less than 40 stems per square foot where yield is 75% of maximum according to University of Wisconsin data.  Yield is maximized at 55 plants per square foot.  Stands may need replacement regardless of average plant or stem density if plants are not uniformly distributed or if many bare spots exist.  Weeds may become a problem in a thinning alfalfa stand and may be difficult to control.  Reduction in forage quality may occur in sparse stands due to thicker stems.

Alfalfa plants contain compounds that are toxic to germinating seedlings of alfalfa.  These compounds be in a class of chemicals called medicarpins.  The leaves and flowers contain higher concentrations of medicarpins than the stems and roots.  A newer alfalfa stand has less medicarpins than an older stand.  The injury to germinating alfalfa seedlings can range from slight stunting to death of the plant.  To avoid autotoxicity, delay seeding alfalfa into a field previously in alfalfa by at least 2 weeks after tillage or 3 weeks after herbicide-kill of the old stand.  Killing the alfalfa stand with an herbicide and then harvesting the crop may result in hay of poor quality.

Renovating an alfalfa stand involves some light form of tillage and seeding into the existing crop.  This practice is rarely successful.  The seeds may germinate and seedlings become established for a few weeks or months.  Eventually, however, the seedlings may succumb to autotoxicity or to competition from established plants.  Seedlings may become established in areas more than 8-16 inches away from other plants, outside of their zone of influence for autotoxic compounds and competition for light, nutrients, and water.  So, large areas at the tail end of the field that have been killed by scald (high temperature flooding injury) are often successfully renovated.

Additional reading

Hall, M.H., J.A. Jennings, and G.E. Shewmaker.  2004.  Alfalfa establishment guide.  Accessed September 21, 2017.

Mueller, S.C, C.A. Frate, and M.C. Mathews.  2008.  Alfalfa stand establishment.  Pp. 39-58 in Irrigated alfalfa management for Mediterranean and desert zones, C.G. Summers and D.H. Putnam eds.  UC Agriculture and Natural Resources, Oakland, CA.  Publication 3512.  Accessed September 21, 2017

Tesar, M.B., and V.L. Marble.  1988.  Alfalfa establishment.  Pp. 303-332 in:  Alfalfa and alfalfa improvement, A.A. Hanson, D.K. Barnes, and R.R. Hill, Jr., eds.  American Society of Agronomy, Madison, WI.  Publication 29.

Undersander, D., C. Grau, D. Cosgrove, J. Doll, and N. Martin.  2011.  Alfalfa stand assessment:  Is this stand good enough to keep?  Accessed September 21, 2017.


Phosphorus Fertilizer for Alfalfa

Michael J. Ottman & Ayman M. Mostafa

The University of Arizona, College of Agriculture & Life Sciences, Cooperative Extension

Phosphorus is the primary mineral nutrient likely to be required by an alfalfa crop in Arizona. A single application of phosphorus fertilizer at planting time may be sufficient for the life of the stand in certain situations. However, on soils with a high phosphorus fixing capacity, a single application of phosphorus fertilizer at planting time may not be sufficient. On these soils, phosphorus fertilizer may be necessary on an annual basis, or even several times each growing season. The availability of phosphorus fertilizer in certain soils decreases over time to such an extent the soil may be deficient in phosphorus a few months after fertilizer application, thus warranting multiple applications in a season.

Phosphorus deficiency in alfalfa is characterized by reduced plant height and size of the leaves (Fig. 1). The leaflets may tilt upwards.   The crop may also have a dark blue-green appearance. Alfalfa usually does not exhibit purpling of the leaves as occurs with phosphorus deficiency in many other crops. Phosphorus deficiency in alfalfa is difficult to detect since the reduction in growth that occurs is not apparent unless the crop can be compared to alfalfa that has adequate phosphorus.

P deficiency

Phosphorus can be supplied to alfalfa on soils with a high phosphorus fixing capacity by 1) applying high rates of fertilizer, 2) using multiple applications of fertilizer, 3) banding the fertilizer, 4) applying manure as a phosphorus source, 5) using sources of phosphorus fertilizer that are complexed with humic, fulvic, or other organic acids, and 6) lowering the soil pH. Applying high rates of phosphorus fertilizer may be inefficient since phosphorus fertilizer can be rapidly fixed by the soil, but there will be a brief time of high phosphorus availability after the application. Multiple applications of phosphorus fertilizer, such as every few months, may be needed each season, particularly in the spring, to maintain soil test levels in the adequate zone. Banding fertilizer is not a common practice in alfalfa, but applying a liquid formulation in a narrow band on top of the soil may be a way to reduce phosphorus fertilizer fixation by limiting the amount of soil the fertilizer comes in contact with. The phosphorus in manure is tied up in organic compounds and does not have as much chance to become unavailable in the soil as inorganic fertilizer. Several fertilizer products are on the market in which the phosphorus is complexed with various organic acids, thus making the phosphorus less likely to be fixed in the soil, similar to manure. Lowering the soil pH can make soil phosphorus more available, particularly on soils with a pH between 8.0 and 8.5. Table 1 contains major phosphorus fertilizers used in Arizona.

P formula

Currently, we do not have strong research data to show differences among fertilizers for supplying phosphorus to alfalfa. Therefore, our recommendation is to apply phosphorus fertilizer based on cost per pound of phosphorus or based on convenience of applying fertilizer in the irrigation water in the case of liquid fertilizers.

Additional Resources:

Ottman, M.J., J. Rovey, A.M. Mostafa & W. Burayu. 2015. Phosphorus Fertilizer Rate Effect on Alfalfa Yield and Soil Test P, Buckeye, 2014. University of Arizona Cooperative Extension AZ1672.

Ottman, M. 2012. Alfalfa Nutrient Requirements.

Ottman, M.J., T.L. Thompson, M.T. Rogers, & S.A. White. 2000. Alfalfa Response to Forms of Phosphorus Fertilizer. Forage and Grain Report: College of Agriculture and Life Sciences, University of Arizona.