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Fertilizing alfalfa seed stands improves yields

Balanced fertility not only improves yield, but can also improve stand competitiveness and longevity.


March 5, 2008
By Donna Fleury

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In western Canada, approximately 173,000 acres of alfalfa are grown for seed production, at a value of more than $60 million annually, including alfalfa leafcutter bees. In some areas, seed yields are often lower than expected and in some stands, productivity can be substantially reduced after three years. Growers and researchers are looking for ways to improve yields and stand longevity in alfalfa seed stands. Increasing stand longevity reduces the costs of production associated with stand termination and reseeding, and reduces concerns of soil and water erosion.

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Alfalfa seed experimental site comparing balanced fertilization. Photo Courtesy Of AAFC.

Much of the available research is focused on alfalfa hay production, with little information specific to seed production. Therefore, the Saskatchewan Alfalfa Seed Producers Development Commission (SASPDC) initiated a project in partnership with Dr. Sukhdev S. Malhi, research scientist at Agriculture and Agri-Food Canada (AAFC) Research Station in Melfort. “We wanted to determine the effect of a balanced soil fertility regime on alfalfa seed yield, stand competitiveness and stand longevity,” explains Wayne Goerzen,  executive director of SASPDC. “The study focussed on dryland alfalfa seed production, which covers a large area of Manitoba, Saskatchewan, and the Peace region of Alberta and British Columbia.”

Optimizing yields through balanced fertility is important for alfalfa seed crops, and as with many crops, if one nutrient is deficient in the soil, crop growth will be poor even if other
nutrients are abundant. “For production of alfalfa and other legumes, nitrogen (N) is not required because they fix their own N,” explains Malhi. Consequently, no N fertilizer is required for legumes provided they are properly inoculated with appropriate rhizobium bacteria.

Previous research shows the focus for alfalfa should be on correcting deficiencies in phosphorus (P),  potassium (K), sulphur (S) and in a few cases, some micronutrients such as boron (B). Phosphorus is important for root development. Sulphur is an essential nutrient for N-fixing bacteria and affects both yield and quality of seed in legumes. Potassium also stimulates N-fixation and is important for decreasing the incidence of winter injury. In most areas deficiencies of micronutrients are rare; however, boron deficiencies are suspected on sandy soils low in organic matter.

In 2000, field experiments were initiated on established alfalfa stands in north-eastern Saskatchewan, and continued to 2007. “We compared various fertilizer treatments, including no fertilizer to various combinations of P and K fertilizers, and S or B at different sites, depending on the nutrient deficiency in the soil,” explains Malhi. The fertilizers were surface broadcast in early spring, in mid to late April every year from 2000 to 2007 on the same plots. All of the plots were harvested annually for seed yield in late October or November. For some plots, alfalfa dry matter yield was harvested every year, including two cuts – one in June to early July and a second cut in late August or early September. The research also included field survey trials and soil sampling of ‘good’ and ‘bad’ areas in alfalfa seed fields to identify reasons for poor alfalfa seed yields.

“The growing conditions varied substantially from year to year, giving us a wide range of results,” explains Malhi. The research results to-date are still preliminary, with the final report expected in 2008. “The results from 2000, 2001 and 2003 indicate there was an increase in seed yield from fertilization in some fields when soil had low levels of available nutrients. In 2002, 2004 and 2005 seed yields were low due to drought in the growing season and/or early fall frost conditions, and there was no yield response to applied fertilizer nutrients.

In some cases, yields were so low the plots were not even harvested. “However, in 2006 we had one of the best years ever, with some of the highest yields we’ve ever seen,” says Malhi. “We had high yields on fields without fertilizer treatment, with yields as high as 400kg/ha, as well as improved seed yield responses on fields with fertilizer treatments.”

Table 1. Alfalfa seed yield
with and without fertilizers
at the Porcupine Plain study
site in 2006, a year with optimum growing and pollination conditions.
 TreatmentYield (kg/ha) Seed
 No fertilizer391
PK416
KS517
PS494
PKS448
PKSB469

Use a balanced fertility program based on soil tests
From these observations, Malhi encourages growers to use balanced fertility as a way to optimize alfalfa seed yield and quality. Soil sampling is important, with samples taken to a soil depth of 60cm for N and S, and 15cm for P and K to find if any nutrients are lacking in the soil. “If you are consistently getting good yields and good crops with your fertilizer management, then continue to apply fertilizer,” says Malhi. “However, even after conducting soil tests, it is still difficult to predict accurately if a profitable seed yield response to fertilization will occur, particularly when the soils are testing marginal in some nutrient levels.

“Therefore, one important strategy for optimizing the returns from fertilizer application is to consider doing some on-farm experiments,” says Malhi. Leave a strip without fertilizer in the field to compare treated and untreated strips and determine the resulting seed yield and quality. The strips should be harvested separately to get a good comparison of yields. Malhi adds, if growers are getting an increase in alfalfa seed yield and soils are deficient, they may want to consider fertilization of the whole field every year.

“Broadcasting fertilizer on alfalfa stands should be done very early in the spring when the soil is moist and the fertilizer has time to move down to the roots,” explains Malhi. Some nutrients such as P are less mobile and will take longer to move down to the roots than other nutrients such as sulphur.

The alfalfa seed yield response to fertilizer is impacted by several factors including soil type, nutrient availability in the soil, precipitation and other weather conditions, such as temperature, frost or drought. “Stand age and plant density will have an impact on the response to fertilizer applications, as will the type and formulation of fertilizer applied, the timing and the method of application.”

The benefits of balanced fertility go beyond improved seed yield, to increasing the longevity of the stand. A good competitive crop will suppress weeds, increase crop tolerance to early spring frosts and reduce the risk of winterkill. These crops will also have better resistance to disease and insect pest pressures. “The longer growers can keep a stand productive, the less often they will have to reseed,” adds Malhi. “Alfalfa seed fields are typically terminated by tillage and herbicides, which increases the cost of production and can result in substantial N losses and soil erosion.” Good balanced fertility can improve the economic returns for growers by increasing seed yield and stand longevity.

Fertilizing forage grass seed stands requires different strategies
Balanced fertility is also important for forage grass seed stands. However, grass stands require different fertilizer management strategies than legume crops. “Unlike legumes, grass seed stands respond well to N fertilizers,” says Malhi. Nitrogen management produces the greatest grass seed yield increases; however, yields will not increase if P, K and S are limiting or out of balance.

“With many grass seed species, early fall fertilizer applications have proven to be much better than spring applications,” explains Malhi. “If the ground is frozen, the N fertilizers will stay on the surface and can be subject to run-off or losses from volatilization, and fall applied N may be not as effective.” Unlike annual crops, fertilizers for forage grasses should be applied earlier in the fall to allow the nutrients time to move down into the roots and to prevent losses. The fertilizer moves down to the plant roots, where the nutrients are stored until needed early in the spring. Grasses set their seed or reproductive tillers very early in the spring, so a spring fertilizer application tends to stimulate vegetative growth over culm formation.

“Growers have few options for broadcast N sources, with urea or urea ammonium nitrate solution the best option. “Urea is vulnerable to N loss through ammonia volatilization when surface applied, therefore application timing is important.” However, grasses efficiently absorb water from the soil, minimizing the risk of N leaching or denitrification. All N sources can be equally effective when applied either just prior to rainfall or below the soil surface. -end-


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