Seeding Rate Effects in Alfalfa
Dwain W. Meyer
North Dakota State University

Roundup Ready alfalfa was released in June 2005. Roundup Ready alfalfa's primary advantage will be vastly improved annual and perennial weed control in both seedling and established stands and more consistent stand establishment, especially in drier areas. A significant factor in moving to the Roundup Ready technology is the increased cost of seed, which runs about $6.50/lb. With the excellent weed control supplied by Roundup, what kind of seeding rate is necessary to establish productive stands?

We established an experiment in 2001 at Fargo, ND, to evaluate the optimum seeding rate for establishing alfalfa without a companion crop. Nine seeding rate treatments were used including 1, 2, 4, 6, 8, 10, 12, 16, and 32 lb/acre. The variety used was Garst 645-II. All plots were broadcast seeded and rolled once with a corrugated roller (an old Brillion seeder) on May 2, 2001. Environmental conditions following seeding were very good. Weeds were controlled with a single application of Pursuit at 3 fl oz/acre with MSO. Weed control was excellent. Two harvests were taken in the seeding year with the first on July 24 at 20% bloom and the second August 24 at 10% bloom. Four harvests were taken each year thereafter with the first taken at very early bud, the second at late bud, the third at 30% bloom, and the fourth at 50% bloom, generally in early October. A similar experiment was seeded in 2002, except the first harvest in the seeding year was lost.

Seeding-year forage yields in 2001 ranged from 3.78 tons/acre at the 10 lb/acre seeding rate to 3.72 tons/acre at the 6 lb/acre seeding rate. All forage yields of seeding rates above 6 lb/acre fell within this range. Even the 4 lb/acre seeding rate yielded 3.59 tons/acre.

DAK34-17RR Roundup Ready alfalfa was seeded at 5, 7.5, 10, 15 and 20 lb/acre in 2006 using the same techniques as the 2001 experiment. The environmental conditions in 2006 have been very dry, less than an inch of rain from the seeding date until harvest. First-harvest forage yields were not significantly different among seeding rates and ranged from 0.59 tons/acre for the 5 lb/acre seeding rate to 0.74 tons/acre for the 15 lb/acre seeding rate. The additional $65/acre seed cost for the higher seeding rate has not been recovered by greater forage yield in any of the three experiments.

Forage yield was not affected by the seeding rate in the seeding and first year of production (Table 1). The 7.5, 10, 15, and 20 lb/acre seeding rates established 19, 22, 25, 30, and 38 plants/ft2 initially, respectively, but an above-ground stand count on 27 April 2007 indicated only 6.8 to 9.7 plants/ft2 for all seeding rates, a typical decline in plant density from the seeding to first year of production. Forage yield was above 7.0 tons/acre in the first year of production with no significant difference among seeding rates across harvest dates or for seasonal total. Forage quality is presently being determined.

Roundup Ready alfalfa seeded with an oat or barley companion crop and removed as forage at the late milk stage in 2006 did not establish an adequate stand due to the droughty conditions. This suggests that producers should use clear seedings in drier areas to improve the rate of successful stand establishment. If a companion crop is needed to reduce wind erosion on sandy soils or water erosion on hilly land, and it is a droughty year, spray the field with Roundup when the cereal is 6 to 10 inches in height to ensure stand establishment. In good rainfall years, the cereal can be harvested for forage and anticipate a good alfalfa stand establishment.

Forage yields following the seeding year averaged over the four years of the 2001 seeding and three years of the 2002 seeding are presented in Figure 1. Forage yields increased with increasing seeding rate up to the 6 lb/acre rate. Forage yields with higher seeding rates were very similar to the 6 lb/acre rate. Therefore, increasing seeding rate above the 8 lb/acre seeding rate had no effect on forage yield on 1- to 4-year old established stands under a high yield environment.

Some producers argue that they need to seed at 15 to 18 lb/acre to produce finer stems and increase the quality of the forage. We did not evaluate forage quality on the 2001 or 2002 seeding rate experiments; however, previously we had evaluated forage quality as influenced by plant density. Alfalfa stands were established at 1, 2, 3, 4, 5, 9, 34, and 45 seedlings/square foot. Forage quality in the first and third harvest years was unaffected by plant density! If we could not show differences in forage quality with stands as low as 1 plant/square foot, there is no way forage quality difference will be shown when 19 to 38 seedlings/square foot were established in the 2006 experiment.

In conclusion, there is little justification for seeding rates above 8 lb/acre for seeding without a companion crop. Put your time and effort into preparing a good seedbed rather than increasing the seeding rate to compensate for a poor seedbed. Use the Roundup Ready alfalfa to improve weed control, but use modest seeding rates to help hold down the establishment cost.