Seeding Rates for Crabgrass
Crabgrass (Digitaria spp.) is an annual warm season grass that volunteers in many "natural" grasslands and can be readily managed to volunteer in many cultivated forage approaches such as double cropping with winter annual crops of wheat, rye, etc. Seeding for new stands is often needed to start the pasture process or to thicken naturalized stands. We conducted a study to determine seedling density and forage production at various seeding rates and methods.
The study was done on a shallow Winthorst fine sandy loam soil near Ardmore. A clean cropland area was prepared to simulate an excellent seedbed in real-farm conditions. Seed was from a field of a natural composite ecotype of crabgrass (D. ciliaris) of a moderate forage growth type. Planting was done on June 5, at seeding rates ranging from 1 lb to 20 lb pure live seed (PLS)/ac.
Soil tests indicated a high availability and adequate amounts of phosphorous and potassium, adequate calcium, and a pH of 6.0-6.2. The crabgrass was top-dressed on June 28 with 68 lb actual nitrogen (N)/ac.
Weeds were controlled with 2,4-D at the 1 lb/ac active ingredient rate to insure a clean stand. Armyworms were controlled with Dylox insecticide (trichlorfon) using the label method for hand spraying. Armyworm problems are not usual, but they have occurred in local crabgrass pastures and plots perhaps three or four times in 25 years.
Light irrigation was done to simulate a "usual" summer and to keep the grass green and acceptably growing. Forage harvests were made with a rotary blade bagging mower. Residue left was 3 inches in height.
Seedlings/sq. ft ranged from five at the 1-lb-broadcast rate, to 43 at the 20-lb rate (Table 1). In general, the number of seedlings per pound of seed declined as rates increased. We surmise that some of that decline may have been from autoallelopathic (self-toxic) response.
The overall average was 3.9 seedlings/sq. ft/lb of seed. This may be a good guideline for judging stand density and planting rate needs. This correlates to about one established seedling per five live seeds planted. However, many more seedlings emerged later.
Crabgrass is like many forage grasses in that the whole stand does not emerge at first. Some seedlings emerge with early rains during proper warm temperatures and more comes out regularly with additional rains until the stand is full or time is gone. I have seen this go on for three to four months in new plantings. Volunteer stands, which are often thicker, tend to emerge and complete filling of the stand earlier. Rowed plantings at the same rate of seed produced more plants per pound of seed than broadcast plantings.
The 5- and 10-lb PLS broadcast seeding rates produced earliest forage at 1,045 and 1,506 lb/ac, which was about double the early forage compared to the 1- and 2-lb rates (Table 2). The rowed plantings likewise showed a similar advantage in early forage yields over the lighter broadcast rates. Yields of the second harvest did not vary greatly likely due to all stands being relatively equal by this time. Of the practical seeding rates, the 5- and 10-lb PLS broadcast rate and the 5-lb PLS rowed rate produced the best total yields. These seeding rates showed a 14% to 26% advantage over the lower yielding seeding rates.
Pounds of total forage per pound of N, without a check plot deduction, ranged from 43 to 56 lb of crabgrass forage/lb of N. The 68 lb/ac of actual N was not nearly enough to optimize or maximize first year production.
In summary, the most practical seeding rates of crabgrass considering seedling density, ground cover, grass height, and production tended to be the 2 lb and 5 lb PLS/ac broadcast and 2 lb PLS/ac rowed plantings. The 2 lb and 5 lb PLS/ac seeding rates produced a good total yield. By the end of summer, all seeding rates produced good stands as such. But, these plots were managed to be very pure without serious limiting factors and thus the lower seeding rates did better than would otherwise be expected.
The seeding rates of 1 lb to 5 lb PLS/ac tended to be the most economical. The choice largely depends on the goals the operator has for the first stand. Is it needed as soon as practical? Or, is later okay? All things considered in research and in practice, the rate of 3 lb PLS/ac seems to be a good general rate and a good compromise between the best agronomic response and costs for establishment in real farm circumstances.
A much more detailed report is available upon request.