ISSUE 2   May 22, 2008

DONíT FORGET PHOSPHORUS FOR ALFALFA PRODUCTION

Phosphorus is the most limiting nutrient for alfalfa production in North Dakota. Alfalfa removes four to five times more nitrogen and potassium than phosphorus, but soils in North Dakota generally are high in potassium and legumes, like alfalfa, fix their own nitrogen. A ton of alfalfa removes about 12 lb/ton of phosphate (P2O5), so if your soil test indicates a low to very low phosphorus level (Table 1), consider applying phosphorus at the removal rate. Therefore, if you expect a 3 ton/acre alfalfa yield, apply 36 lb/acre of phosphate or if irrigating and you expect a 6 ton/acre yield, apply 72 lb/acre of phosphate.

Table 1. Nutrient recommendations for alfalfa

   

Soil Test phosphorus, ppm

   

VL

L

M

H

VW

Yield
Goal

Bray-l
Olsen

0-5
0-3

6-10
 

11-15
8-11

16-20
12-15

21+
16+

ton/a

 

---------------lb P2O5/acre----------------

2
4
5
6

 

35
65
85
100

25
50
60
70

15
30
40
45

10
10
15
15

0
0
0
0

Bray-I P recommendation = (18.57-0.93 STP)YG
Olsen P recommendation = (18.57-1.16 STP) YG

Deficiency of phosphorus is common in alfalfa fields. Last fall, Brian Zimprich, Ransom County Extension Agent, and I toured several alfalfa fields in Ransom county looking for potential fertility problems. We stopped at six fields that appeared to have differences in fall growth across the field. Soil samples of the poor growth area indicated a phosphorus deficiency in all six fields visited. In addition, three of the six appeared to be deficient in sulfur also. One additional field just west of the Sheyenne National Grassland on Hwy 27 tested deficient in phosphorus, sulfur, and potassium. A similar tour in western and southern Cass County with John Kringler found only one field with deficiency symptoms. Phosphorus deficiency was also found just north of Grand Forks in 2006.

A phosphorus fertility experiment on alfalfa was conducted at Buffalo, ND, in 2006 and 2007 on a soil testing 2 ppm (4 lb/acre), a very low-testing site. Forage yields increased with increasing phosphorus rate (Table 2). Forage yield showed the typical law of diminishing returns with a doubling for 40 lb/acre fertilization and 146% increase with 80 lb/acre during 2006 to 2007. Forage yields were greater and responded to greater fertilization in 2007 than 2006 due to greater rainfall in 2007 and a dry 2006.

Table 2. Forage yield of alfalfa with phosphorus fertility at Buffalo, ND in 2006 and 2007 (2ppm soil test)

Phosphorus treatment

2006

2007

Total

lbP/acre

-tons dry matter / acre

0
20
40
60
80

LDS 0.05

1.89
2.62
2.96
3.36
3.44

0.42

2.23
4.64
5.60
6.17
6.71

0.46

4.12
7.26
8.56
9.53
0.15

 

With todayís cost of phosphorus, the economics of phosphorus fertilization must be considered. Obviously, even the 1 ton/acre increase in 2006 at the Buffalo site with hay selling for $60/ton would have been economic with the 3.4 tons/acre increase in 2007 highly economic. But, what about soils testing higher in phosphorus? The experiment at Buffalo is located on a hillside with 5 to 7% slope. The forage yield of the fertilized plots appeared as good as unfertilized low areas of the field (although no yield test was performed). This suggests that a topographic or grid sampling with variable application of fertilizer would have been the most cost-efficient fertility program. If nothing else, apply about 50 to 60% of normal removal rate to the whole field and double the application on the highly deficient areas.

Some consideration should be given to adjusting the fertilization rate based on seasonal rainfall. For example, the greater forage yield in 2007 due to higher rainfall justified a greater fertilization rate than that of 2006. Therefore, consider a second application in above-normal rainfall years since the increased productivity would more than justify the additional fertility and application costs.

The cheapest phosphorus fertilizer generally is MAP or DAP, both of which have nitrogen in addition to the phosphorus. Application of nitrogen to alfalfa does not increase or decrease alfalfa yield so based the cost of the fertilizer on the phosphorus component only. If the "alfalfa" field has a significant component of grass, the nitrogen may increase yield slightly.

Dwain W. Meyer
Extension Forage Specialist
Dwain.Meyer@ndsu.edu  

 

SOYBEAN REPLANTING ISSUES

If a soybean crop has poor emergence, which can be caused by various reasons, producers often are faced with the decision of whether or not to replant the field. There are a number of factors to consider when replanting is being contemplated. Replanting will be expensive as there are costs for preparing the seed bed, new seed, planting the seed, potential extra herbicides and donít forget the extra labor. Replanted soybeans normally have a lower yield potential as the length of the growing season is reduced. A two to three week delay in re-seeding usually leads to a substantial yield reduction. There is also additional risk in replanting because there is no guarantee that the seeding and emergence conditions will be better than during the first planting. However the soil temperatures may be higher when re-seeding, which may reduce the time till the plants emerge.

The soybean plant has an amazing capability to compensate for low plant populations. Most of the soybeans grown in this region are indeterminate, which means they keep on growing and flowering till late in the season. Beans can fill in gaps by branching out and producing more pods per plant. The number of seeds and weight of seed can adjust to good growing conditions. Low populations and even irregular or uneven stands can still provide acceptable yields.

The most important aspect of evaluating if a field needs to be replanted is the plant distribution throughout the field. If soybeans are planted in 30 inch rows and there are large gaps between the plants within the row, there may not be enough plants nearby to compensate for the reduced stand. With narrow row spacing it is more likely that plants are more evenly distributed in the field. Even with some gaps within the row, the remaining plants may be able to fill in and close the canopy.

If the conditions have been dry and the stands are irregular, the cause may be limited water availability for the seed to germinate. You need to evaluate the field to see if there is un-germinated seed that may still germinate if conditions improve.

In conventional soybeans, weed control may be an issue to consider when making re-planting decisions. If the gaps are large between the remaining plants, you can expect weeds to take over these areas. With Roundup ready soybeans, an extra herbicide application may be needed to control the weeds.

Some producers replant certain areas within the field where there are inadequate stands. If the soybean is seeded in rows, you could consider planting another row in- between the poorly germinated rows. The planter will need to be shifted a few inches from its original site, otherwise the wheels of the tractor will drive over existing rows. Any potential mechanical weed control would not be possible using this system. The soybeans which were planted first will have a head start and there will be a difference in maturity of the two planting dates. Throughout the season the original planted soybeans will shade the later seeded plants, resulting in weaker plants and potential lodging. The yield advantage of this system is generally smaller than growers anticipate.

In general terms, the additional costs of re-planting are often higher than the yield loss of a soybean field with irregular stands.

Table. Percent yield loss of soybean affected by stand reduction (all stand figures in 1,000 plants/acre)

Original

Remaining stand

Expected

120

110

100

90

80

70

50

Stand

% yield reduction

125

1

3

6

10

14

18

30

110

 

0

3

7

11

15

27

Source: Adapted from page 99 Soybean Production Field Guide for ND and NW MN, 2002.

Hans Kandel
Extension Agronomist
hans.kandel@ndsu.edu

 

EARLY CROP DEVELOPMENT IS SLOWED BY COOL WEATHER IN 2008

This spring, conditions have generally been favorable for planting crops except where extremely dry conditions currently prevail. Drought is becoming a concern in a growing area of the state . Additionally, cool temperatures are slowing development of emerged crops and/or delaying emergence. So how do temperatures this spring compare with previous seasons?

Assuming a planting date of April 15th, wheat growing degree days accumulations (GDDs) this season are 210 to 340 GDDs behind 2007 and 25 to 160 GDDs behind the long-term average, depending on the location in the state (see following table). Typically, we can expect to accumulate 20 to 25 wheat GDDs during this time of the year, so in terms of calendar days the small grain crops are running about seven to ten days behind last year and one to six days behind the long term average. Though the cooler weather we are experiencing this season slows development (probably at least one leaf stage behind the long-term average), it is also means that we are losing less water to evapo-transpiration which is good news given the dry conditions we are experiencing. Furthermore, yield potential development in small grains is favored by cool temperatures during vegetative growth, particularly if other factors are not limiting.

Corn GDD accumulations this season (assuming a 1 May planting date) also significantly lag behind those for the same period in 2007 and for the long-term average. Corn GDDs are calculated using a 50 degree base (compared to 32 degrees for small grains), so they accumulate more slowly than wheat GDDs. Since corn GDDs typically accumulate during mid-May at the rate of about 9 to 10 per day, corn development is running about a week behind last year in terms of calendar days but only two or three days behind the long-term average. It usually takes about 125 corn GDDs before corn emerges, so we should start to see some of the early planted corn beginning to emerge soon. Corn emergence is favored by relatively warmer temperatures and the colder temperature we have experience is not only delaying emergence, but it will likely decrease the uniformity in the timing of corn emergence. Uniformity of emergence in corn is a basic component of a high yielding crop.

Wheat and corn growing degree days (GDD) for 2008 compared to 2007 and
normals (30 year average) for selected locations in ND (data obtained from NDAWN).

Location

Wheat GDDs 2008*

Wheat GDDs 2007

Normal Wheat GDDs

Corn GDDs 2008**

Corn GDDs 2007

Normal Corn GDDs

Carrington

467

791

627

114

185

165

Dickinson

536

746

561

120

177

138

Fargo

525

865

651

130

209

152

Minot

523

789

573

115

187

135

* Accumulated wheat GDDs were calculated assuming a 15 April planting date.
** Accumulated corn GDDS were calculated assuming a 1 May planting date.

Joel Ransom
Extension Agronomist for Cereal Crops
joel.ransom@ndsu.edu


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