Fertilization of Extremely Grazed and Moderately Grazed Mixed-Grass Prairie with Slow Release Phosphorus and Urea

J. M. Volk, W.T. Barker, P. Nyren and D. Whitted

NDSU Animal and Range Sciences Department, and Central Grasslands Research Extension Center
 



Table of Contents



Introduction


Many studies have reported on fast release nitrogen and phosphorus fertilization of rangeland, but few have reported on slow release fertilizers or changes in botanical composition as a result of fertilization, which may be as important as changes in forage production. Response of extremely and moderately grazed mixed-grass prairie vegetation to annual applications of slow release phosphorus (P) and slow release urea (N) was studied during a four-year period at the Central Grasslands Research Extension Center (CGREC) near Streeter, North Dakota. Fertilizer rates of 0, 24, and 48 pounds/acre of elemental P were applied May 1 and June 20. Fertilizer rates of 33 and 66 pounds/acre of elemental N were also applied June 20. Both the phosphorus and nitrogen fertilizers are slow release fertilizers encapsulated with a synthetic polymer.



Methods


This study was conducted at the CGREC located in Kidder and Stutsman counties, North Dakota. Two sites of 148 ft by 197 ft were blocked, one on the extremely grazed pasture and the second on the moderately grazed pasture of the grazing intensity trial established in 1988. Each exclosure was placed in an area with similar topography, plant communities, and soil types in the moderately and extremely grazed pastures.


The extremely grazed pasture was characterized by having 20% of the current year’s crop production remaining at the end of the grazing season, or stocked at a historic average of 2.75 AUMs/ac since 1988. The moderately grazed pasture had 50% of the standing crop produced remaining at the end of the grazing season, or stocked at a historic average of 0.96 AUMs/ac since 1988.


The experiment was a complete randomized block design. Within each grazing intensity site, four replications (69 ft by 49 ft) for each of six fertilizer treatments and one control randomly allocated to each replication.

Individual plot size was 9.8 ft by 49.2 ft. The study area was fenced to exclude grazing. Annual applications of 24 lb P/ac and 48 lb P/ac on 1 May and 20 June with slow release P fertilizer, and 33 lb N/ac and 66 lb N/ac on 20 June with slow release N fertilizer were made using an All Terrain Vehicle (ATV) pulling a Gandy fertilizer spreader.


Over the course of the summer, four clipping sets were taken of five plant species to measure the uptake of phosphorous. Clipping dates occurred approximately between the fifth and seventh of June, July, August and September. The five species clipped were western wheatgrass (Agropyron smithii), green needle grass (Stipa viridula), Kentucky bluegrass (Poa pratensis), smooth brome (Bromus inermis), and western snowberry or buckbrush (Symphoricarpos occidentalis). Also a separate clipping was performed to establish above-ground production on each of the treatments.

 


Results


Extremely Grazed Trial

Total production on the extremely grazed trial resulted in no fertilizer treatment effects in 2000, 2001, and 2002 (Table 1). Total production in 2003 resulted in the 33 lb/ac and 66 lb/ac N treatments being significantly higher than the control and phosphorus treatments. Possible explanations of the treatment effects in the fourth year of the study or 2003 versus the first three years could be related to the low plant vigor of the extremely grazed pasture or from the slow release of the fertilizer. A year effect was seen with 2000 and 2002 having the lowest production and 2001 being lower than 2003, which had the highest production. All four years were significantly different in cool season grass production although there were no treatment effects in 2000, 2001, and 2002. As in total production, the nitrogen treatments were significantly higher in cool season grass production than the control and phosphorus treatments in 2003. Warm season grasses, forbs, and shrubs had no significant changes over the four years although year effects were reported in forbs and shrubs.


The longer the plants can meet the minimum cattle requirements the longer the cattle can stay on the range. The minimum cattle phosphorus requirements for a 1,200 lb lactating cow are 0.19, 0.18, 0.17, and 0.14 respectively for the months of June, July, August, and September. During the grazing season the control was deficient of phosphorus 42% of the time (Table 2). The 33 lb N/ac was deficient 31% of the time and 66 lb N/ac was deficient 42% of the time, versus the May 1 applications of 24 lb P/ac being deficient 11% and 48 lb P/ac 7% of the time. The June 20 applications of 24 lb P/ac were deficient 16% and 48 lb P/ac 13% of the time. It appears that the earlier (May) application is more beneficial than the higher lbs/ac applied.


Few studies have reported on changes in botanical composition as a result of fertilization, which may be as important as changes in forage production.Forbs on the extremely grazed fertilization exclosure resulted in a year effect with 2000, 2001, and 2002 all being different (Figure 1). 2002 had the lowest forb diversity and 2001 had the highest forb diversity. A treatment effect was also observed with 66 lbs N/ac having the lowest forb diversity and the control having the highest forb diversity over the three year period. The phosphorus treatments and the 33 lb N/ac were similar.

 

 

Table 1. Aboveground production (lb/ac) for fertilizer treatments on extremely grazed mixed grass prairie at Central Grasslands Research Extension Center, Streeter, North Dakota 2000-2003.

 

 

Fertilizer Treatments

  

Year

Control

May 1

24 lb

P/ac

May 1

48 lb

P/ac

June 20

24 lb

P/ac

June 20

48 lb

P/ac

June 20

33 lb

N/ac

June 20

66 lb

N/ac

Cool- season

grass

2000r

1285ab

1192ab

1205ab

1085a

1082a

1215ab

1366abc

2001y

1924abcdef

1972bcdef

1943bcdef

1934abcdef

1973bcdef

1540abcd

2319def

2002x

1401abc

1562abcd

1400abc

1605abcde

1467abc

1711abcde

1934abcdef

2003z

2546f

2428ef

2629f

2205cdef

2362def

3562g

3996g

Warm-season

grass

2000

48

163

68

203

24

44

57

2001

7

23

13

180

57

133

82

2002

366

33

120

115

234

203

28

2003

129

72

136

206

57

61

303

Forb

2000y

641

521

562

593

530

632

726

2001y

686

443

766

554 

473

921

640

2002x

188

391

482

323

327

377

215

2003z

1071

884

1101

1112

889

920

987

Shrub

2000x

60

29

91

28

102

54

51

2001x

36

74

9

67

70

80

68

2002x

17

26

30

25

51

71

43

2003y

181

211

118

195

323

394

62

Total

2000x

2035a

1907a

1927a

1910a

1739a

1946a

2202ab

 

2001y

2654abc

2514ab

2733abcd

2737abcd

2573ab

2675abc

3110bcde

 

2002x

1974a

2013a

2034a

2068ab

2080ab

2363ab

2221ab

 

2003z

3929

3598cde

3986

3719de

3633cde

4939f

5349f

1Means in a plant class followed by a different letter differ (P<0.05).

2 Means in the year column in each plant class followed by a letter differ (P<0.05).


 

 

 

 

Moderately Grazed Trial

Total production on the moderately grazed trial resulted in no treatment effects over the four years (Table 3). A year effect resulted in 2002 being significantly lower than 2000, 2001, and 2003. Cool season grasses, warm season grasses, forbs, and shrubs had similar treatment effects although year effects were prominent in each plant class. Phosphorus, being a limiting factor in plant growth, and the second most important nutrient behind nitrogen, should be a concern late in the grazing season in regards to the plants being able to meet the minimum cattle requirements of phosphorus. During the grazing season the control was deficient of phosphorus 37% of the time (Table 2). The 33 lb N/ac was deficient 44% of the time and 66 lb N/ac was deficient 49% of the time, versus the May 1 applications of 24 lb P/ac being deficient 12% and 48 lb P/ac 2% of the time. The June 20 applications of 24 lb P/ac were deficient 11% and 48 lb P/ac 9% of the time. It appears that the higher lbs/ac applied is more beneficial than the earlier date applied. This is opposite what was found on the extremely grazed trial where the earlier application seemed more beneficial than the higher pounds applied. Overall, the moderately grazed trial and the extremely grazed trial were similar in percent time phosphorus deficient from 2000 to 2002. The moderately grazed trial resulted in a phosphorus deficiency on the nitrogen treatments 47% of the grazing season. The phosphorus treatments were deficient 9%, of the season and the control was deficient 37% of the grazing season. The extremely grazed trial resulted in a phosphorus deficiency on the nitrogen treatments 37% of the grazing season. Phosphorus treatments were deficient 12% of the season and the control was deficient 42% of the grazing season. Forbs on the moderately grazed fertilization exclosure had a year effect with 2001 and 2002 having significantly lower forb diversity than 2000. A treatment effect was also observed with 33 lbs N/ac having higher forb diversity than 24 lbs P/ac June 20, 66 lbs N/ac June 20, control, and 48 lbs P/ac May 1 (Figure 2).

 

 

 

 

 

Conclusion


There are many questions that need to be answered concerning the effect of production and species diversity, whether it is with phosphorus or nitrogen fertilizer. During the past three years changes have been very slow and not statistically significant although in the fourth year of the trial, effects are starting to present themselves and trends are starting to form. The growing season of 2004 will be the fifth and final season of the fertilization study and should determine if botanical changes are in fact taking place and if the herbage production differences with phosphorus and nitrogen are affected by the wide range of precipitation levels over the past four years. A full report on botanical change will be presented in 2004.