Selenium Status of North Dakota and South Dakota Range Forages Throughout the Grazing Season


T. L. Lawler, J. B. Taylor, J. S. Caton, and J. W. Finley


 

Table of Contents



Introduction

 

In addition to the well known antioxidant effects, recent research has revealed possible anticancer properties of selenium (Clark et al., 1996; Davis et al., 1999; Wanger et al., 2000). These effects of selenium occur at supranutritional levels of intake (3 – 4 fold recommended daily intake) and appear to be somewhat dependent upon the chemical form of selenium consumed. Beef is a major (if not the greatest) contributor of selenium in North American diets (Finley, 1999). Selenium concentrations in beef muscle seems contingent on the selenium concentration of the feedstuffs consumed; beef allowed to graze North Dakota rangelands high in selenium appear to have a higher amount of selenium in the muscle that beef grazing areas lower in available selenium (Hintze et al., 2001). Limited data are available that fully describe the amount and form of selenium that are present in forages throughout the grazing season. Therefore, the objectives of this study are to assess the selenium status of forages from low, moderate and high selenium rangelands and monitor possible changes (form, concentration, location, etc.) in selenium behavior throughout the grazing season. The following report is a brief summary of findings from the first of three collection periods scheduled to occur during each grazing season of 2001, 2002 and 2003.

  

Materials and Methods

 

Locations and Sites. Three locations, representing high, moderate and low status ranges, were selected for the study. Locations were selected based on presumed selenium status of the range forages. A ranch near Milesville, SD was selected for the high selenium location, the Central Grasslands Research Extension Center, Streeter, ND was selected for the moderate selenium location and the Ekre Ranch Research Station, Walcott, ND was selected for the low selenium location (Figure 1). At each location, four sites were selected for forage sampling. Each site was 90 ft. in diameter (6362 sq. ft.). All sites combined estimated approximately 0.58 acre within each location.

 

Figure 1. Location of the three sampling locations in North Dakota and South Dakota. (HIH = Milesville, SD; MOD = Central Grasslands Research Center; LOW = Ekre Ranch Research Center).


 

Sampling Date and Protocol. The days from May 29 – June 3, which corresponded to early spring growth of the range forages, were selected as the initial sampling period of an ongoing (multi-year) study to assess selenium status change of the three locations over the summer grazing period. At each location, ten 0.67 sq. ft. sample areas were identified inside each 90 ft. diameter site (Figure 2). All 2001 forage growth (previous years’ growth was discarded) greater than approximately 0.25 in. in height was harvested from each sampling area and pooled together with the other sampling areas within each site. Pooled forage samples from each site were then divided into forb and grass categories and further subdivided into stem

and leaf fractions.

 

Figure 2. Layout of sites (1, 2, 3 and 4; diameter = 90 ft.) within each location. Ten sampling areas (C 0.67 sq. ft.) were identified within each site. All year 2001 forage growth above 0.25 in. in height was collected from each sampling area and pooled for total weight estimate within each site.

 


Analysis. For each site, total forage, grass, forb, grass leaf, grass stem, forb leaf and forb stem weights were measured and recorded following moisture depletion in a forced air oven at 122oF for 48 hours. Samples were further processed and analyzed for selenium as described by Hintze et al. (2001).

 

 

Results

 

As mentioned previously, the results reported herein are part of a repeated sampling project to be conducted throughout the grazing season over several years, and the following data are from the first collection period during the first project year. As such, arithmetic means (and standard deviations) were calculated for each location (based on the four sampling sites) and NO statistical tests were preformed. Therefore, potential differences discussed below are ONLY numerical.


Mean biomass estimates of locations are reported in Table 1. Total 2001 growth forage harvested was numerically similar between locations with the low selenium location having the highest yield followed by the high selenium location. No forbs were present in the high selenium and moderate selenium locations. Forbs were only observed at the low selenium location and were approximately 11.2% of the total biomass. Fraction of leaf and stem were similar for grasses at each location. Stem was approximately 70% of total grass harvested at all locations, while only 60% of forb collected at the low selenium location.


Table 1. Total biomass1 and fraction2 estimates (+ SD) of forage from high, moderate and low selenium locations.


Item

Location

Near Milesville,

SD

(high)

CGREC,

Streeter, ND

(moderate)

Ekre Ranch

 Walcott, ND

(low)

Total Biomass1, lbs

853.9 + 102.3

676.9 + 36.2

935.7 + 80.2

    Grass2, %

100

100

88.8

          Leaf, %

31.3

24.3

30.0

          Stem, %

68.7

75.7

70.0

    Forb2, %

-

-

11.2

          Leaf, %

-

-

39.3

          Stem, %

-

-

60.7

1Biomass is the total weight of forage (Spring 2001 growth only) collected from four sites within each location. 2Fraction estimates are as follows: Grass and Forb are percentages of Total Biomass and Stem and Leaf are percentages of respective Grass or Forb.

 

Total selenium yields (ug/location) within each site for all locations are presented in Figure 3. As expected, the high selenium location was numerically higher than the moderate and low locations (292.7 vs. 18.3 and 67.7 ug/location, respectively). Unexpectedly, the low selenium location yielded more total selenium than did the moderate site. However, upon close observation of Figure 3, it is apparent that one site within the low location yielded greater than 20 fold more selenium than other sites which contributed much variation. Although not as great, high variation was also observed at the moderate and high selenium locations. The coefficient of variation for selenium yield within sampling sites at each location was 56.3, 92.1 and 176.8% for high, moderate and low sites, respectively.

 


Figure 3. Selenium distribution among sites and average (± SD) contribution within high (HIH: near Milesville, SD), moderate (MOD: Central Grasslands Research Extension Center, Streeter, ND) and low (LOW: Ekre Ranch Research Center, Walcott, ND) status rangeland.

 

 

Selenium concentrations of leaf and stem (Table 2) are similar for both the high and moderate locations with the leaf and stem fractions of the high selenium location being numerically highest when compared to other locations. Surprisingly, selenium content of leaf and stem fraction of the low selenium location appeared different with the numerically highest concentration found in the stem fraction of the grass but reversed in the forb fractions. As with the total selenium yield discussed above, these numerical differences in the mean selenium concentrations of the low selenium sites’ forage fractions are difficult to interpret due to the high standard deviations.

 

Table 2. Selenium concentrations1 (+ SD) of forage fractions at high, moderate and low selenium locations.


Item

Location

Near Milesville,

SD

(high)

CGREC,

Streeter, ND

(moderate)

Ekre Ranch

 Walcott, ND

(low)

Grass leaf, ppm

5.12 + 3.01

0.33 + 0.60

0.14 + 0.17

Grass stem, ppm

6.05 + 2.19

0.37 + 0.40

1.03 + 1.84

Forb leaf, ppm

-

-

1.33 + 1.71

Forb stem, ppm

-

-

0.57 + 0.65

1Concentrations are arithmetic means and standard deviations of forage samples collected and pooled from four sites within each location.

 

 

Conclusions


As can be readily observed, the variation between sites for total selenium yield and concentration within plant fractions is great. Moreover, these data are only a small part of an ongoing project to assess behavior and status of selenium in forages throughout the grazing season. Therefore, inferences in reference to the objectives cannot be made at this time. Additional sampling dates occurred during mid-July and the later part of September and analyses of the samples and data are in progress. In addition, this project is scheduled to be repeated throughout the grazing seasons of 2002 and 2003.
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