NMSU: Use of Plants in Seasonal Grazing Trials on Chihuahuan Desert Rangeland
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Use of Plants in Seasonal Grazing Trials on Chihuahuan Desert Rangeland


Research Report 733
M. Nsinamwa, Former Graduate Student (currently an instructor in Botswana College of Agriculture, Gaborone, Botswana)
R. F. Beck, Professor, Department of Animal and Range Sciences
R. P. McNeely, Senior Research Assistant, Department of Animal and Range Sciences
College of Agriculture, Consumer and Environmental Sciences New Mexico State University


Abstract

Short-term grazing trials were conducted to determine seasonal diet differences and forage plant use patterns on mesa dropseed (Sporobolus flexuosus Thurb. ex Vasey) and snakeweed (Gutierrezia sarothrae [Pursh] Britt. Rusby) rangelands. Trials were conducted at the Chihuahuan Desert Rangeland Research Center in southern New Mexico during June and September in 1992 and 1993. Each trial lasted 6 days. Stocking rate was 4 cows/ha/trial, except for June 1993 when 2 cows were used.

Objectives of the study were to: measure changes in diets selected by cows as forage availability changed; determine if either moderate grazing or heavy grazing in June (end of the dry season) or September (end of the growing season) caused changes in plant composition, herbage production, and use; compare use techniques for measuring mesa dropseed.

Distinct preference patterns were shown by cows for some species. As preferred species disappeared (e.g., hog potato [Hoffmanseggia glauca (Ort) Eifert] and paperflower [Psilostrophe tagetina (Nutt.) Rydb.]), other plants became important in the diets, such as fluffgrass [Dasyochola pulchella (Kunth) Muell.-Arg.] and desert zinnia (Zinnia grandiflora Nutt.). Dropseed, the most abundant grass, comprised large portions of the diets in all seasons. Total above ground biomass was maintained under moderate and heavy stocking in grazed paddocks in September. Total biomass was reduced nearly 50% on the June, heavy-grazed paddocks, with grasses declining 36% and 47% on the moderately and heavily grazed paddocks, respectively. The heavy stocking rate appeared to have more influence on plant biomass present in June 1993, as compared to that present in September 1993. Because paddocks were small, trampling and scuffing caused the disappearance of many individual plants. Use measured on mesa dropseed with the before-andafter grazing technique was considered more representative than using height-weight charts.


Table of Contents

Materials and Methods
    Description of the Atudy Area
Methodology
    Paddock Design and Grazing
    Plant Characteristics and Grazing Use
Results and Discussion
    Diets versus Forage Availability
    Use
Summary and Conclusions
Literature Cited


Forage availability and acceptability are important determinants of diet selection by herbivores. On grazing lands with abundant available forage, cows selectively graze the most desirable plant parts, usually leaves. As percentage of dry material increases, cows are unable to select entirely green diets without decreasing their total intake. Generally the total amount of herbage in a plant community is greater than what cows willingly graze, because of differences in palatability of plants and plant parts (Bailey et al. 1996).

Preferential grazing of leaves and other plant parts affects whether plants survive. Briske and Richards (1994) propose that the importance of grazing on plant growth and plant survival may be more closely related to changes in competitive interactions and abilities among plants than the direct effect of removing photosynthetic material. Grazing animals alter competition among plants by selective plant use resulting from plant characteristics that cause preferences or avoidances (Briske 1991). Evaluating several studies on range grasses, Briske and Richards (1994) reported that plant growth was more dependent on current photosynthesis than on carbohydrate reserves unless soil water and temperature were unfavorable. However, in arid environments like southern New Mexico, it is probable that carbohydrate reserves are important for regrowth and necessary for plants to be effective competitors. Each plant has its own peculiar response to defoliation, and the amount of defoliation affects its ability to survive. For example, the amount of active meristem, root respiration, and nutrient absorption are positively affected by defoliation intensity. But another factor to consider is the age of the photosynthetic material grazed, because young leaf material is more photosynthetically active than older leaves (Briske and Richards 1994). Trampling by grazing herbivores also is a factor that needs to be considered, because it affects the microenvironment and causes physical damage that alters the plants’ ability to compete. As a result of vegetation composition changes caused by herbivory, many rotational grazing systems and plans have been developed for different grazing seasons that attempt to counteract the loss of grazing plants (Beck 1980).

The appropriate grazing plan and the correct stocking rates are both important for rangeland perpetuation (Taylor et al. 1997), livestock well-being, and the operator’s economic stability. Continuous or seasonal overstocking is certain to result in deteriorated rangeland resources. Low stocking rates may or may not result in improved rangeland conditions, but they generally mean more forage, allowing individual animals to be more selective in their diets and increasing maximum animal production.

The study’s purpose was to determine the influence of moderate and heavy grazing before and after the summer rainy season on plant composition changes, and selective use of forage species. It also considered how decreases in forage availability cause changes in cattle diets on mesa dropseed (Sporobolus flexuosus Thurb. ex Vasey) and snakeweed (Gutierrezia sarothrae [Pursh] Britt. & Rusby) rangelands. A secondary objective was to evaluate use measurements on mesa dropseed with two techniques. Mesa dropseed on this rangeland is a key species.


Materials and Methods

Description of the Study Area

The study paddocks were at New Mexico State University’s Chihuahuan Desert Rangeland Research Center, approximately 38 km north of Las Cruces, New Mexico. The area is a semiarid rangeland characterized by soils (mostly Petroargids and Petrocalcids) ranging from fine sandy to sandy loam texture. Annual average temperature is 16° C, with an average maximum daytime temperature during June of 37° C, and an average maximum daytime temperature during January of 13° C. Spring is the windiest and driest season, with strong winds occurring only in conjunction with storms during the remainder of the year. Annual precipitation during the study was 59% and 12% above the long-term average in 1992 and 1993, respectively (table 1). Summer precipitation was near the average in 1992, and 25% above average in 1991 and 1993. The unusually large amount of rain during the 1991—1992 winter and in May 1992 resulted in a large grass and forb standing crop before the start of the study.

Table 1. Monthly and total precipitation (mm) averages of two rain gauges within 1.5 km of study areas.

Month Years
1991 1992 1993 1930–1992*
  ------------------ (mm) ------------------
January 10 38 35 12
February 11 4 13 11
March 15 8 2 7
April 0 21 2 6
May 3 88 8 8
June 7 15 20 13
July 71 25 88 41
August 63 86 65 45
September 57 8 6 37
October 0 26 10 23
November 24 11 12 12
December 112 46 14 16
Total 372 376 265 236
Seasonal precipitation (July, Aug., Sept.) 191 120 159 127
*Long-term averages are for all gauges on the Chihuahuan Desert Rangeland Research Center.

The major grasses found in study paddocks were mesa dropseed, threeawns (Aristida spp.), black grama (Bouteloua eriopoda [Torrey] Torrey) and fluffgrass (Dasyochloa pulchella [Kunth] Steudel). The major half-shrubs were broom snakeweed and desert zinnia (Zinnia graniflora Nutt.). Some mesquite (Prosopis glandulosa Torrey) grew in the area. A large number of forb species were present.

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Methodology

Paddock Design and Grazing

Two 1-ha paddocks (replicates) were each stocked with 4 cows for 6 days in June 1992 (fig. 1). In September 1992, another replicated pair of 1-ha paddocks were each stocked with 4 cows for 6 days. In 1993, the same paddocks were grazed at the same times as in 1992—for 6 days in June and 6 days in September. Only 2 cows were used for the grazing in each paddock in June 1993, and 4 cows grazed in each paddock during the September trials. Different cows of similar ages were used in each grazing trial. Cows had access to the entire paddock until mesa dropseed (used as a key species) was grazed to near 35% use, which usually took about 4 days. A fence was then put across the paddock, and cows grazed on the remaining 0.5 ha for 2 days to obtain 65% use on mesa dropseed. One cow in each replication was observed for 1 hour each day (generally in the morning) during the trials. The number of bites the cows took of each plant species were recorded for that hour (Mofarreh et al. 1997). Bite count data were summarized by species as a percentage of the total diet eaten that day.

Fig. 1: Illustration of replicated paddock layout.

Figure 1. Layout of one replicated paddock. Cows grazed on a side (June or September) until the side was moderately grazed. At that time, a fence was constructed (dotted line) and cows were restricted to one half of the side until it was heavily grazed.

Plant Characteristics and Grazing Use

Five belt transects (0.5 × 10 m) were permanently established in each grazing treatment (a total of 10 transects in each season and replication) for measuring plant attributes. Transects were specifically located in areas with an abundance of herbaceous plants, so there would be a maximum number of each species to follow through the study. Grass, shrub, and forb densities were determined on the 5 transects. Basal area was estimated for all plants growing on the transects. Plant attributes were measured on the transects before and after each grazing trial.

Grazing use estimates were determined by two different methods. A height-weight chart for mesa dropseed was developed for each grazing trial. This technique was used during the grazing trials to determine the use level on mesa dropseed while the cows were grazing. The before-and- after grazing technique was used to estimate percent disappearance (use) by weight for all species (including mesa dropseed). Before and after each grazing trial, different sizes of each plant species were clipped at ground level, put into separate bags, oven dried at 50° C and each plant weighed individually. Only current years’ growth was measured. For annual and perennial forbs, generally the entire plant was collected. For grasses and half-shrubs, only green leaves and stems were measured. The basal area of each plant was recorded before clipping. Dry-biomass for each species was calculated in grams per square centimeter of basal area. Dry-biomass/cm2 was then multiplied by the total basal area (in cm2) for each species found on the 5 transects. The results were then converted to kilograms per hectare. The difference in weight before and after grazing was expressed as a percent disappearance (use). Before-grazing biomass was used as an estimate of total forage available. Standard errors were calculated for the means for each species in each paddock for each season. No samples were taken from the control paddocks in June 1993.

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Results and Discussion

Aboveground biomass available for grazing across the 4 trials was comprised of 4 perennial grasses, 20 to 30 forbs (mostly annuals), and 5 shrubs and half-shrubs, including soaptree yucca (Yucca elata Engelm.) (table 2). Aboveground biomass was similar for moderate and heavy grazed paddocks at the study’s start in 1992 (table 3). In June 1992, the production of dropseed, threeawn, and snakeweed was unusually high for that early in the summer. This high production occurred because of above-average rainfall in April and May (table 1) which followed above-average rains in midwinter. In this area, spring months generally are dry, and little herbaceous growth occurs until the start of the summer rains in late June to mid-July. In September 1992, the available forage production was similar in the paddocks prior to grazing and was more typical of production estimates from other studies (Mofarreh et al. 1997; De Alba Becerra et al. 1998). Rainfall in 1993 was approximately equal to the long-term average, and plant biomass availability was again similar to the long-term average production. By June 1993, the effect of grazing and less rainfall during the previous year was noticeable as reduced available biomass (table 3). In September 1993 there was not a clear pattern regarding the availability of grasses and forbs between the moderately and heavily grazed pastures. The lack of a clear pattern can be attributed to there only being one year between sampling dates and also starting the study with a large amount of available plant biomass due to high rainfall. Also, the technique for calculating biomass contributed to the potential variation observed. The estimations of the basal areas of plants growing in the frames were generally consistent. The exceptions were with plants such as snakeweed, that tended to lean over on to the soil surface. Or sometimes the lowest branches were covered by drifting sand and were assigned more basal area than just the area occupied by the single stem. The higher estimates were assigned because of the larger areas dominated (occupied) by the plant.

Table 2. Number of species in each plant class available and eaten by cows in June 1992, 1993, and September 1992, 1993.

Year Season Class of
plants
Total no. of
spp. available
Total no.
of spp. eaten
1992 June Grasses
Forbs
Shrubs
5
22
5

4
18
3

1993 June Grasses
Forbs
Shrubs
5
20
5
5
11
3
1992 September Grasses
Forbs
Shrubs
5
30
5
5
14
3
1993 September Grasses
Forbs
Shrubs
4
22
5
4
10
1

Table 3. Average aboveground biomass (kg/ha ± SE; mean of two replications) by species before grazing on grazed and control paddocks in June and September, 1992 and 1993. Control paddocks were not sampled in June 1993.

Species Stocking
rate
June September
1992 1993 1992 1993
kg/ha SE kg/ha SE kg/ha SE kg/ha SE
Grasses
Threeawns
(Aristida spp.)
control
moderate
heavy
257
171
53
112
102
38

247
85

133
27
243
196
105
66
66
60
271
155
49
74
58
16
Black grama
(Bouteloua eriopoda)
control
moderate
heavy
0
47
0
0
47
0

65
0

65
0
2
28
10
2
28
10
0
18
8
0
18
8
Fluffgrass
(Dasyochloa pulchella)
control
moderate
heavy
47
14
33
14
4
13

13
26

4
8
46
18
19
24
10
8
77
7
13
24
3
5
Hall’s panicum
(Panicum hallii)
control
moderate
heavy
0
0
7
0
0
6

0
2

0
2
0
0
3
0
0
3
0
T
T
0
0
0
Mesa dropseed
(Sporobolus flexuosus)
control
moderate
heavy
499
479
364
139
140
144

192
230

69
127
247
356
140
74
86
28
240
195
208
105
55
76
Subtotal control
moderate
heavy
804
710
458
97
89
69

516
343

49
43
538
598
277
57
69
28
588
375
280
58
41
39
Perennial Forbs
Desert holly
(Acourtia nana)
control
moderate
heavy
67
0
1
43
0
1

0
T

0
0
21
0
2
14
0
1
16
1
1
14
1
1
Twin leaf
(Cassia bauhinioides)
control
moderate
heavy
7
6
5
3
3
2

3
1

2
1
11
3
4
6
2
2
5
1
1
2
0
0
Leatherweed croton
(Croton potsii)
control
moderate
heavy
516
250
146
152
118
76

140
126

52
51
176
160
98
62
52
47
252
34
246
75
13
72
Hog potato
(Hoffmanseggia glauca)
control
moderate
heavy
23
5
7
20
5
5

6
6

6
3
13
10
7
8
8
5
16
1
1
14
1
1
Paperflower
(Psilostrophe tagetina)
control
moderate
heavy
31
23
1
31
15
1

18
1

11
1
29
22
3
28
13
3
0
42
23
0
35
23
Silverleaf nightshade
(Solanum elaeagnifolium)
control
moderate
heavy
32
3
20
19
3
16

0
4

0
3
24
21
7
13
12
4
11
2
1
8
1
0
Tetraclea
(Tetraclea coulteri)
control
moderate
heavy
9
3
37
5
3
27

2
1

1
0
7
2
0
4
1
0
6
1
0
4
1
0
Other control
moderate
heavy
19
8
7
18
7
6

21
14

14
12
7
2
2
7
1
1
6
39
31
6
15
24
Subtotal control
moderate
heavy
704
297
224
62
30
17

191
154

17
15
288
220
122
20
19
12
312
122
305
31
7
30
Annual Forbs
Gray goosefoot
(Chenopodium incanum)
control
moderate
heavy
4
4
11
4
4
7

0
0

0
0
0
1
1
0
1
1
0
0
0
0
0
0
Spectaclepod
(Dimorphocarpa wislizenii)
control
moderate
heavy
119
0
2
79
0
2

1
3

0
3
18
0
0
12
0
0
2
0
7
2
0
5
Abert’s buckwheat
(Eriogonum abertianum)
control
moderate
heavy
14
1
9

10
1
6


0
0

0
0
17
17
7
13
17
5
6
1
3
4
1
1
Russian thistle
(Salsola tragus)
control
moderate
heavy
4
11
16
3
11
7

0
0

0
0
9
185
6
9
165
4
0
2
9
0
2
7
Other control
moderate
heavy
11
3
1
9
3
1

1
1

1
1
0
23
11
0
20
6
4
3
5
2
2
4
Subtotal control
moderate
heavy
151
18
39
22
2
3

2
4

1
1
0
227
25
4
35
2
13
6
24
1
1
2
Half-Shrubs
Snakeweed
(Gutierrezia sarothrae)
control
moderate
heavy
589
934
1154
216
497
563

965
738

407
344
324
215
391
138
163
168
261
472
222
115
219
119
Plains zinnia
(Zinnia grandiflora)
control
moderate
heavy
49
40
39
48
38
36

10
14

9
12
41
12
2
32
11
2
12
8
5
12
7
5
Subtotal control
moderate
heavy
638
974
1193
270
497
563

975
751

477
362
365
228
393
141
102
195
273
480
227
125
232
108
Herbaceous total control
moderate
heavy
1659
1026
721
203
201
121

709
500

150
98
826
1044
424
155
125
74
913
503
608
166
109
90
TOTAL control
moderate
heavy
2297
2000
1913
145
213
253

1869
1258

213
162
1235
1272
818
112
93
82
1186
983
835
118
110
64
T = > .05 kg/ha aboveground production

This same phenomenon was encountered with other plants, such as grasses. During the grazing trial, these plants were often disturbed by the trampling of cows. Therefore, when basal area was measured after the trial, some plants had less basal area covered by soil, and a smaller total basal area was measured.

The other reason for not getting a clear picture of biomass for each species relates to how the multiplier used in conversion was determined. The plants selected to calculate the multipliers were sometimes larger and taller than the average plants growing on the transects. This allowed the numbers used for the multipliers to be larger than what should have been used, considering the size of the plants growing on the transects. These two factors—overestimating the basal areas and using larger than appropriate multipliers—generally resulted in an overestimation of production for snakeweed, dropseeds, and threeawns, but not for other species.

Diets versus Forage Availability

Grasses were the major forage component for cattle in all 4 grazing trials, followed by forbs and then shrubs (table 4). All 4 perennial grasses were eaten in 3 of the 4 trials, while the number of forbs eaten was fewer in trials after June 1992 (table 2). This reduction in forb content was due to lower production available (table 3) and the plants’ generally smaller stature. Because of the large amount of rainfall in April and May 1992 there was a relatively large variety of spring and summer forbs available that the cattle sampled (table 2), but did not eat in the quantity they did in June 1993 (table 4). Larger percentages of bites eaten for fewer species indicate less abundance of grasses and forbs in June 1993 (table 3) and lower palatability of many forbs because of drier conditions. De Alba Becerra et al. (1998) noted similar findings with heifers grazing in the Chihuahuan Desert, and reported forbs comprised small portions of cattle diets in the nongrowing season.

Table 4. Average percentage of bites eaten of common plant species. Cows were observed each day for 1 hour in each replication in June and September in 1992 and 1993.

Category3 Percent
Days1 Days1
1 2 3 4 5 6 1 2 3 4 5 6
  June 1992 June 1992
Grasses
   Mesa dropseed 55 74 73 90 65 43 54 34 24 27 22 28
   Blackgrama 0 0 0 0 0 0 0 2 2 0 1 0
   Fluffgrass 0 0 0 0 3 4 1 1 1 7 7 5
   Threeawns 0 0 1 1 2 27 0 18 21 24 32 41
Subtotal 55 74 74 91 70 74 55 55 48 58 62 74
Forbs
   Desert holly 0 0 0 0 0 3 0 2 14 0 7 1
   Hogpotato 6 2 3 1 0 0 2 1 T T 0 0
   Leatherweed croton 0 2 7 3 19 11 14 26 24 29 23 15
   Paperflower 11 5 3 1 T2 T 12 3 4 1 T T
Subtotal 17 9 13 5 19 15 29 33 42 30 31 17
Shrubs
   Broom snakeweed 0 0 0 0 0 0 0 0 0 0 0 0
   Desert zinnia 0 1 2 1 3 2 3 4 4 6 4 9
   Yucca 1 T 0 1 3 5 8 1 2 T 0 0
Subtotal 1 2 2 2 6 7 11 5 6 7 4 9
Other annuals 18 12 10 3 5 3 2 7 2 2 2 0
Other perennials 9 3 2 1 0 0 3 2 2 1 0 0
Other shrubs 0 0 0 0 0 0 0 0 0 2 2 T
Subtotal 27 15 12 4 5 3 5 9 4 5 4 T
Total 1004 100 101 102 100 99 100 102 100 100 101 100
  September 1992 September 1993
Grasses
   Mesa dropseed 76 48 66 65 44 44 50 43 43 40 28 26
   Blackgrama 4 27 12 5 3 4 8 9 7 2 0 0
   Fluffgrass 1 T 2 1 2 0 T 4 10 12 13 15
   Threeawns 1 1 4 10 24 31 0 0 1 11 17 30
Subtotal 82 77 85 81 73 79 59 57 60 65 59 71
Forbs
   Desert holly 0 0 0 0 0 0 0 1 1 3 5 1
   Hogpotato T 2 T T 0 0 5 2 T 2 T 0
   Leatherweed croton 3 4 3 7 14 12 0 5 14 17 20 23
   Paperflower 7 5 2 1 T 1 19 8 2 2 0 0
Subtotal 10 11 5 8 14 13 24 16 18 24 26 24
Shrubs
   Broom snakeweed 0 0 0 0 0 0 0 0 0 0 0 0
   Desert zinnia 1 5 1 3 2 0 6 14 16 4 15 5
   Yucca 0 0 1 1 1 1 0 0 0 0 0 0
Subtotal 1 5 2 4 3 1 6 14 16 4 15 5
Other annuals 6 7 7 5 10 7 10 10 5 6 0 0
Other perennials 1 0 0 0 0 0 2 4 1 1 0 0
Other shrubs 0 0 1 2 0 0 0 0 0 0 0 0
Subtotal 7 7 8 7 10 7 12 13 6 7 0 0
Total 100 100 100 100 100 100 101 100 100 100 100 100
1 Cows had access to both moderately and heavily grazed paddocks for days 1–4 and were restricted to heavily grazed paddocks on days 5 and 6.
2 T < .05%
3 Scientific names for plants listed can be found in table 3.
4 Subtotals may not add to 100% due to rounding.

Generally, more bites were counted per hour in the heavily grazed paddock on days 5 and 6 as compared to the first 4 days when the cattle had access to the entire paddock (both moderate and heavily grazed areas). This could be attributed to a change in bite size, which becomes smaller as forage availability becomes less (Bailey et al. 1996). Or, as species preference changed, the cattle ate smaller-sized plants and took more bites in order to satiate their requirements. Arnold (1964) reported that highest biting rates were observed when forage was limited, especially when there was green growth.

The diets selected by the cows depended to a large degree on a species’ abundance. Considering his work with crested wheat grass pastures (Agropyron spp), Scarnecchia et al. (1985) suggested that biting rate and grazing time are affected by leaf:stem ratio and forage maturity as well as forage density. Dropseed was the most abundant species (table 3) and no general pattern of preference was seen for it in any of the trials (table 4). In all trials, dropseed plants made up the bulk of the total forage eaten. However, dropseed plants provide green growth earlier and respond more quickly to summer showers than other warm-season grasses growing in this region.

For other species, availability and preference were important factors in the diets selected. The cows had been grazing on a similar vegetation type before being put on the study paddocks. Diet selected on the first day probably represented preferred species. As these species become less abundant other species became more important. Black grama, for example, was not eaten in June 1992, even though it was green and flowering. In June 1993, when there was less forage and most plants were dry, black grama was heavily used (95% use on the black grama on the heavily grazed paddock) (table 6). Black grama was readily eaten in September of both years. Fluffgrass, a species generally considered not important for cattle, was eaten in all trials, with most consumption in the latter part of each trial. Threeawns were eaten in all trials with overall percentage of the diets increasing toward the end of the trials. Shortness of threeawn plants and presence of seedheads caused the animals to take many bites from around the base of the plants without obtaining much material in any individual bite.

Forb consumption showed similar patterns to that of grasses with some being eaten more at the beginning of the trials. As they disappeared, other species became more common in diets. Both paperflower (Psilostrophe tagetina [Nutt.] Rybd.) and hog potato (Hoffmenseggia glauca [Ort] Eifert) were readily eaten at the beginning of each trial, but the percentage in the diets declined during each trial (table 4). Percent of croton (Croton potsii [Klotzsch] Muell.-Ang.) in the diets generally increased from the first to last day in all trials. Desert holly (Acourtia nana [Gray] Reveal & King) in diets tended to increase for both trials in 1993, even though biomass was low (table 3).

Generally, average shrub aboveground biomass was greater than forbs’ or grasses’ biomass (table 3). Broom snakeweed and desert zinnia, both half-shrubs, were the two major shrubs in the study area. Yucca and some mesquite were present in the paddocks, but did not occur in the plots used for measuring biomass. Highest aboveground shrub biomass was in June 1992, with 974 kg/ha and 1,193 kg/ha before grazing on the moderate-and heavily-grazed paddocks, respectively. Shrub biomass was generally lower in the paddocks grazed in September than in June.

Broom snakeweed was abundant in June and September in both years, but the cows were not observed eating it in any of the grazing trials. Desert zinnia biomass fluctuated from season to season even when other forage species became limited. Its production was greater in moderately grazed paddocks than on heavily grazed paddocks. Results indicate that both shrubs responded similarly to seasonal changes, but differently to grazing because one was eaten and the other was not. The highest bite count of desert zinnia was recorded in September, 1993, and the least amount in the diets was recorded in June 1992. Although results showed some fluctuations from season to season, generally the percentage of desert zinnia in diets was higher during the latter part of each grazing trial.

Low rainfall in spring 1993 coupled with grazing effects from 1992 trials might be the cause of low forage availability and, hence, diet changes in the June and September 1993 trials. Changes observed in diets during each trial are most likely related to total available biomass and palatability of available forage. Because of the shortness of each grazing trial there was little time for physiological changes that might affect palatability or plant compositional changes other than plant losses due to hoof action. As preferred species were eaten, the ratio of palatable (green forage material) to unpalatable (old, dry plant material) decreased and other plant species offered better opportunities for foraging. Stature of plants also appeared to be a factor with smaller plants offering less forage material per bite at the end of each trial. Different patterns of preference found for forbs and desert zinna within and between trials were in part due to their response to the available rainfall patterns. Timing of the rain events prior to grazing trials influenced composition as well as size of individual plants, and both contributed to differences in diets observed between June and September trials. These observed diet selection patterns were described by Stuth (1991) in discussing animal preference in relation to availability. In his discussion, he categorized one group of plants as having secondary preference, meaning plants that change from being avoided to preferred by the herbivores as herbage biomass declined. Generally, these plants are considered to have some morphological constraints affecting grazing selection. Threeawns and fluffgrass (possibly because of awns), leatherweed croton (possibly because of leaf texture), and paperflower (possibly because of taste) are plants in this study with morphological constraints.

Use

Calculating plant use by before-and-after weight difference includes loss of weight due to the hoof action and trampling of individual plants of the different species. The soils were relatively sandy. As a result of the animals walking across the small paddocks, many plants were broken off or scuffed out of the ground. An example of this (table 5) shows the difference for mesa dropseed use calculated by before-after and height-weight techniques. The height-weight technique gives a fairly clear distinction between the percent use on the moderately and the heavily grazed portions of the paddocks for all seasons. The use estimates are generally higher with the before-and-after weight technique, probably because many small plants were trampled and scuffed out or pulled out and grazed. The total density and basal area were less for a species after grazing. In September 1993, use was lower when calculated by weight in both moderate- and heavy-grazed paddocks. This may have been due to less green or new forage compared to the dry, old forage. In calculating use with the before-and-after technique, only differences in green forage weights were used. However, dropseed use with the height-weight charts used total heights that sometimes included old, dry leaves and stems—not just green material. Therefore, measuring grazed heights of dropseeds would include dry stems and leaves that had been grazed, and percent use would be higher than if only green material was considered as in the before-andafter calculation.

Table 5. Dropseed use determined by differences between weight before and after grazing and by height-weight ratios at two stocking rates.

Year Season Level of
grazing use
Percent use by
Before and
after weight
Height-weight
ratio
1992 June moderate
heavy
47
67
38
68
1993 June moderate
heavy
63
63
39
54
1992 September moderate
heavy
6
74
37
65
1993 September moderate
heavy
23
59
40
70
Use by weight takes into account disappearance as well as amount eaten.

Table 6. Use of grasses, forbs, and shrubs determined by weight differences before and after grazing at two stocking rates.

Species Stocking
Rate
June 1992
Use (%)
June 1993
Use (%)
Sept.1992
Use (%)
Sept.1993
Use (%)
Grasses
   Dropseeds moderate
heavy
47
67
63
63
66
74
23
59
   Blackgrama moderate
heavy
−1
−1
50
95
33
50
26
56
   Fluffgrass moderate
heavy
0
−1
33
50
0
50
0
18
   Three-awn moderate
heavy
14
17
25
19
26
36
24
41
Forbs
   Aberts buckweed moderate
heavy
0
70
0
0
50
67
31
68
   Desert holly moderate
heavy
0
0
0
100
0
0
0
0
   Goosefoot moderate
heavy
83
83
0
0
80
83
0
0
   Hogpotato moderate
heavy
75
88
100
100
70
67
38
83
   Leatherweed-cotton moderate
heavy
50
65
0
0
70
0
40
56
   Paperflower moderate
heavy
26
63
77
33
0
0
50
17
   Russian thistle moderate
heavy
33
80
0
0
0
60
0
0
   Silverleaf nightshade moderate
heavy
0
57
0
100
50
50
44
50
   Tetraclea moderate
heavy
0
0
100
100
33
0
100
0
   Twinleaf moderate
heavy
75
75
50
100
0
0
100
67
Shrubs
   Broom snakeweed moderate
heavy
5
4
11
9
18
−0.4
7
5
   Desert zinnia moderate
heavy
80
78
0
67
40
70
30
58
*Percent use was calculated from weights before and after.
*Negative numbers indicate increase in weight from before to after grazing (considered as no use).

Forb and half-shrub use is not well documented in literature. There is no satisfactory technique to determine use over an extended period. The height-weight technique does not appear practical for this group of plants because of their growth forms. The before-andafter weight technique can only be used for short grazing periods as used in this study. The 100% use obtained on some species in some trials indicate that the plants disappeared (eaten or trampled out) during the trial. The 0% use for some plant species on some paddocks may be because the species were not available (annual) or because no bite count was recorded. For example, the animals sometimes ate a species on the heavily grazed paddock and not on the moderately grazed paddock, even though it grew in both paddocks. Aberts buckwheat and silverleaf nightshade are examples of this.

Another problem with determining grazing use for forbs is that many disappeared during the trials. It is difficult to determine whether the cows pulled them out of the ground and ate them, or whether they were scuffed out of the ground. By watching the animals graze, it is possible to observe patterns of grazing selection and avoidance. Snakeweed was never observed to be eaten during any trial, yet some use is indicated with the before-and-after technique. This disappearance is most likely due to trampling.

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Summary and Conclusions

Average biomass of grasses decreased with both levels of stocking after the first year. Forb and shrub biomasses also declined in the June grazed paddocks after one year, while they remained nearly similar or increased in the September grazed paddocks. Cattle diets changed throughout each of the 6-day grazing trials. Dropseeds and threeawns were important grass species in all seasons, while black grama and fluffgrass were only important in September diets. Leatherweed croton was the important forb species. Paperflower was a forb species that was preferred and eaten at the beginning of each trial, but very little was eaten during the latter part of each trial. Desert zinnia was eaten during each trial, comprising up to 16% of the diet in September 1993. Yucca blooms were readily eaten when available in June. A small amount of yucca leaves was eaten each September.

The study also showed that more forage production can be maintained under a moderate stocking rate, regardless of the season. The lowered forage production in the heavily grazed paddocks was partially a loss of plants due to direct trampling and physical disturbance of sandy soil. The before-and-after use technique measured disappearance of small plants, seedlings, annual grasses and forbs that were trampled by hoof action. In contrast, use of height-weight charts on mesa dropseed gave a good indication of use of established plants. However, it did not take into account disappearance of dropseed seedlings. Problems encountered during this study in measuring use are aptly described by Sharp et al. (1994) in their review about use and whether it has value as a management tool. Using the height-weight technique on dropseed to indicate when to close the moderately grazed paddock to grazing and let the cows graze only on the heavy-grazed paddock worked well. But it also is obvious that the height-weight technique (table 5) did not represent the amount of dropseed biomass that disappeared.

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Literature Cited

Arnold, G. W. 1964. Factors within plant associations affecting the behavior and performance of grazing animals. p. 133–154. In: D. C. Crisp (ed.). Grazing in terrestrial and marine environments. Blackwells Scientific Pub. Adlard and Son. Limited, Dorking, England.

Bailey, D. W., J. E. Gross, E. A. Laca, L. R. Rittenhouse, M. B. Coughenour, D. M. Swift, and P. L. Sims. 1996. Mechanisms that result in large herbivore grazing distribution patterns. J. Range Manage. 49:396–400.

Beck, R. F. 1980. Impacts of grazing systems on range vegetation. In: K. C. McDaniel and C. D. Allison (ed.). Proc. Grazing Management Systems for Southwest Rangelands Symposium. Range Improve. Task Force New Mex. St. Univ., Las Cruces, New Mex.

Briske, D. D. and J. H. Richards. 1994. Physiological responses of individual plants to grazing: Current status and ecological significance, p. 147–176. In: M. Vavra, W. A. Laycock, and R. D. Pieper (ed.). Ecological implications of livestock herbivory in the West. Soc. Range Manage,. Denver, Colo.

Briske, D. D. 1991. Developmental morphology and physiology of grasses. p. 85–108. In: R. K. Heitschmidt and J. W. Stuth (ed.), Grazing management an ecological perspective. Timber Press, Inc. Portland, Ore.

De Alba Becerra, R., J. Winder, J. L. Holechek, and M. Cardenas. 1998. Diets of 3 cattle breeds on Chihuahuan Desert rangeland. J. Range Manage. 51:270–275.

Mofareh, M. M., R. F. Beck, and A. G. Schneberger. 1997. Comparing techniques for determining steer diets in northern Chihuahuan Desert. J. Range Manage. 50:27–32.

Scarnecchia, D. L., Amastasios, S. N., and J. C. Malechek. 1985. Effects of forage availability on grazing behavior of heifers. J. Range Manage. 38:177–180.

Sharp, L., K. Sanders, and N. Rimbey. 1994. Management decisions based on utilization – Is it really management? Rangelands 16:38–40.

Stuth, J. W. 1991. Foraging behavior. p. 65–83. In: R. K. Hertschmidt and J. W. Stuth (ed.). Grazing management an ecological perspective. Timber Press, Inc. Portland, Ore.

Taylor, C. A. Jr., M. H. Ralphs, and M. M. Kothmann. 1998. Vegetation response to increasing stocking rate under rotational stocking. J. Range Manage. 50:439–442.

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Printed and electronically distributed June 1999, Las Cruces, NM.