Conditioning cattle to graze broom snakeweed () M. H. Ralphs and R. D. Wiedmeier

J Anim Sci 2004. 82:3100-3106.

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Downloaded from jas.fass.org by on September 29, 2009. Conditioning cattle to graze broom snakeweed (Gutierrezia sarothrae)

M. H. Ralphs1 and R. D. Wiedmeier

ARS, USDA Poisonous Research Laboratory, and Animal, Dairy, and Veterinary Science Department, State University, Logan 84341

ABSTRACT: Broom snakeweed (Gutierrezia sar- spring grazing trial, there was no snakeweed consumed othrae) is the most widespread range weed in North in the free-ranging part of the trial; however, when America. We attempted to positively condition cattle to the pasture size was decreased, the heifers started to graze broom snakeweed to create a biological tool to consume snakeweed as alternative forages became less decrease the competitive ability of snakeweed in a plant abundant. In the second small pasture trial, heifers in community. Fifteen yearling heifers were divided into the positively conditioned group consumed more snake- weed than those in the control group (16 vs. 5% of bites, three treatment groups receiving different supple- P < 0.001). In the fall grazing trial, little snakeweed was ments: 1) cornstarch, 2) starch with ground snakeweed, consumed in the free-ranging part of the trial. When the and 3) a control (no supplements). Heifers were fed pasture size was decreased, both positively conditioned fresh snakeweed, and then were gavaged with the re- and control groups increased snakeweed consumption spective supplements to provide positive feedback to up to 35% of bites. In the small pastures of both the enhance their acceptance of snakeweed. The starch spring and fall grazing trials, 36 to 59% of snakeweed group consumed more snakeweed in the pen condition- were grazed. Cattle can be forced to graze snake- ing trial (P = 0.02). The starch and control groups were weed in a short-duration, high-intensity grazing then taken to the field for two grazing trials. In the strategy.

Key Words: Broom Snakeweed, Diet Training, Gutierrezia sarothrae, Positive Conditioning, Short-Duration Grazing

2004 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2004. 82:3100–3106

Introduction ungulates (Pieper, 1989) and is toxic to livestock, caus- ing abortions (Dollahite and Anthony, 1957). Platt Broom snakeweed (Gutierrezia sarothrae (Pursh) (1959) ranked this range weed as one of the most unde- Britt. & Rusby) is the most widespread range weed sirable plants in the various regions of the West. in North America. Early investigations associated the Part of the problem of rangeland degradation has increase of broom snakeweed with overgrazing, which been the role of selective livestock grazing, which can decreased desirable vegetation and allowed broom suppress desirable species in the plant community and snakeweed to increase where it already existed and to free up resources for undesirable weedy species. To invade deeper soils and more productive sites (reviewed counteract this, differential grazing habits, prefer- by McDaniel and Torrell, 1987). However, snakeweed ences, and selective abilities of livestock species may has even increased in healthy plant communities in the allow them to exert selective grazing pressure against absence of grazing (Chew, 1982; Hennessy et al., 1983). some weeds that would result in effective control Other disturbances (such as fire, drought, and chaining) can also cause an increase in snakeweek density (USFS, (Brock, 1988). Furthermore, livestock may be positively 1937; Parker, 1939; Arnold et al., 1964). Broom snake- conditioned to graze weeds (Quimby et al., 1991). The weed is very competitive with desirable grasses and diet selection theory of Provenza (1995) states that ani- greatly suppresses forage production (Ueckert, 1979; mals learn which plants to eat or to avoid based on McDaniel et al., 1982). It is not palatable to most large postingestive consequences. Foods that provide ade- quate nutrients are preferred, whereas those that cause nausea or illness are avoided. The objective of this ex- periment was to determine whether cattle could be posi- 1 Correspondence: 1150 E. 1400 N. (phone: 435-752-2941; e-mail: tively conditioned to graze broom snakeweed by dosing [email protected]). Received January 9, 2004. them with cornstarch to provide positive energy Accepted June 16, 2004. feedback.

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Downloaded from jas.fass.org by on September 29, 2009. Conditioning cattle to graze snakeweed 3101 Materials and Methods of tap water and pumped through the stomach tube in the starch and starch plus snakeweed treatments. In Effects of Starch on Ruminal pH the starch plus snakeweed treatment, dry snakeweed was finely ground and mixed with 4 L of water and The objective of this preliminary trial was to deter- pumped into the rumen. Heifers were given 100 g of the mine the amount of starch that could be fed without ground snakeweed the first day, then 200 g thereafter. causing acidosis. Clinical signs of acidosis start to occur Controls received 2 L of tap water. The starch dose when ruminal pH falls below 5.5 (the normal range increased daily from 5% of the NEm requirement the of ruminal pH is 6.5 to 7.0; Huntington, 1998). Two first day, to 10%, then 15% for 3 d, and 20% for 3 d. ruminally cannulated Holstein steers (981 kg) were fed Following the 8-d conditioning phase, acceptability was a maintenance diet of chopped alfalfa (17.6% CP, 1.14 tested on d 10 in a single-choice trial. Acceptability of Mcal/kg NEm; as-fed basis) and then dosed with increas- snakeweed was tested again on d 32 and 33 following ing amounts of starch through the ruminal cannula (0 the spring field grazing trail and again on d 94 before to 25% NEm requirements). Starch was mixed with tap the fall grazing trial. The quantity of snakeweed con- water in 5% increments of NEm requirements (0, 155, sumed was a measure of its palatability. 310, 619, 929, 1,239, 1,450 g), and steers were dosed with a higher rate each day for 7 d. Ruminal fluid was siphoned through a tube in the cannula, and ruminal Spring Grazing Trial pH was measured before dosing and subsequently at The starch (positive condition) and control groups 1, 2, 4, 6, and 8 h after dosing. The daily ration of hay were taken to snakeweed-infested rangeland for field was fed following dosing with starch, and intake was grazing trials. The first study site was in the Howell monitored as an indication of illness caused by acidosis. Valley, 15 km west of Tremonton, UT (long 41.724°949′N, lat 112.408°959′W). It was a degraded Conditioning Trial sagebrush site dominated by snakeweed, Sandberg Fifteen Angus and Hereford × Angus heifers that had bluegrass (Poa sandbergii), and cheatgrass (Bromus no previous exposure to snakeweed were stratified into tectorum). Cheatgrass had matured and dried up, and three weight groups (333, 363, and 380 kg; BCS = 6.0), Sandberg bluegrass was senescent. Remnant perennial bunchgrasses, such as needle-and-thread (Stipa com- and the NEm was calculated for each group (6.0, 6.4, and 6.6 Mcal/d; NRC, 2000). Within each weight group, ata) and crested wheatgrass (Agropyron cristatum) heifers were allocated randomly to three treatment were still green. Scarlet globemallow (Sphaeralcea coc- groups to receive the supplemental energy: 1) starch, cinea) was the principal green forb, but red-stem filaree 2) starch plus ground snakeweed, or 3) control that (Erodium cicutarium) was mature. Snakeweed stems received no supplements. For the second treatment, were fully extended but still vegetative. ground snakeweed was added to starch in an effort to A 5.4-ha field was divided into two pastures with increase acceptance of snakeweed by the heifers. We electric fencing. The two groups of heifers were ran- reasoned that snakeweed in the rumen and exposure domly allocated to a pasture, and then the pastures to its taste during regurgitation and chewing of the cud were switched each day to minimize pasture bias. Diets may enhance their familiarity to it. were estimated by a bite count technique (Lehner, The heifers were fed the same chopped hay as the 1987). Each heifer was observed for 5-min periods and Holstein steers in the preliminary trail. Intake was the number of bites of the following forage classes was restricted to 80% of the NEm requirement to ensure recorded: perennial bunchgrasses that were still green, they were hungry and motivated to eat, and to put them dry annual grasses, forbs, and snakeweed. Observa- in a negative energy balance to enhance their response tions were made during the early morning and late to supplemental starch. evening grazing periods. Three to seven observations The heifers were trained to eat from locking stan- were made on each heifer daily. The positive condi- chions for individual feeding. Snakeweed was picked tioned group received 0.5 kg/heifer of steamflaked corn fresh each morning and individually offered to each each morning and night to enhance the metabolism and heifer for the 8-d trial. Ten- to fifteen-centimeter excretion of terpenes. Green grass was present at the lengths of the new growth were clipped from the plants beginning of the trial to provide adequate protein. Ade- and offered intact in amounts beginning at 100 g and quate energy and protein are required for detoxification increasing up to 700 g to ensure the heifers did not and excretion of terpenes (Foley et al., 1995). The study run out of snakeweed during the 10-min offerings. The was conducted from June 9 to 24. quantity of snakeweed was weighed and refusals were After 8 d and no snakeweed consumption, the pasture weighed back. Consumption was calculated by dif- sizes were decreased to 0.4 ha in an attempt to quickly ference. utilize the available forage and force the cattle to graze Following feeding, heifers were restrained in a snakeweed. There was enough herbaceous forage exclu- squeeze chute and gavaged by stomach tube with the sive of snakeweed to last the heifers for 4 d. A second respective treatments. The starch was dissolved in 2 L set of small pastures (0.4 ha each) was fenced to repeat

Downloaded from jas.fass.org by on September 29, 2009. 3102 Ralphs and Wiedmeier the forced grazing trial. During these intensive grazing Statistical Analyses -trials, both groups received 0.5 kgؒheifer−1ؒd−1 of steam flaked corn each morning and were offered a Crystalyx In the preliminary starch trial, individual steers were 14% (Ridley Block Operations, Mankato, MN) CP/mo- the experimental units. Ruminal pH was compared lasses supplement to enhance digestion (Cook and Har- among the starch doses over the time after dosing by ris, 1968) and detoxification (Foley et al., 1995) of snake- a repeated-measures mixed ANOVA using compound weed. The Crystalyx tub was weighed before and after symmetry covariance structure (SAS Version 8e; SAS the trial and the difference was divided by the number Inst., Inc., Cary NC). The difference between treatment of days and heifers in each group to estimate daily doses was determined by linear contrasts. In the pen conditioning and field grazing trials, individual heifers consumption. Crystalyx consumption averaged 0.5 − − were the experimental units. Snakeweed consumption .kgؒheifer 1ؒd 1 during the 8-d pen conditioning trial was compared Fall Grazing Trial among the three supplement groups with repeated- measures mixed ANOVA using compound symmetry The trial was conducted near Bothwell, 10 km north- covariance structure. Differences among supplement west of Tremonton, UT (long 41.744°483′N, lat treatments were determined by linear contrasts. 112.248°698′W). The site was a degraded big sagebrush Snakeweed intake during each of the subsequent pref- site (5% canopy cover) dominated by dry cheatgrass, erence tests on d 10, 32 and 33, and 94 was compared jointed goatgrass (Aegilops cylindrica), and medu- among groups by mixed model ANOVA. Snakeweed sahead (Taeniatherum caput-medusae). Purple three- consumption of the treatment groups in the grazing awn (Aristida purpurea) occurred on a ridge in the cen- trials was analyzed by repeated-measures mixed AN- ter of the pastures and its regrowth was the only OVA comparing locations, treatment groups, and their green forage. interaction. The location × group interaction was sig- The pasture design was similar to the spring grazing nificant (P < 0.001); therefore, the model was reduced trial. A 5.1-ha area was divided into two pastures. The and the locations were analyzed separately by compar- same heifer groups were allocated randomly to a pas- ing treatment groups and periods in a repeated-mea- ture, after which the pastures were switched each day sures ANOVA over days within the periods. The periods to decrease pasture bias. Diets were quantified by bite were 1) large pasture with unlimited grass, 2) small counts. After 4 d, there was limited consumption of pasture with limited grass, and 3) small pasture with snakeweed, so the pasture size was decreased to 0.85 ha limited grass. Standing crop of forage classes was ana- to create a forced grazing trial. There was only enough lyzed using a mixed model ANOVA comparing locations herbaceous forage for4dtotrytoforce the heifers to and pastures within locations. Standing crop at the graze snakeweed. A second small-pasture forced-graz- beginning and end of the studies was compared by lin- ing trial (0.85 ha each pasture) was started ond8and ear contrasts. Density of snakeweed plants was com- lasted 4 d. Both groups of heifers were offered 0.5 kg pared between the locations and among pastures within of corn each morning and had access to the Crystalyx locations by mixed model ANOVA. The number and protein supplement. The heifers consumed 0.9 percentage of grazed snakeweed plants in the small kgؒheifer−1ؒd−1 of the 14% CP molasses supplement de- intensively grazed pastures were compared between scribed previously. locations and treatment groups by mixed-model ANOVA. Standing Crop, Snakeweed Density, and Nutrient Content Results Standing crop was estimated in each of the large Preliminary Starch Trial pastures at the beginning of both the spring and fall There was a general decline in ruminal pH during trials, and in each of the small pastures at the end of the day (P < 0.001), indicating a buildup of VFA as the the trials. Ten 0.125 m × 0.5 m quadrats were systemati- hay digested (Figure 1). There was no difference in pH cally placed through each pasture at 10-m intervals. between the starch rates until the 20% NE level was The following forage classes were clipped at ground m reached. Only the high doses of 20 and 25% NE dif- level: green grass, dry grass, forb, snakeweed, and lit- m fered from the other rates (P = 0.03). However, even ter. Samples were dried in a forced air oven at 60°C for when the pH dropped to 6.2 to 6.4 at these high rates, 48 h, weighed and analyzed for CP by means of total the steers did not decrease hay intake or show any N (Leco model FP-528; Leco Corp., St. Joseph, MI), and signs of acidosis. Because the starch levels gradually NDF (Ankom200 fiber analyzer; Ankom, Fairport, NY). increased at 5% increments, the ruminal microbes Snakeweed density was estimated at the end of each seemingly adjusted without causing illness. seasonal trial. Twenty 1-m2 quadrats were systemati- cally placed at 10-m intervals along paced transects Pen Conditioning Trial throughout the pastures, the number of snakeweed plants grazed and ungrazed was counted in each quad- The starch group consumed more snakeweed (182 ± rat, and the percentage of grazed plants was calculated. 25 g compared with 93 ± 27 g for the control group and

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Figure 2. Snakeweed consumed by heifers in three sup- plemental energy groups during the 8-d pen conditioning Figure 1. Effect of increasing starch (as a percentage of trial, and in subsequent tests without reinforcement NEm) on ruminal pH in a preliminary experiment. from starch.

74 ± 30 g for the starch plus ground snakeweed group; < P = 0.02). There was also a day effect (P 0.001), indicat- ferred the limited amount of bunchgrasses that were ing that all groups increased snakeweed consumption still green (22% of bites) and the forbs (14% of bites), over time. However, the increase was not continuous but dry cheatgrass and sandberg bluegrass dominated (Figure 2). Following an increase in snakeweed con- the diets (64% of bites; Table 2). sumption one day, intake would drop the next. No dif- In Period 2, when the pasture size was decreased, ferences were detected for acceptance of snakeweed on heifers increased consumption of snakeweed as the test d 10 after conditioning, on d 32 or 33 after the other forage diminished (Figure 3), but there was no grazing trial, or on d 94 before the fall grazing trial. difference between treatment groups (7% of bites). There was a difference in snakeweed consumption Standing Crop and Snakeweed Density between treatment groups in Period 3 (P < 0.004). The in the Field Grazing Trials positively conditioned group consumed snakeweed for There was no difference in total standing crop be- an average of 16% of bites compared with the control tween the two locations at the beginning of the studies group at 5% of bites. There was also a group × day (Table 1). However, snakeweed standing crop was interaction (P = 0.001). The positively conditioned group greater at Bothwell (P = 0.02), and dry grass was increased snakeweed consumption over time (up to 34% greater at Howell (P = 0.01; Table 1). There was a of bites), whereas the control group varied their con- difference in standing crop of all forage classes between sumption between days without any distinct trend. the beginning and end of each study (P < 0.02), but no differences were detected in standing crop between the Fall Grazing Trial pastures at the end of the small pasture trials in Period 2 and 3. In the large pasture grazing trial in Period 1, the Snakeweed density was three times greater at Both- positively conditioned group consumed more snake- 2 < well than at Howell (10.4 vs. 3.2 plants/m ; P 0.001). weed than the control group (5 vs. 1% of bites, P = 0.03). The number of snakeweed plants grazed was also There was also a group × date interaction (P = 0.02), greater at Bothwell (3.8 vs. 1.85/m2, P < 0.001); how- indicating that the positively conditioned group in- ever, the percentage of plants grazed was greater at creased snakeweed consumption over time, whereas the Howell (59 vs. 36%) because of the greater density at control group ate very little over the 4-d trial (Figure Bothwell. There was no difference between the treat- 4). In the two small intensively grazed trials in Periods ment groups in the number or percentage of snakeweed 2 and 3, there were no differences in snakeweed con- plants grazed. sumption between groups. However, snakeweed con- Spring Grazing Trial sumption increased in both groups over time as alterna- tive forages were depleted, and peak consumption During Period 1 in the larger pastures, there was reached 35% of bites. Dry cheatgrass averaged 82% little snakeweed consumed (Figure 3). The heifers pre- of diets.

Downloaded from jas.fass.org by on September 29, 2009. 3104 Ralphs and Wiedmeier Table 1. Standing crop of forage classes at the beginning and end of grazing at two lo- cations

Location Time Green grass Dry grass Forb Snakeweed Total

kg/ha Howell Begin 76 ± 25 389 ± 51a 43 ± 17 308 ± 79b 815 ± 115a End 0** 141 ± 21** 0 112 ± 29** 254 ± 31** Bothwell Begin 0 235 ± 28b 0 612 ± 96a 847 ± 112a End 0 181 ± 11** 0 337 ± 31** 519 ± 35** **Standing crop of all forage classes differed from beginning to end of the trials (P < 0.01). a,bWithin a forage class column, standing crop values at the beginning of each trial that do not have a common superscript letter differ (P < 0.01).

Nutritional Content of Forage take would decrease the next. All three groups exhibited this trend, although not all on the same days. Perhaps Snakeweed averaged 40% DM during the pen condi- the heifers were experiencing negative feedback from tioning trial. During the spring grazing trial, snake- the snakeweed when it was consumed at high levels. weed averaged 8.6% CP and 48% NDF (DM basis). The ground snakeweed added along with starch in Green grasses averaged 8.3% CP and 69% NDF, and Treatment 2 did not increase acceptance of snakeweed. the dry grasses averaged 5.6% CP and 73% NDF. This group was gavaged with 100 g of ground snakeweed Snakeweed was comparable to green grass in CP, but the first day, and then 200 g each day thereafter. The was lower in fiber. In the fall grazing trial, CP content DM content of the fresh picked snakeweed was 40%, so of snakeweed was 6.9% and NDF was 53% compared this group would have received a total of 230 g of air- with dry cheatgrass (4.8% CP and 70% NDF). Snake- dried snakeweed. In comparison, the starch group con- weed was higher in nutrient quality than the poor qual- sumed an equivalent of 120 g of air-dried snakeweed at ity cheatgrass, but its secondary compounds seemingly its peak. It seems that the deleterious effects of snake- rendered it unpalatable. weed overcame the positive effects of starch in the starch plus ground snakeweed group and may have contributed Discussion to the cyclic consumption patterns in the other groups. Individual terpenes in tarbush (Estell et al., 2002) and Heifers were positively conditioned to increase con- sagebrush (Dziba, 2004) reduced or stopped feed intake sumption of snakeweed in the pen conditioning trial. in sheep. Diterpene acids in ponderosa pine, which are However, consumption of snakeweed was cyclic, follow- similar to snakeweed (Gardner et al., 1999), adversely ing an increase in snakeweed consumption one day, in- affect ruminal microbial populations (Weidmeier et al.,

Figure 3. Snakeweed consumption between the posi- Figure 4. Snakeweed consumption between the posi- tive-conditioned and control groups of heifers at Howell tive-conditioned and control groups of heifers at Bothwell during spring grazing trial. Period 1 was in large pastures during fall grazing trial. Period 1 was in large pastures with unlimited grass. Periods 2 and 3 were subsequent with unlimited grass. Periods 2 and 3 were subsequent trials in small pastures as alternative forages became less trials in small pastures as alternative forages became less abundant and heifers were forced to graze snakeweed. abundant and heifers were forced to graze snakeweed.

Downloaded from jas.fass.org by on September 29, 2009. Conditioning cattle to graze snakeweed 3105 Table 2. Forage classes in cattle diets with different seasons and pasture sizes at two locations

Location/season Period/pasture size Group Green grass Dry grass Forb Snakeweed Litter

% of bites Howell 1/Large Positive 22 ± 1.9 65 ± 2.2 12 ± 1.3 1 ± 0.3 — Spring Control 21 ± 2.5 64 ± 2.7 14 ± 1.8 0 — 2/Small Positive 11 ± 1.6 77 ± 1.9 5 ± 1.0 7 ± 1.3 — Control 7 ± 1.0 79 ± 2.3 6 ± 1.3 8 ± 1.7 — 3/Small Positive 2 ± 0.4 76 ± 2.1 6 ± 1.0 16 ± 2.1a — Control 9 ± 20 80 ± 2.3 4 ± 0.8 5 ± 1.2b — Bothwell 1/Large Positive 6 ± 2.5 89 ± 2.7 — 5 ± 1.4a — Fall Control 3 ± 1.9 96 ± 1.9 — 1 ± 0.2b — 2/Small Positive — 75 ± 3.7 — 17 ± 3.4 8 ± 1.7 Control — 65 ± 5.2 — 22 ± 4.5 12 ± 3.4 3/Small Positive — 80 ± 3.0 — 16 ± 2.7 4 ± 1.1 Control — 85 ± 2.5 — 9 ± 1.7 6 ± 1.3

a,bWithin a column and period, means that do not have a common superscript letter differ (P < 0.001).

1992) and N intake and digestibility (Pfister et al., 1992). brush production by 20% and increased grass and forb Secondary compounds in snakeweed may act directly by production 30% in Idaho (Laycock, 1967). In the live oak decreasing palatability, reduce ruminal microbial popu- savanna of west , Taylor et al. (1980) reported that lations and digestibility, or physiologically reduce heifers heavily browsed mesquite, live oak, sacahuista, intake. and prickly pear at the end of 21-d grazing periods in Although the starch group consumed more snakeweed a high-intensity, low-frequency grazing system. At the during the pen conditioning trial, there was no difference same location, a short-duration grazing system using 3- in snakeweed intake between the groups in the subse- d grazing periods caused cattle and sheep to heavily quent test days. In fact, snakeweed intake decreased graze sachauista and prickly pear (Ralphs et al., 1990), over time. Without the positive reinforcement from the which decreased the size and production of sacahuista starch treatments at the time snakeweed was consumed, by 40% after 5 yr. On shortgrass prairies of Colorado, its palatability or acceptance decreased. heavy grazing intensity eliminated half shrubs, such as Our results suggest that energy from starch supple- broom snakeweed, fringed sagewort, and slender eriogo- ments enhanced the initial acceptance of snakeweed in num (Hart, and Ashby, 1998). the pen, but did not result in long-term preference for Snakeweed contains a variety of diterpene acids, fla- it when the heifers were taken to snakeweed-infested vones, and saponins (Dollahite et al., 1962; Roitman et rangeland. When other forage was available in the field al., 1994). However, there were no clinical signs of intoxi- grazing trials, the heifers selected grasses and forbs in cation in the intensive grazing trails, even though our preference to the snakeweed. The positively conditioned heifers consumed snakeweed for up to 35% of their bites. group consumed more snakeweed than the control group The relatively large amount of snakeweed consumed in in Period 3 of the small intensively grazed trial at How- the intensive grazing trials contrasts with the relatively ell, and in the larger pasture in Period 1 at Bothwell. small amount that apparently limited intake in the pen However, the lack of consistency makes it difficult to conditioning trial and tests. Perhaps the dry grass con- conclude that positively conditioning with starch will sumed along with snakeweed and the extended grazing reliably increase consumption of snakeweed, even under time allowed for dilution of the secondary compounds. high grazing pressures. Other studies have conditioned The supplemental energy and protein also might have preferences for unpalatable foods under controlled condi- enhanced detoxification. tions in pens using energy sources such as glucose (Bur- ritt and Provenza, 1992; Ralphs et al., 1995), propionate Implications (Villalba and Provenza, 1996, 1997a), starch (Villalba and Provenza, 1997b), and supplemental N (Villalba and Cattle increased consumption of snakeweed when it Provenza, 1997c). However, this test of positive condi- was paired with supplemental energy from starch in a tioning under field grazing conditions was not successful. pen conditioning trial. However, when taken to snake- Grazing pressure was the overriding factor that forced weed-infested rangeland, they refused to graze snake- our heifers to graze snakeweed in the small intensively weed under free-ranging conditions. This finding sug- grazed pastures. Other studies reported that increasing gests that supplemental energy can enhance acceptance stock density with an accompanying increase in grazing of snakeweed when no other choices are available, but pressure increased use of less palatable shrubs such as preference for snakeweed was not maintained in the shadscale, black sage (Pieper et al., 1959), big sagebrush, field when alternative forages were available. When the and rabbitbrush (Cook et al., 1962) on Utah’s western pasture size was decreased and alternative forages were desert. Heavy fall grazing by sheep decreased big sage- depleted, heifers were forced to graze snakeweed. Short-

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