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. 2019 Feb 17;97(3):1424–1432. doi: 10.1093/jas/skz002

Sex-dependent differences for larkspur (Delphinium barbeyi) toxicosis in yearling Angus cattle1

Benedict T Green 1,✉, John W Keele 2, Dale R Gardner 1, Kevin D Welch 1, Gary L Bennett 2, Daniel Cook 1, James A Pfister 1, T Zane Davis 1, Clint A Stonecipher 1, Stephen T Lee 1, Bryan L Stegelmeier 1
PMCID: PMC6396263  PMID: 30772914

Abstract

Larkspur (Delphinium spp.) poisoning is a long-term problem for cattle grazing on rangelands of western North America. Results from preliminary experiments have suggested that differences in larkspur toxicity may exist between heifers and bulls. The objective of this study was to compare the physiological responses of yearling Angus heifers, steers, and bulls with a standardized dose of Delphinium barbeyi and to test the hypothesis that the response is sex dependent. Clinical signs of intoxication, including muscle coordination and function, were measured 24 h after oral dosing with larkspur by walking the cattle at a pace of 5 to 6 km h−1 for up to 40 min on an oval dirt track. Due to the experimental methods used, the variation in susceptibility to larkspur was not quantifiable for walking times of 0 or 40 min or more. Larkspur susceptible animals that were not able to walk (0 min; 36% of the animals) or larkspur resistant animals that walked the entire test period of 40 min (9% of the animals) resulted in censored or truncated data. The statistical methods (censReg and lmec) were used to adjust for data truncation or censoring. The heifers were only able to walk −8.9 ± 3.9 min (65.5% censored on the left) compared with 13.2 ± 3.7 min for bulls and 15.9 ± 2.7 min for steers. When heifers were compared with bulls and steers together, heifers walked 23.4 ± 4.5 min less (P < 0.0001). Serum alkaloid concentrations were measured immediately before walking, and deltaline concentrations averaged 266 ± 28, 131 ± 20, and 219 ± 28 ng mL−1 for all heifers, steers, and bulls, respectively, and serum methyllycaconitine concentrations averaged 660 ± 46, 397 ± 32, and 612 ± 34 ng mL−1 for all heifers, steers, and bulls, respectively. The relative risk of a zero walk time for yearling heifers is 330% that of yearling bulls (P = 0.0008). These results suggest that yearling Angus heifers are more susceptible to larkspur intoxication and, when possible, heifers should be kept from grazing larkspur-infested rangelands as a simple management tool to reduce the risk of fatal poisoning.

Keywords: Angus, Delphinium, methyllycaconitine, N-(methylsuccinimido) anthranoyllycoctonine, sex dependent, toxicokinetic

INTRODUCTION

Larkspurs (Delphinium spp.) have been a long-term problem for cattle grazing on rangelands of western North America (Marsh et al., 1916). Annual livestock losses on larkspur-infested rangelands may vary greatly from 2% to 5% up to 15% (Pfister et al., 1999). A simple, practical solution for larkspur poisoning is needed by western U.S. cattle ranching operations. In an effort to provide solutions for larkspur poisoning of cattle, a research program was developed to identify the genetic basis for larkspur tolerance in cattle. Initial studies focused on steers, but more recently, the emphasis has shifted to intact bulls and heifers to support selection of susceptible and resistant parents to create lines of susceptible and resistant Angus cattle which resulted in the discovery of sex-dependent effects. Larkspur alkaloids and their toxicity have been intensively studied in cattle, goat, lizard, and rodent models, but none of the studies have reported sex-dependent responses (Nation et al., 1982; Manners et al., 1991, 1995; Welch et al., 2008, 2009, 2010, 2012, 2013, 2017; Cook et al., 2011). Grazing studies have shown that naïve and young animals generally begin consuming larkspur before older, more experienced animals; however, no studies have investigated sex-dependent differences (Pfister and Gardner 1999; Pfister et al., 2011). The observations of heifer susceptibility from the selection experiment have led to the hypothesis that there are sex-dependent differences in the responses of yearling Angus cattle to larkspur poisoning. An incomplete block design consisting of steers, heifers, and bulls tested over multiple years was used to compare the responses of the three groups to an 8 mg kg−1 BW dose of N-(methylsuccinimido) anthranoyllycoctonine type (MSAL-type) alkaloids.

MATERIALS AND METHODS

The experiments were performed at the Poisonous Plant Research Laboratory, in Logan, Utah. All animal work was done under veterinary supervision with the approval of the Utah State University Institutional Animal Care and Use Committee, and the Guide for the Care and Use of Agricultural Animals in Research and Teaching, third edition, 2010, standards were followed.

Plant Material

Delphinium barbeyi in the early flowering stage was collected during July, 2003 near Manti, UH (N lat 39° 03.154, W long 111° 30.752′, collections number 03-12) at an elevation of ~3,000 m above sea level. A voucher specimen was deposited at the Utah State University, Intermountain Herbarium (#237494). The plant material was processed and stored as previously described (Green et al., 2009a); the alkaloid composition of the dried plant material is stable. The long-term storage of dried ground larkspur plant material does not modify alkaloid compositions (Cook et al., 2009).

Animals

Yearling Angus steer (n = 60; 366 ± 15 kg) responses to larkspur were tested during 2009, 2010, 2014, and 2016; some of the steer responses to larkspur have previously been published in Green et al. (2014, 2018). Yearling Angus bull (n = 33; 315 ± 6 kg) responses to larkspur were tested during 2016 and 2018, and yearling Angus heifers were tested during 2017 and 2018 (n = 30; 280 ± 11 kg). Experiment 1 (Figure 1) consisted of all animals tested before 2018. The 15 Angus bull calves and 15 Angus heifer calves used in Exp. 2 were born during the spring of 2017 and were obtained from the U.S. Meat Animal Research Center in Clay Center, NE. A timeline of this work is presented in Figure 1. For Exp. 2, the bulls and heifers were kept in the same feedlot, fed the same diet, trained to walk using a lead rope at the same time, and then tested for their responses to larkspur during the spring of 2018. One bull was lost to endotoxemia and was replaced with a yearling Angus bull raised at our laboratory to keep the group sizes equal. All cattle were maintained on alfalfa hay with free choice mineral supplement (iOFIX T-M salt block, Morton Salt, Chicago, IL).

Figure 1.

Figure 1.

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A timeline depicting the progression of research from Exp. 1, and Exp. 2 with Angus cattle. The Angus steer walk times from 2009, 2010, and 2014 were first reported in Green et al. (2014). The steer walk times from 2016 were reported in Green et al. (2018). These walk times were used in fixed and mixed model analyses as described in the Material and Methods section and presented in Table 1.

Dosing

Feed was withheld overnight before each animal was dosed with larkspur. For Exp. 1, the cattle dosed as previously described (Green et al., 2014). For Exp. 2, at ~0700 hours. Three bulls and three heifers were weighed, restrained in a squeeze chute, and orally gavaged with a standard alkaloid dose of 8 mg kg−1 BW MSAL-type alkaloids in the form of dry, finely ground larkspur in ~7 to 10 L of tap water (Green et al., 2014). This dose normally only elicits clinical signs of intoxication (muscle weakness and reluctance to walk) after moderately resistant cattle have been exercised (Green et al., 2014).

Exercise Protocol

All of the cattle in this study were obtained as weanlings and were halter broke and trained to walk using a lead rope as they matured to yearlings. This exercise protocol is similar to that of Cook et al. (2011) and Green et al. (2014). Twenty-four hours after oral dosing, groups of haltered yearlings (three bulls and three heifers) were tethered with a lead rope to a fence rail before exercise. Some cattle showed severe clinical signs of intoxication at this time such that they were not able to walk behind the tractor. Therefore, they were given a zero walk time, their halters removed and then released back into their original pen. The cattle not showing severe clinical signs were then transferred to a specially devised 2.5 m tow bar secured with a trailer tie to their halter (the quick release panic snap was attached to the halter and the trigger snap to the tow bar) and then walked behind a tractor at 5 to 6 km h−1 (3 to 4 mph) on a fenced dirt track until they exhibited larkspur-induced muscle weakness, or for a maximum of 40 min. Animals with a walk time of 40 min were classified as resistant for the purposes of this study. If an animal exhibited larkspur-induced muscle weakness, the tractor was stopped, the time to clinical signs was recorded, and the animal was quickly released using the panic snap (elapsed time, ≤30 s), after which the exercise was continued for the remaining yearlings. Any yearling which exhibited clinical signs was allowed to rest until the animal recovered sufficiently to return to their pen. This process continued until all animals exhibited clinical signs or they walked for 40 min.

Serum Alkaloid Analyses

Approximately 7 mL of blood was collected by venipuncture from the heifers just before oral dosing and then 24 h after dosing, just before walking. The blood was allowed to clot at room temperature and the serum was separated from the clot by centrifugation at 2,000 × g for 20 min at 4 °C. The serum was then stored frozen at −80 °C until chemical analysis. Chemical analysis of the serum for norditerpenoid alkaloids found in larkspur included: methyllycaconitine, and deltaline as previously described and was performed at the same time in the same laboratory (Welch et al., 2017).

Data Analysis

Walking times and serum alkaloid concentrations are expressed as the mean ± SE, and the threshold for significance was set at P < 0.05. For the analysis of serum alkaloid concentrations, a one-way ANOVA with Tukey’s multiple comparisons test in Prism version 6.03 (GraphPad Software, San Diego, CA) was used. For the analysis of weight differences between heifers and bulls, a Mann–Whitney test of untransformed data in Prism was used because the heifer weights were not normally distributed. A comparison of the walking times between heifers and bulls for Exp. 2 was done with an unpaired t-test in Prism after a square root transform of the data to equalize the variances caused by censoring of the walk times at 0 and 40 min.

Mixed and Fixed Model Analyses

Due to pen space limitations, all of the larkspur responses of the heifers, steers, and bulls were not measured during the same year. The walk times of the cattle from Exp. 1 and Exp. 2 were therefore analyzed as an incomplete block design using the R packages (R Core Team, 2013), lmec (mixed), and censReg (fixed) were used to allow comparison among bulls, heifers, and steers and adjusting for censoring of animals with 0 and 40 min walk times (Tobin, 1958; Vaida et al., 2007; Greene, 2008; Kleiber and Zeileis, 2008; Vaida and Liu, 2009). Mixed model analysis with censoring (lmec) was used to account for year of the observation as a random nuisance factor. The use of censored regression models like censReg allowed for the accounting of censoring at 0 and 40 min and an unbiased estimation of sex differences. Similar methods have been used to assess risk in drug toxicity studies and human immunodeficiency virus studies (Matos et al., 2015; Berges et al., 2018).

Risk Analysis

Relative risk was quantified as the occurrence of a risk variable after exposure compared with the likelihood of its occurrence in a control group (Andrade, 2015), and was analyzed using the MedCalc Software online (MedCalc Statistical Software version 16.4.3 (MedCalc Software bvba, Ostend, Belgium; https://www.medcalc.org)). For this analysis, the number of animals with a zero walk time was the bad outcome, and the number of animals with a >0 walk time was the good outcome. The heifers were considered the exposed group and the bulls the control group (https://www.medcalc.org/calc/relative_risk.php, accessed 8/19/2018).

RESULTS AND DISCUSSION

Experiment 1

An ongoing experiment (Exp. 1) at this laboratory to identify factors that affect larkspur susceptibility in cattle has documented differences in susceptibility for cattle breed and age (Green et al. 2014, 2018). When bulls and steers were tested during 2016, there were no differences in walk times between the 2 groups. The first observed Angus heifer responses to larkspur occurred in 2017 (Figure 1), the 15 heifers tested during that year walked 0.7 ± 0.3 min. These observations led to the Exp. 2 with age-matched heifers and bulls obtained from the U.S. Meat Animal Research Center.

Experiment 2

In Exp. 2 with Angus heifers and bulls, the heifers were more severely affected, and most could not be walked, ~70% of the heifers had a zero walk time (Figure 2a). Those that could walk had limited endurance resulting in shorter walk times. Unadjusted for censoring, the heifers in Exp. 2 walked an average of 4.1 ± 2.1 min, and the bulls walked an average of 11.7 ± 3.3 min (Figure 2a). These walk times were different by t-test (P = 0.032). However, this analysis does not take into account data censoring which is why mixed and fixed model analyses accounting for censoring (lmec, censReg) were used and the results of these analyses are described below.

Figure 2.

Figure 2.

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Walk times (a) and serum alkaloid concentrations (b) in 15 heifers and 15 bulls tested during Exp. 2. (a) A scatter plot with the mean ± SE walk times 24 h after receiving a dose of dried ground Delphinium barbeyi (8 mg kg−1 BW N-(methylsuccinimido) anthranoyllycoctonine type alkaloids). To determine the walk times, the cattle were walked behind a tractor at 5 to 6 kph on a dirt track until they exhibited larkspur-induced exercise fatigue or for 40 min. Walking times were compared with an unpaired t-test. (b) The serum concentrations of deltaline and methyllycaconitine (ng mL−1) in yearling cattle at 24 h after oral dosing and obtained just before the cattle being exercised. Serum alkaloid concentrations were compared using a one-way ANOVA with Tukey’s multiple comparisons test. Heifer and bull serum methyllycaconitine, and deltaline concentrations were not different.

Exp. 2: Serum Alkaloid Analysis

Serum samples were obtained at 24 h after oral dosing from the heifers and bulls in Exp. 2 and serum alkaloid concentrations were not different (Figure 2b). Serum deltaline concentrations at 24 h post-dosing averaged 276 ± 45 and 291 ± 47 ng mL−1, for heifers and bulls, respectively, and serum methyllycaconitine concentrations averaged 799 ± 62, and 674 ± 54 ng mL−1, for heifers and bulls, respectively. However, interpretation of the serum alkaloid concentrations must be carefully considered due to the relatively small sample size of Exp. 2, and could change if the number of animals increased. When the methyllycaconitine and deltaline concentrations are compared within each sex, for Exp. 2, they were different (P < 0.01, heifer deltaline vs. heifer methyllycaconitine; P < 0.01, bull deltaline vs. bull methyllycaconitine). This was expected, previous work at this laboratory has reported similar observations from serum toxicokinetics research. Specifically, Green et al. (2009b) have shown that the half-life of deltaline is shorter than methyllycaconitine and is eliminated from the body more quickly resulting in lower serum deltaline concentrations at 24 h after dosing (elimination half-life; deltaline, 8 ± 1 h vs. methyllycaconitine, 21 ± 4 h, P < 0.05, N = 5 Angus steers). We were particularly interested in serum alkaloid concentrations in the heifers and bulls, because recent research has shown that Angus steers eliminate larkspur alkaloids from their serum more effectively as they age (Green et al., 2018). Specifically, for 2-yr old Angus steers, there was a 99% decrease in serum deltaline concentrations and a 75% decrease in serum methyllycaconitine concentrations at 24 h post-dosing compared to when they were yearlings. These changes in serum alkaloid concentrations were associated with walking time changes from 16.0 ± 5.3 min as yearlings to 40 min as 2-yr olds. The lack of similar differences in serum alkaloid concentrations in heifers 24 h after dosing compared to bulls suggests that toxicokinetic differences between the sexes did not play a role in the reduced walk times of the heifers.

Mixed and Fixed Model Analyses

Mixed and fixed model analyses were used to analyze the walking times data from cattle tested in Exp. 1 and Exp. 2 together. This dataset included a total of 123 Angus cattle. The non-adjusted walk times of the three groups were: 2.9 ± 1.2 min (30 heifers), 16.6 ± 1.9 min (60 steers), and 15.5 ± 2.4 min (33 bulls), and are displayed in Figure 3a for comparison.

Figure 3.

Figure 3.

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Walk times (a) and serum alkaloid concentrations (b) in heifers, steers, and bulls sampled in Exp. 1 and Exp. 2 combined. (a) A scatter plot with the mean ± SE walk times of 30 heifers, 60 steers, and 33 bulls, 24 h after receiving a dose of dried ground Delphinium barbeyi (8 mg kg−1 BW N-(methylsuccinimido) anthranoyllycoctonine type alkaloids). Fixed model analysis with censoring (censReg) was used to compare heifer walk times (n = 30) to the consolidated walk times of the bulls and steers combined (n = 93). (b) The serum concentrations of deltaline and methyllycaconitine (ng mL−1) in 30 heifers, 23 steers, and 33 bulls at 24 h after oral dosing and serum obtained just before the cattle being exercised. Serum alkaloid concentrations were compared using a one-way ANOVA with Tukey’s multiple comparisons test. The serum methyllycaconitine concentrations for both heifers and bulls were significantly different from the steers (P < 0.01). Heifer and bull serum methyllycaconitine concentrations were not different.

Mixed model analysis with censoring (R package, lmec) was used as an initial analysis (model 1, Table 1). The variance in walk times due to year was small, so we adjusted for both left and right censoring with a fixed model (R package, censReg) including only sex (heifers, bulls, and steers) in the model (models 2 and 3, Table 1). The walk times between bulls and steers were not different in any of the three models used to compare walk times so bulls and steers were combined for models 2 and 3.

Table 1.

Mixed and fixed model analysis of Angus cattle walk times from Exp. 1 and Exp. 2 combined, presented as adjusted walk times

Analysis Heifer walk times1 (min)n = 30 Bull walk times1 (min)n = 33 Steer walk times1 (min)n = 60 Estimated differences between heifers vs. bulls and steers (min) P-value heifers vs. bulls and steers Estimated differences between bulls vs. steers (min) P-value bulls vs. steers walk times
Model 1 −7.26 ± 3.23 13.03 ± 3.35 14.89 ± 2.48 −21.22 ± 3.83 1.58 × 10−7 −1.86 ± 4.07 0.65
Model 2 −7.16 ± 3.35 12.77 ± 3.19 14.94 ± 2.35 −21.02 ± 3.83 1.90 × 10−7 −2.17 ± 3.95 0.58
Model 3 −8.89 ± 3.90 13.18 ± 3.68 15.93 ± 2.72 −23.44 ± 4.47 5.94 × 10−7 −2.75 ± 4.57 0.55
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1

The variation in walk times was not quantifiable for times of 0 or 40 min resulting in censored data. Censoring of data for extreme animals reduces variation, and adjusting for this censoring (censReg and lmec) increased variation causing negative mean walking times for the heifers.

Model 1 left censored at zero; year fit as random, analyzed using lmec.

Model 2 left censored at zero; year was not fit, analyzed using censReg.

Model 3 left censored at zero, right censored at 40; year was not fit, analyzed using censReg.

Due to the experimental methods used cattle that were unable to be exercised were given a 0 walk time, and resistant cattle were stopped from walking after 40 min. However, not all animals which walked 0 min were the same. For example, some animals exhibited clinical signs of intoxication in their pen at 24 h after dosing and were not disturbed. Others walked to an area behind a squeeze chute where they exhibited severe clinical signs, and some exhibited clinical signs while being haltered or led to the dirt track for exercise. All of these cattle were classified as susceptible and scored 0 min for their walk time resulting in censored or truncated data. Adjusting for this censoring increased the variation among animals as well as the means of the groups, causing negative estimates of mean walking times for the heifers which were quantified in minutes similar to the measured walking time (Table 1). Using these statistical methods, heifers were only able to walk −8.89 ± 3.90 min compared with 13.18 ± 3.68 min for bulls and 15.93 ± 2.72 min for steers. Heifers walked 23.44 ± 4.47 min less than the average for bulls and steers (P = 5.94 × 10–7), while walking times for bulls and steers did not differ (P = 0.55). Based upon these analyses, the negative walk times of the heifers indicated that they were very susceptible to larkspur intoxication.

In previous larkspur experiments at this laboratory with Angus steers, Holstein steers, Line 1 Hereford steers, and mixed breed heifers, none of the cattle exhibited the severe clinical signs of intoxication observed in this experiment with these Angus heifers (Green et al., 2009a, 2009b, 2011, 2012, 2013b, 2018).

Exp. 1 and Exp. 2: Combined Serum Alkaloid Concentrations

To increase sample sizes for the statistical comparison of serum alkaloid concentrations, heifers, bulls, and a subset of the steers from Exp. 1 and Exp. 2 were analyzed together (Figure 3b). The cumulative serum deltaline concentrations averaged 266 ± 28, 131 ± 20, and 219 ± 28 ng mL−1, and serum methyllycaconitine concentrations averaged 660 ± 46, 397 ± 32, and 612 ± 34 ng mL−1 for 30 heifers, 23 steers, and 30 bulls, respectively (Figure 3b). Heifers and bulls had larger serum methyllycaconitine concentrations than did steers (Figure 3b; P < 0.01). These combined results from Exp. 1 and Exp. 2 with larger numbers of cattle suggests there were no significant differences in serum alkaloid concentrations between the heifers and bulls. This experiment was the first to orally dose large numbers of heifers, steers, and bulls with dried ground larkspur and measure their serum alkaloid concentrations at 24 h after dosing. Previous work at this laboratory has documented differences in methyllycaconitine elimination half-life values between Holstein steers (11.8 h, Green et al., 2013b) and Angus steers (15.6 h, Green et al., 2012). We have no explanation as to why the serum alkaloid concentrations differ in Angus steers compared with Angus bulls and Angus heifers.

Serum Alkaloid Concentrations and Sex

There were no differences in serum alkaloid concentrations between heifers and bulls from either Exp. 1 or Exp. 2. There are many well-documented toxicokinetic differences between males and females in the literature. For example, there is sex-dependent expression of genes coding for xenobiotic metabolizing and transport enzymes, and differences in male and female BW, and body fat, all of which affect xenobiotic toxicokinetics (Zanger and Schwab, 2013). In this research, there was a difference between the body weights of the heifers and bulls (P = 0.0077, 30 heifers, 33 bulls). However, this difference does not appear to have affected the toxicokinetics of the larkspur alkaloids between heifers and bulls because there were no differences in serum alkaloid concentrations between the sexes (Figure 3b). This suggests that an alternative mechanism is responsible for the sex dependent decrease in walking times and that larkspur alkaloid metabolite concentrations in serum are not a factor in toxicosis.

One possible explanation for the differences other than toxicokinetics could be differential expression of nicotinic acetylcholine receptors (nAChR), the receptors to which larkspur alkaloids bind (Green et al., 2013a). For example, Welch et al. (2009) identified nAChR involvement in differential responses to methyllycaconitine with 10 different inbred mouse strains. In that study, median lethal dose (LD50) values for methyllycaconitine in the 10 strains were determined, with the A/J mouse strain being most susceptible (LD50 = 3.3 mg kg−1) and the 129 mouse strain being most resistant (LD50 = 5.8 mg kg−1). When the protein expression of nAChR subunits was compared, the 129 mouse strain had twice the amount of α7 nAChR subunit expression and significantly greater (P < 0.05) expression of α3 and α5 nAChR subunits than the A/J strain. It was concluded that the susceptibility of A/J mice to methyllycaconitine may be due to a lower expression of nAChR subunits. It is tempting to speculate that there may have been differences in nAChR expression in the yearling heifers which increased their sensitivity to larkspur alkaloids and that these receptor differences are estrogen dependent. Researchers have documented sex-dependent differences in the vasodilatory effects of nicotine in the kidneys of female rats which was abolished by the estrogen-receptor antagonist tamoxifen and mediated by changes in the expression of α7 nAChRs (El-Mas et al., 2011). There are also sex-dependent differences in the expression of β2 nAChR subunits in humans which may be responsible for sex differences in nicotine and tobacco smoking use (Cosgrove et al., 2012). Further research is needed to determine whether the sex-dependent differences in cattle are due to estrogen-dependent modulation of nAChR expression or whether there is another mechanism behind the differences.

Risk Analysis

In addition to measuring the walk times and serum alkaloid concentrations, the relative risk of a zero walk time was calculated for heifers and bulls. There were 21 zero walk time heifers vs. seven zero walk time bulls (Figure 3a). Heifers relative to bulls were associated with a 3.3-fold increased relative risk of a zero walk time (P = 0.0008, 95% CI 1.6 to 6.6, 30 heifers vs. 33 bulls). This greater relative risk emphasizes the importance of careful management needed when grazing heifers on larkspur-infested rangelands.

Grazing Management

Current management options for grazing larkspur-containing pastures are based on the relative palatability to cattle, plant growth stage, and the concentration of toxic plant alkaloids without consideration of cattle sex, breed, age, or other susceptibility factors (Gardner et al., 2002, 1997; Gardner and Pfister 2009; Green et al., 2014). Work by Pfister et al. (1999) has identified a toxic window indicating when cattle should not be grazed on larkspur-infested rangelands (Figure 4). This window was developed primarily based on grazing studies with steers, and also with the assumption that all cattle respond to larkspur in a similar manner. The results presented in this manuscript suggest that yearling heifers must be more carefully managed than either bulls or steers. However, it is currently not known what might be the width (i.e., number of weeks) of the “toxic window” for heifers during which they should be removed from larkspur-containing pastures and when late-season grazing would be safe to begin. In addition, plant factors such as maturity and toxicity of the larkspur must be considered. Even so, it seems reasonable to assume that the time frame for the toxic window will expand when considering grazing highly susceptible Angus heifers on larkspur-infested rangelands. Clearly, the toxic window management recommendation needs further refinement for specific populations of cattle such as replacement heifers. Increasing the specificity of larkspur management recommendations using genetic markers is a long-term goal of this research, marker-based screening like that used in human personalized medicine (Abrahams, 2008), would allow for the identification of the right animal, for the right larkspur-infested pasture, at the right time.

Figure 4.

Figure 4.

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The Delphinium toxicity and palatability relationship in cattle from Pfister et al. (1999) which is used as management guide for toxic larkspur-infested pastures. The toxic window is the growth stage of the plant when the concentration of toxic alkaloids is sufficient to intoxicate cattle, and larkspur consumption by cattle begins to increase due to increased palatability. Currently, there are low-risk grazing windows early in the season before flowering and late in the grazing season after the seed pods shatter. It is not yet known what the toxic window would be for extremely susceptible cattle like yearling Angus heifers, but it is reasonable to assume that the time period for the toxic window would expand for this highly susceptible population of cattle.

Summary

The results from this research suggest that yearling Angus heifers are the most larkspur-susceptible cattle of any tested to date. There were no differences in serum alkaloid concentrations between the heifers and bulls at 24 h after oral dosing which suggests that larkspur alkaloid toxicokinetics are similar between the two groups of animals. This research indicates that Angus heifers should be kept from larkspur-containing rangelands during the toxic window as a simple management tool to reduce the risk of larkspur poisonings.

ACKNOWLEDGMENT

The authors wish to thank Isabelle McCollum, Rex Probst, Jessie Roper, Kermit Price, Ed Knoppel, Chuck Hailes, and Scott Larsen for their expert technical support of this research.

Conflict of interest statement. None declared.

Footnotes

1

This research was supported by United States Department of Agriculture, Agricultural Research Service.

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