Crabgrass Integration in New Jersey Horse Pastures

Maintaining horses on pasture can be beneficial for financial and environmental sustainability of equine operations. Providing pasture access can reduce costs associated with supplemental feeding as well as the potential for nutrient leaching, which can occur when horses are confined in dry lots with concentrated manure accumulation and low levels of vegetative cover. Horse health and well-being may also be positively impacted by pasture access, which provides more natural feeding conditions and promotes more voluntary exercise in comparison to confinement in stalls or dry lots.

However, cool-season grasses commonly utilized as pasture forages in temperate regions of the U.S. often undergo a period of low-productivity during hot, dry summer months, which is commonly termed the "summer slump." Reduced forage growth during the summer slump can lead to overgrazing and create management challenges for horse producers. In contrast to cool-season grasses, growth of warm-season grasses is most vigorous during this same summer slump period.

Therefore, a grazing management strategy that incorporates warm-season grasses into traditional cool-season equine rotational grazing systems has the potential to bridge the forage gap during the summer slump and increase season-long pasture production.

Cool-Season vs. Warm-Season Grasses

Pasture forages in temperate regions of the U.S. are mainly perennial cool-season grasses such as orchardgrass [Dactylis glomerata (L.)], tall fescue [Lolium arundinaceum (Schreb.) Darbysh.], Kentucky bluegrass [Poa pratensis (L.)], timothy [Phleum pratense (L.)], and perennial ryegrass [L. perenne (L.)]. These grasses possess a C₃ photosynthetic system that functions most efficiently between 60°F and 75°F, which corresponds to temperature ranges during cooler periods of the growing season in the north-central to northern U.S. (i.e., late spring to early summer and fall). Under the elevated temperatures that occur during the summer months, photorespiration for these C₃ plants increases and carbon fixation declines, leading to a period of semi-dormancy.

Warm-season grasses such as bermudagrass [Cynodon dactylon (L.) Pers.], crabgrass [Digitaria sanguinalis (L.) Scop.], sudangrass [Sorghum bicolor (L.) Moench spp. drummondii], sorghum-sudangrass [S. bicolor (L.) x S. bicolor var. sudanense], pearl millet [Pennisetum glaucum (L.) R. Br.], and teff [Eragrostis tef (L.)] are more commonly grown in the warmer climates of the southern U.S. Warm-season grasses utilize an alternate C₄ photosynthetic system, for which the optimal temperature range is 85°F to 95°F. Differences in the photosynthetic system and leaf anatomy of C₄ plants concentrates carbon dioxide within the plant cells, which effectively limits photorespiration and allows for greater yields under higher temperatures like during hot summer months.

In addition to differences in leaf anatomy and photosynthetic processes, cool- and warm-season grasses also store soluble carbohydrates differently. Cool-season grasses have the capacity to accumulate relatively large quantities of sugars and fructans, particularly during periods of cooler temperatures. Warm-season grasses do not produce fructan, instead relying on sugars and starch for carbohydrate storage. Sugars and starches are also typically found in lower concentrations in warm-season vs. cool-season grasses. Overall, these differences in carbohydrate storage mechanisms often result in greater non-structural carbohydrates [NSC; water-soluble carbohydrates + starch] concentrations for cool-season grasses in comparison to warm-season grasses.

Integrated Warm- and Cool-Season Grass Grazing Systems

An integrated rotational grazing approach incorporates forages with complementary seasonal growth patterns into one rotationally or sequentially grazed system. In temperate regions of the U.S., this would mean that cool-season grasses would be grazed during the spring and fall, while warm-season grasses would be grazed during the summer slump period. This strategy has primarily been evaluated for grazing management of cattle in the Midwest. These prior studies have reported higher summer and/or season-long yield for warm-season species in integrated systems, particularly under drought conditions (Tracy et al., 2010; Ritz et al., 2020). However, the generally lower nutritional quality of warm-season grasses resulted in no advantages for milk production or growth performance in grazing cattle (Moore et al., 2004; Ritz et al., 2021). However, forage preferences, grazing behaviors, nutritional requirements, digestive physiology, animal management goals, and drivers of enterprise profitability are vastly different between cattle and horse operations. An integrated rotational grazing strategy may be well-suited to nutrition and management goals of horse operations, as horses are fed to maintain or control body weight and sustain athletic performance rather than to maximize growth. Furthermore, lower-NSC forages may be desirable for obese horses and horses with a history of metabolic dysfunction (i.e., insulin resistance, laminitis, etc.).

Improved Forage Crabgrasses as Equine Pasture Forages

Many warm-season annual grasses commonly utilized for cattle grazing are not suitable for grazing by horses due to prussic acid production and the potential for other forage-related disorders. Additionally, most traditionally cultivated warm-season perennial grasses commonly grown in the southern U.S. are cold-sensitive and cannot survive winters in cooler temperate regions. These factors may limit integration of certain warm-season grasses into temperate horse grazing systems.

Although common crabgrass, a warm-season annual, is traditionally thought of as a weed, improved varieties have been bred for increased forage production and quality. Several varieties are available on the market, and high summer yields of good quality forage have been reported for these improved forage varieties. Given its lack of forage-related disorders, crabgrass may represent a viable summer forage option for horse pastures. Under conditions designed to simulate equine grazing in Maryland, Jaqueth et al. (2021) found that an improved forage crabgrass variety, Red River, had lower persistence and traffic tolerance compared to the warm-season perennial grasses zoysiagrass [Zoysia japonica] or cold-tolerant bermudagrass. Annual yield for Red River crabgrass was moderate in comparison to warm-season perennial grasses.

In a study conducted in New Jersey, integration of another forage crabgrass, Quick-N-Big, into cool-season grass equine rotational grazing systems improved pasture production during the summer slump period in comparison to a traditional cool-season system (Weinert-Nelson et al., 2021). Forage quality samples from this study showed that the Quick-N-Big crabgrass was low in NSC, and thus may serve as a potential source of pasture forage for horses where dietary NSC levels are of concern; however, in this study NSC also remained below 10% for cool-season grasses during the summer slump. Additionally, horses grazing on the integrated crabgrass system did receive adequate nutrition to maintain horse body condition and were not different compared to horses maintained in a traditional cool-season rotational grazing system. Therefore, the primary benefit of integrated rotational grazing is more likely to be the increased forage yield rather than limiting NSC intake or managing weight/body condition for grazing horses.

Methods for crabgrass establishment have also been evaluated. In integrated grazing systems, warm-season pasture sections are typically established in monoculture or as a mixture of warm-season grasses, with any existing pasture forage eliminated through burning, herbicide, and/or traditional tillage prior to planting. This method of establishment mitigates competition with existing cool-season grasses, which can negatively impact initial establishment, yield, and long-term persistence for the warm-season grasses. However, interseeding warm-season grasses into existing cool-season grass stands would improve land-use efficiency of available pasture areas.

Furthermore, no-till establishment of warm-season grasses in monoculture requires an application of glyphosate to eliminate the existing forage, whereas interseeding warm-season grasses into cool-season grass stubble can be done following harvest or grazing. This reduction in labor and herbicide expenses may be attractive to producers. However, interseeding Quick-N-Big crabgrass into existing cool-season pasture grasses in this manner did not produce similar yields compared to monoculture establishment and yields for the interseeded pastures were not significantly greater than for cool-season grasses alone (Weinert-Nelson, 2021; Figure 3).

Horses are also known to be preferential in their forage selection. To address this, a study in New Jersey assessed grazing preferences for horses grazing crabgrass and found that when horses were assessed in experimental small plots (as is common practice in equine grazing research), horses spent less time grazing crabgrass in comparison to cool-season grasses. Conversely, when horses were evaluated in a full pasture setting under normal grazing management conditions using remote automated monitoring technology, grazing time did not differ by forage (Figure 4). The chewing rate was greater, however, when horses were grazing cool-season grasses, indicating greater grazing vigor when consuming cool-season grasses vs. crabgrass (Weinert-Nelson, 2021).

Overall Conclusions and Recommendations

Improved forage varieties of warm-season annual crabgrass are high-yielding during hot, dry months and can be incorporated into an integrated rotational grazing system and used as a more productive alternative to cool-season grasses during the summer slump. Crabgrass can provide adequate nutrition to maintain body condition for grazing horses. However, using crabgrass in an integrated rotational grazing system may not offer benefits if the primary goal is to limit NSC consumption or encourage weight loss. Additionally, grazing horses may consume crabgrass less vigorously, suggesting a lesser preference in comparison to cool-season pasture grasses.

References and Additional Reading

1. Bouton, J. H., B. Motes, M. A. Trammell, and T. J. Butler. 2019. Registration of ‘Impact’ Crabgrass. J. Plant Regist. 13.1:19-23. doi: 10.3198/jpr2018.04.0019crc
2. Guretzky, J.A., J.D. Volesky, M.B. Stephenson, K.R. Harmoney, and J.L. Moyer. 2020. Interseeding annual warm-season grasses into temperate pasturelands: forage accumulation and composition. Crop Ecol. Physiol. 112:2812-2825. doi: 10.1002/agj2.20250
3. Jaqueth, A.L., T.R. Turner, M.E. Iwaniuk, B.J. McIntosh, and A.O. Burk. 2021. Relative traffic tolerance of warm-season grasses and suitability for grazing by equine. J. Equine Vet. Sci. 103:103244. doi: 10.1016/j.jevs.2020.103244
4. Jensen, K.B., P. Harrison, N.J. Chatterton, B.S. Bushman, and J.E. Creech. 2014. Seasonal trends in nonstructural carbohydrates in cool‐and warm‐season grasses. Crop Sci. 54.5:2328-2340. doi: 10.2135/cropsci2013.07.0465
5. Moore, K. J., T. A. White, R. L. Hintz, P. K. Patrick, and E. C. Brummer. 2004. Sequential grazing of cool‐and warm‐season pastures. Agron. J. 96.4:1103-1111. doi: 10.2134/agronj2004.1103
6. Ritz, K. E., B. J. Heins, R. Moon, C. Sheaffer, and S. L. Weyers. 2020. Forage yield and nutritive value of cool-season and warm-season forages for grazing organic dairy cattle. Agronomy. 10.12:1963. doi: 10.3390/agronomy10121963
7. Ritz, K. E., B. J. Heins, R. D. Moon, C. C. Sheaffer, and S. L. Weyers. 2021. Milk production, body weight, body condition score, activity, and rumination of organic dairy cattle grazing two different pasture systems incorporating cool-and warm-season forages. Animals. 11:264. doi: 10.3390/ani11020264
8. Teutsch, C. 2006. Warm-season annual grasses for summer forage. Publication 418-004. Communication and Marketing, College of Agriculture and Life Sciences, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA. vtechworks.lib.vt.edu/…/SPES-88.pdf (PDF)
9. Tracy, B. F., M. Maughan, N. Post, and D. B. Faulkner. 2010. Integrating annual and perennial warm‐season grasses in a temperate grazing system. Crop Sci. 50.5:2171-2177. doi: 10.2135/cropsci2010.02.0110
10. Weinert-Nelson, J.R. 2021. Integrated warm- and cool-season grass equine rotational grazing systems: pasture production, grazing behavior, metabolism, and the gut microbiome. Diss. Rutgers, The State University of New Jersey, New Brunswick, NJ.
11. Weinert-Nelson, J.R., W.A. Meyer, and C.A. Williams. 2021. Yield, nutrient composition, and horse condition in integrated crabgrass and cool-season grass rotational grazing pasture systems. Transl. Anim. Sci. 5:1-18. doi: 10.1093/tas/txab208

December 2021