Forage mineral concentration with patch-burn grazing

Patch-burn grazing is a livestock management practice that provides a wide range of benefits to ecosystem conservation and livestock production. Mineral nutrition is important for livestock health and performance; however, the impact fire has on mineral concentration of forages in the northern Great Plains remains unknown. In this study, we determined how burning affects the mineral concentration of available forage through the growing season. Data were collected on mixed-grass rangeland at the Central Grasslands Research Extension Center in southcentral North Dakota during 2017 and 2018. Vegetation was clipped from recently burned patches and unburned patches on thin loamy ecological sites at the same sampling locations in the spring and late summer. All samples were analyzed for calcium, phosphorus, copper and zinc concentration. Burning increased forage mineral concentration across all minerals. Copper, phosphorus and zinc were greater in burned patches, compared with unburned patches, at the beginning and end of the growing season. Calcium  concentration was not different between burned and unburned patches during the spring but was greater in burned patches by late summer. Increased mineral concentration in forage on burned areas has the potential to reduce mineral supplementation costs and improve cow performance through enhanced immune function and reproduction.

Patch-burn grazing is a rangeland management practice that concentrates grazer activity in recently burned patches within large rangeland pastures (Fuhlendorf et al., 2017). Patch burning benefits livestock production by altering the physiology of vegetation, which creates an increased crude protein content and a reduced fiber content (Spiess et al., 2020). Patch burning also maintains or increases cattle weight gains (Scasta et al., 2016) and buffers livestock production during drought (Spiess et al., 2020).

Mineral nutrition is an important consideration to maximize cattle production and maintain ranch sustainability through influence on reproduction, health and growth (Suttle, 2010). However, rangeland forage does not always satisfy the mineral requirements of grazing cattle throughout the grazing season, decreasing forage utilization and performance. While free-choice mineral supplementation can be used to meet requirements of grazing animals, this practice is costly for producers and results in high variability of intake between animals. Although minerals are an important component of livestock nutrition, no studies have examined the impacts of patch-burn grazing on forage mineral concentration. Therefore, the objective of this study was to determine if patch burning increases mineral concentration of forage in rangeland pastures.

This study was conducted at the Central Grasslands Research Extension Center in south-central North Dakota. Pastures are mixed-grass prairie consisting of native and introduced C3 grasses, native C4 grasses, forbs, legumes and shrubs.

Samples were collected in 2017 and 2018 on four pastures managed with patch-burn grazing. These pastures underwent a spring burn treatment (April) in which a quarter of the pasture (40 acres) is burned each spring, creating a four-year fire return interval.

Forage sampling occurred on recently burned and unburned patches during the spring (May) and late summer (September), which corresponded to when cattle started the grazing season and within a month of the end of the grazing season. Patches classified as recently burned received a fire treatment in the sampling year.

To determine forage mineral content at the beginning and end of the grazing season, above-ground biomass was clipped from a 25-centimeter (cm) by 25-cm frame during late spring (May-June) and late summer (August-September). All plant material above the crown was clipped to minimize contamination from soil and litter but still include the live and standing dead material. Samples were collected from thin loamy ecological sites to minimize the effect of different soil type on mineral content.

After clipping, samples were dried for 48 hours at a temperature of 60 C in a forced air oven, ground with a Willey Mill using a 1-millileter (mm) screen and stored in bags for chemical analysis. Samples were analyzed for calcium (Ca), phosphorus (P), copper (Cu) and zinc (Zn) using atomic absorption spectrophotometry.

Phosphorus, Cu and Zn concentration were greater during late spring and late summer in the forage regrowth after fire, as compared with forage in unburned patches (Figure 1). In both years, Ca was only greater in the recently burned patch during late summer sampling. Forage Cu concentration in the recently burned patch was variable between years but remained higher than in unburned patches. Year to year difference is apparent in P, Cu and Zn with the late-season, recently burned mineral concentration being much lower in 2018 than 2017. Patch-burn grazing increased forage mineral concentration in recently burned patches for the extent of the grazing season. Increases in forage mineral concentration following fire can be attributed to reduced age of plant tissue, increased leaf-to-stem ratio and nutrients distributed over less biomass in post-fire vegetation (Van de Vijver et al., 1999).

Figure 1. Average mineral concentration of forage in burned and unburned patches in patch-burn grazing pastures. Horizontal lines indicate recommended mineral concentrations based on NASEM, 2016, Nutrient Requirements of Beef Cattle. Calcium and P recommendations are based upon a 572-kilogram (kg) cow with 8 kg/day peak milk production and 40 kg calf birth weight. The gray dashed line in the calcium and phosphorus represents recommendations for April calving during the early grazing season and the black dash-dot line represents late summer recommendations. The dashed line for copper and zinc represents the general season-long minimum recommendations.

Mineral concentration in the recently burned patch was generally adequate to meet recommended levels of Ca, P and Zn for lactating cows based on an April-calving 1,300-pound cow (National Academies of Sciences, Engineering, and Medicine [NASEM], 2016). Forage in the unburned patch was often below recommended levels for P and Zn.

Requirements for Ca and P are dependent upon cow size, physiological state and milk production. Therefore, requirements will change between individual animals and throughout the grazing season.

What is important to note is that meeting the recommended mineral levels does not indicate that requirements are being met. Factors such as the location of the mineral in the plant, chemical form and mineral interactions all influence the bioavailability of minerals. Copper was deficient based on recommended levels (NASEM, 2016) across both treatments, and higher concentration in the burn patch might not be substantial when antagonistic interaction with molybdenum, sulfur and iron are considered. Our results suggest that patch-burn grazing can increase mineral concentration of forage in the recently burned patch. Increased mineral concentration in forage on burned areas has the potential to reduce mineral supplementation costs and increase cow performance. Understanding mineral trends may provide producers with insight into the best supplementation strategies and mineral formulations to maximize performance and profitability.

The authors acknowledge the North Dakota State University Central Grasslands Research Extension Center and North Dakota Agricultural Experiment Station for financial support of this project. Thanks to Micayla Lakey and the staff at the Central Grasslands Research Extension Center for assistance with forage collection.

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Spiess, J.W., D.A. McGranahan, B. Geaumont, K. Sedivec, M. Lakey, M. Berti, T.J. Hovick and R.F. Limb. 2020. Patch-burning buffers forage resources and livestock performance to mitigate drought in the northern Great Plains. Rangel. Ecol. Manag. 73:473–481.

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