Spring Burning of Native Tallgrass Pastures Influences Diet Composition of Lactating and Non-Lactating Beef Cows15 January 2013
Grass condition is more of a factor in grazing behviour than lactation status according to research unveiled by Kansas State University.
Diet selection is a dynamic process because of seasonal changes in animal and plant
characteristics. Nutrient requirements of grazing animals are a function of physiological
state; moreover, plant characteristics may be altered with prescribed spring burning of
native rangelands. Prescribed spring burning is used to improve the average quality of
pasture forage by removing old growth and making new plant growth more accessible to
Microhistological analysis of fecal material has been a widely used method for quantifying the botanical composition of a grazing animal’s diet since it was first described by Baumgartner and Martin in 1939. Little research has been conducted on how diet selection preferences of lactating beef cows with suckling calves and non-lactating beef cows are influenced by prescribed burning. We hypothesized that during the summer grazing season, lactating cows with calves and non-lactating cows would display distinctive preferences for certain species. Furthermore, we anticipated that these diet selection preferences might be influenced by prescribed burning. To that end, our objective was to characterize differences in diet selection between lactating beef cows suckling calves and non-pregnant, non-lactating beef cows grazing either burned or unburned native tallgrass prairie during summer.
The study was conducted on 8 native tallgrass pastures (approximately 240 acres each) located at the Kansas State University Commercial Cow-Calf Unit. Four of the pastures were burned in mid-April and 4 had no recent burning history. Predominant pasture forage species at this location were big bluestem (Andropogon geradii) and little bluestem (Schizachyrium scoparium), which were grouped together for the purposes of microhistological analysis; sideoats grama (Bouteloua curtipendula); blue grama (Bouteloua gracilis); switchgrass (Panicum virgatum); indiangrass (Sorghastrum nutans); leadplant (Amorpha canescens); heath aster (Symphyotrichum ericoides); dotted gayfeather (Liatris punctata); and purple prairie clover (Dalea purpurea). Grazing commenced May 15.
Results and Discussion
Previous results suggested that lesser maintenance requirements could result in less
selective foraging behaviors by non-lactating compared with lactating ruminants. Previous
research also indicated lactating cows grazed more selectively than non-lactating,
non-pregnant cows; however, we found no treatment differences (P ? 0.11) in the
botanical diet composition between lactating and non-lactating cows (Table 1). Similar
findings were reported that found no differences in diet composition between lactating
ewes and non-lactating ewes.
Cows consumed more (P = 0.01; 74.2 versus 71.8%, respectively) grasses and fewer (P = 0.01; 25.8 versus 28.2%, respectively) forbs on burned pastures compared with unburned pastures (Table 2). Research suggests that unburned pastures have a greater selection of forbs compared with burned pastures because burning reduced forb availability. Cows ate more (P < 0.01) sideoats grama and less (P ? 0.02) switchgrass, leadplant, and purple prairie clover on burned pastures than on unburned pastures.
As the grazing season progressed, selection of switchgrass increased (burn × period effect, P = 0.09) sharply in both burned and unburned pastures, whereas selection of sideoats grama generally decreased (burn × period effect, P < 0.01; Table 3). Selection of leadplant doubled (burn × period effect, P = 0.04) on burned pastures monthby- month, but selection was inconsistent in unburned pastures. Selection of dotted gayfeather ranged from 12.3 to 20.4% of the diet in June, July, and August and diminished to 8.5 to 8.9% in September (burn × period effect, P = 0.05).
Cows selected more (P < 0.01) switchgrass, blue grama, leadplant, and heath aster over time, whereas they selected less (P < 0.01) indiangrass over time (Table 4). Palatability is a major factor driving selection preferences by grazing herbivores and is reduced as plants approach reproductive maturity and dormancy. Under unrestricted grazing conditions, herbivore preference for specific forage plants is known to change over time. The cows used in our study may have modified their diets over time to select greater proportions of plants that were slower to reach maturity. Alternatively, decreased consumption over time may have been related to diminishing availability or regrowth of certain forage plants.
Consumption of all grasses and all forbs changed slightly (P < 0.01, Table 4) from month to month during the grazing season; however, the relative proportions of grasses and forbs remained consistently within the range of 71 to 75% grasses and 25 to 29% forbs.
Effect of Collection Period on Botanical Composition of Diets (%) Selected by Lactating Cows with Calves or Non-lactating, Non-pregnant Cows Grazing the Kansas Flint Hills during Summer
The botanical composition of diets grazed by beef cows during summer in the Kansas Flint Hills was influenced by prescribed spring burning but was not influenced by lactation status. We interpreted these data to suggest that forage selection preferences of beef cows can be altered with spring burning of native tallgrass pastures.
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