Continuous vs. Rotational Grazing of Cool Season Pastures During the Summer Months

Abstract

s / Journal of Equine Veterinary Science 31 (2011) 230-356 285 [2] Undersander DJ, Bertram MG, Clark JR, Crooks AE, Rankin MC, Silveira KG, Wood TM. Forage Variety Update for Wisconsin. 2010. Trial Results Available online at http://www.uwex.edu/ces/forage/ pubs/A1525-2010.pdf. Continuous vs. Rotational Grazing of Cool Season Pastures During the Summer Months G. Webb, S. Webb, C. Duey, and K. Minton Missouri State University, Springfield, MO, US Introduction: Many horse owners in the Ozarks are inexperienced as livestock/pasture managers. This can lead to overgrazing and potential problems for the environment including erosion, sedimentation, and pollution from runoff. Improvement in grazing management, which will aid in maintenance of forage stand density and ground cover, is also essential for the improvement in adjacent stream and ground water quality. Several studies have been conducted on the effects of rotational grazing in cattle. However, little research has been done using horses [1,2]. A four year study was conducted, with the objective to compare the effects of grazing management (continuous vs. rotational) on the amount of available forage when adult horses were grazed on cool season pastures during the summer months. Materials and Methods: This study was conducted during 4 consecutive summers from 2007-2010. The pasture used in the study was located at the Missouri State University Research Center located in Mountain Grove, Missouri and is typical of the Ozark region, with predominantly fescue (Festuca arundinacea) and ladino clover (Trifolium repens L.), and smaller amounts of orchard grass (Dactylis glomerata L.) and bluegrass (Poa pratensis). The pasture used in the study was harvested for hay in late spring to mimic conditions and practices in the industry. A 4.7 ha pasture was divided into one paddock of 1.7 ha which was grazed using a continuous (C) management scheme; the remaining 3.0 ha was divided into four paddocks of 0.75 ha and grazed using a 7 d graze21 d rest rotation (R). The same areas were grazed under the same grazing management scheme (C or R) all 4 years. However, the pasture was not grazed from August to 3 wk after it was harvested for hay in May-June of each year. Portable electrified fencing was used to divide the paddocks. Horses were not supplemented with any additional feed or hay during the study but had access to a trace mineral salt block and water at all times. Mature horses of Quarter horse or Arabian breeding were used all 4 years. Horses were assigned to groups so that within year the number of 454 kg animal units/ha where similar between C and R paddocks. Due to variations in the weights (BW) of horses available for the study, the size of a non-grazed area was manipulated to make the stocking rate similar between years. Based on the BW of horses available for the study 9 horses were used in 2008. In the remaining years, 8 horses were used per year. Stocking rates expressed in 454 kg animal units/ha for the C vs. R grazed pastures were 2.03 vs. 2.05, 1.98 vs. 2.03, 2.06 vs.1.95, and 1.97 vs. 1.93 for 2007, 2008, 2009 and 2010 respectively. Grazing begin 3 wk after the pasture was harvested for hay. Horses were placed in the assigned paddocks on June 18th , July 15th, June 10th and July 3rd in 2007, 2008, 2009 and 2010. Horses were grazed for a total of 8 wk each year and therefore each R grazed paddock was grazed twice. Horse BW were obtained prior to the and after the 8 wk grazing periods. Rump fat depth as determined by ultrasound and body condition scores (BCS) were taken at the beginning, midpoint and end of the 8 wk grazing periods. Available forage was measured every 7 d in the C paddocks and at the beginning and end of each 7 d grazing period for R paddocks using a rising plate meter. In order to calibrate the plate meter, grass clippings were taken by hand from 6 random 0.929 m2 areas from the C paddock weekly and before and after each grazing period (18 per wk) for the R paddocks in 2007 and 2008. Dry matter of grass samples was determined by drying in a forced air oven. The regression equation for converting the rising plate meter readings to grams forage/.0929 m2 was Y 1⁄4 5178 X + 16.78 with an R2 of 0.62 were X 1⁄4 the plate meter reading. Data were analyzed using the GLM procedure of Minitab 16 (Minitab Inc., State College, PA). Type of grazing mangement and wk of grazing were considered fixed effects and year was considered a random effect. The amount of rainfall during the 3 wk prior to the grazing wk was considered a covariate. Results and Discussion: In some years the mean rump fat thickness, BCS and BW of the horses increased and in others it decreased during the 8 wk grazing period. These values were not significantly different. This was possibly because of the high variation in rainfall and available forage during the second 4 wk of the grazing periods and/or low animal numbers used in this study. However, it is an indication that adequate forage was available for animals in both grazing schemes to maintain their BW which changed by an average of 1.8 1.9, 6.8 9.9, -3.4 7.8, and 0 18.6 kg for the C group in 2007-2010 respectively. In comparison horses in the R grazed groups displayed mean BW changes of -3.8 6.3, 20.5 3.2, -7.8 12.5 and 9.1 16.7 in those same years. The amount of available forage at the beginning of each 7 d grazing period was significantly higher in the R paddocks as compared to C paddock (P < .001). There was also a significant year effect (P < .0001) but no significant year by treatment interaction. Conclusion: There is a need for further investigation of how the stocking rate, forage variety and growing season alter the effectiveness of rotational vs. continuous grazing schemes when equines are used as the animal model. Results of this study demonstrate that a 7 d graze and 21 d rest rotation is a feasible grazing scheme when cool season annual pastures are grazed during the summer months and with the stocking densities used in this study. A more frequent rotation may be desirable to maximize forage use and prevent spot grazing during periods of the year when cool season forage is growing more rapidly.