Grazing Management Affects Manure Distribution by Beef Cattle
Introduction: Livestock excrement represents a valuable, recyclable source of soil nutrients on pasture because 60-95% of the nutrients consumed by grazing livestock pass through the digestive tract (Wilkinson and Lowrey, 1973). Dalrymple et al. (1994) attributed 979 lb/acre of added pasture growth and 95 lb/acre of added beef production to cattle excrement deposited on pasture. Shade and watering locations are sites of P and K accumulation in pastures because cattle tend to camp and loaf, and thus defecate and urinate, in these areas (West et al., 1989; Gerrish et al., 1993; and Mathews et al., 1994). Grazing system design parameters such as level of paddock subdivision (rotation frequency), stocking rate, and water accessability may influence the uniformity of return of nutrients to pasture via cattle excrement. The objectives of this experiment were to 1) quantify the proportion of beef cattle manure deposited in water-access lanes, 2) determine the impact of cattle rotation frequency and landscape features on manure distribution on individual paddocks and 3) simulate manure distribution on entire grazing cells.
Materials and Methods: A grazing experiment was conducted for 2 yr at the University of Missouri-Forage Systems Research Center on mixed cool-season grass/legume swards. Grazing systems with 3, 12, and 24 paddocks were rotationally stocked with Gelbvieh X Polled Hereford cows, their Angus-sired calves, and yearling steers. Stocking rates for the 3-, 12-, and 24-paddock systems were as follows: 2.0, 1.6, and 1.25 acres per cow/calf pair equivalent, respectively, from mid-April through mid-July; and 3.3, 2.7, and 2.1 acres per cow/calf pair, respectively, from mid-July through November (1 yearling steer = 0.7 cow/calf pair). This resulted in 9 to 18 cow/calf pairs and 11 to 20 yearling steers per grazing cell. Each of the system treatments included one grazing cell with water available in every paddock and another cell with lane access to water. Detailed sampling of manure distribution was performed in one, three, and three paddocks of each of the 3-, 12-, and 24-paddock cells, respectively, in 1993 and 1994. Paddocks to be sampled were subdivided into 60 to 75 square grids. Paddock size varied from 1.3 acres in the 24-paddock cells to 14 acres in the 3-paddock cells. Grid size thus varied from about 900 ft2 to about 7000 ft2 depending upon paddock size. Counts of manure piles deposited were obtained in a checkerboard fashion using the grids immediately after each grazing period. A defecation event resulting in a minimum ground coverage of 25 in2, whether in one spot or scattered, was counted as 1 manure pile for this experiment.
The number of piles deposited on the paddocks per head per day for each grazing period was calculated based upon the percentage of the paddock area counted and the number of animal unit days per grazing period. Each animal over 400 lbs was considered to be 1 animal unit for this experiment. The difference in piles per head per day deposited on paddocks of grazing cells with the same paddock number but varying in water accessability was attributed to manure deposited in lanes in the lane-access-to-water cells. Sampled paddocks were replicated to simulate entire grazing cells by assigning the manure distribution obtained in sampled paddocks to unsampled paddocks in the respective cells. This assignment was based upon similarity in landscape features when possible; otherwise, assignment was random. Contour maps were then constructed for these simulated cells.
Results and Discussion: Table 1 illustrates the effect of water location on manure distribution. Lane access to water resulted in an average of 13% loss of manure off of the paddocks in the 24-paddock system. At current costs of commercial fertilizer, this would be equivalent to US$910 of fertilizer nutrients per year for a 100-cow herd. In addition, this experiment was conducted on grazing cells of about 32 acres, and sampled paddocks never required more than about 700 ft of lane travel from the paddock gate to water. Greater distances would likely be incurred in a commercial-sized grazing cell and would probably result in greater nutrient loss in lanes. No consistent, significant loss of manure in water-access lanes was detected in the 12-paddock systems (data not shown) probably because cattle never had to travel more than 450 ft of lane from sampled paddock gates to water in our system design. In contrast, a significant loss of manure (avg. of 22%) was observed in the 3-paddock system with lane access to water. In addition to requiring 750 ft of lane travel from the paddock gate to the water tank, the sampled paddock had a tree-lined ditch that held water through much of the grazing season. This was an attractive camping site for cattle, and likely contributed to some of the manure loss attributed to the lane in the 3-paddock system.
Figures 1 to 3 represent simulated contour maps of 2 years of manure accumulation on grazing systems that had water available in individual paddocks. Figure 1 simulates a 32-acre, 3-paddock rotational system with an average grazing period of 10-20 days. The manure distribution observed on the sampled paddock was replicated to simulate the entire cell, inverting the distribution when necessary to maintain proper orientation with water tanks. Greatest manure accumulation was near water and the tree, and a zone of lesser accumulation (<20 piles per 500 ft2) was observed between about 650 and 900 ft east of water. Cattle were frequently observed to be under the tree even when the temperature was cool and comfortable. We surmise that when cattle were loafing in the back third of the paddock, they were attracted to the tree. Thus most of the area in the back third received less manure return and would likely become depleted of soil fertility if cattle removed nutrients from that area through grazing. Figure 2 simulates a 32-acre, 12-paddock system with a grazing period of 2-6 days. Paddocks monitored in this cell were #2, 6, and 10. Paddock #10 had no striking landscape features. Despite a zone of greater accumulation close to water, manure distribution was quite uniform over the majority of that paddock. In contrast, paddock #2 had a draw that frequently held water running diagonally through it. The wetness of the draw resulted in cattle loafing on the slopes to either side of the draw and thus greater manure accumulation in those areas. Since the 3 paddocks sampled in this cell represented 3 distinct landscapes, assignment of distributions obtained to unsampled paddocks in the cell was done based upon similarity in landscape features (eg. the draw in paddock #2 continued through paddocks 5, 8, 9, and 12; thus its distribution pattern was used for all of these paddocks). Figure 3 simulates a 32-acre, 24-paddock system with a grazing period of 1-2 days. Distributions obtained in sampled paddocks of this cell were not distinctly different due to landscape features, so assignment to unsampled paddocks in this cell was random. About 95% of the cell received 40-60 piles per 500 ft2. The increasing concentration of manure piles observed with increasing paddock subdivision (avg. of 21, 27, and 46 piles per 500 ft2 for the 3-, 12-, and 24- paddock cells, respectively) can be explained largely by the corresponding increases in stocking rate. Despite the presence of a gradient of manure accumulation toward water in all systems, the steepness of the gradient in the 24-paddock cell was less than that in the 12-paddock cell, and considerably less than that in the 3-paddock cell. However, landscape differences somewhat confound comparisons among cells. Our findings concur with those of Morton and Baird (1990) who reported greater aggregation of dung patches when sheep grazed a paddock for 4 days as compared to 1 day. In contrast, Mathews et al. (1994) reported no advantage of rotational stocking over continuous stocking for improving uniformity of K return via cattle excrement. However, moveable shade and waterers used in their experiment favored greater uniformity of excretal return than would be expected under typical continuous stocking situations.
In summary, a significant loss of manure off of pasture occurred when cattle were forced to travel a lane to access water. In addition, as rotation frequency increased and landscape variation within a paddock decreased, the uniformity of manure distribution on individual paddocks and entire grazing cells was improved.
Dalrymple, R.L., R. Stevens, T. Carroll, and B. Flatt. 1994. Forage production benefits from nutrient recycling via beef cattle and how to manage for nutrient recycling in a grazing cell. p.269-273. In American Forage and Grassland Council Proc. Lancaster, PA. 6-10 March 1994.
Gerrish, J.R., J.R. Brown, and P.R. Peterson. 1993. Impact of grazing cattle on distribution of soil minerals. p.66-70. In American Forage and Grassland Council Proc. Des Moines, IA. 29-31 March 1993.
Mathews, B.W., L.E. Sollenberger, P. Nkedi-Kizza, L.A. Gaston, and H.D. Hornsby. 1994. Soil sampling procedures for monitoring potassium distribution in grazed pastures. Agron. J. 86:121-126.
Morton, J.D., and D.B. Baird. 1990. Spatial distribution of dung patches under sheep grazing. N.Z. J. Agric. Res. 33:285-294.
West, C.P., A.P. Mallarino, W.F. Wedin, and D.B. Marx. 1989. Spatial variablity of soil chemical properties in grazed pastures. Soil Sci. Soc. Am. J. 53:784-789.
Wilkinson, S.R., and R.W. Lowrey. 1973. Cycling of mineral nutrients in pasture ecosystems. p.247-315. In G.W. Butler and R.W. Bailey (ed.) Chemistry and biochemistry of herbage. Vol. 2. Academic Press, New York.
Table 1. Manure piles deposited per head per day on paddocks in grazing systems differing in paddock number, water accessability, and landscape features (2 yr avg).
Month ------------------------------------------------------------ System May June July Aug Sept Oct AVG ----------------------------------------------------------------------------- ----------------- manure piles/head/day -------------------- 24-paddock, 12.9 12.3 14.1 9.9 10.7 8.5 11.4 water in 24-paddock, 10.7 11.3 12.0 7.4 9.7 8.0 9.9 lane ............................................................................. % lost in lane 17 8 15 25 9 6 13 ----------------------------------------------------------------------------- 3-paddock, 10.0 10.9 8.4 7.1 --- 7.2 8.7 water in, 1 tree 3-paddock, 8.3 7.6 6.2 6.2 --- 5.5 6.8 lane, ditch with trees ............................................................................. % lost in 17 30 26 13 --- 24 22 lane, ditch with trees
1Assistant Professor, Plant Science Department, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9; and Research Assistant Professor, University of Missouri- Forage Systems Research Center, RR1 Box 80, Linneus, MO 64653, respecti