Agronomy Journal 94:860-863 (2002)
© 2002 American Society of Agronomy
NOTES & UNIQUE PHENOMENA
Correcting Measurements of Pasture Forage Mass by Vacuuming the Stubble
Lori J. Unruha and
Gary W. Fick*,b
a Dep. of Crop Sci., North Carolina State Univ., Raleigh, NC 27695
b Dep. of Crop and Soil Sci., Cornell Univ., Ithaca, NY 14853
* Corresponding author (gwf2{at}cornell.edu)
Received for publication May 21, 2001.
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ABSTRACT
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Estimates of forage mass in pasture obtained by harvesting small plots may be too low because fragments of the harvested herbage are dropped into the stubble and not recovered. Our objective was to evaluate stubble vacuuming as a means of retrieving such material, thereby improving the accuracy of forage mass estimation. Our study was conducted on intensively managed dairy pastures dominated by orchardgrass (Dactylis glomerata L.) and white clover (Trifolium repens L.). The stubble was vacuumed following conventional harvesting of small quadrats to a stubble height of 2 cm. Harvested herbage and vacuumed stubble were oven-dried separately and then ashed to correct for soil contamination. The slope of the regression of total organic matter (OM), which included vacuumed material, on the mass of harvested herbage without vacuuming showed that clippings lost into the stubble in this study amounted to 0.045 Mg/ha OM for each Mg/ha unvacuumed herbage dry weight or 0.286 Mg/ha OM for each Mg/ha unvacuumed herbage OM. Because of the extra cost of the procedure, it is not recommended, except for ecological studies where very accurate estimations of OM distribution are required.
Abbreviations: DM, dry matter from 105°C oven drying • DW, dry weight from 70°C oven drying • OM, organic matter • RMSD, root mean squared deviation for regression (also called RMSE)
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INTRODUCTION
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ACCURATE MEASUREMENT of herbage mass is important for both the study and management of pastures and rangelands. Handbooks are available that describe standard procedures and factors to consider in selecting appropriate methods for measuring herbage mass (Davies et al., 1993; Milner and Hughes, 1968). Two early studies suggested that vacuuming would improve accuracy, especially with the sparse vegetation of rangeland (Becker, 1959; Van Dyne, 1966), but further analysis of this refinement to harvesting small plots has not been published. Clipping small quadrats continues to be an essential feature of pasture research, including the calibration of indirect methods designed to reduce the labor requirements of estimating herbage available to grazing livestock (Harmoney et al., 1997; Murphy et al., 1995; Rayburn and Rayburn, 1998). As a part of the process of calibrating a rising-plate meter for estimating pasture herbage mass, we used a shop vacuum cleaner and portable power generator to collect the fine clippings that remained on the harvested plot area following harvest with electric grass shears (Unruh, 1998). The objective of this study was to evaluate vacuuming the stubble as a means of improving estimates of herbage mass in grazed pastures.
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MATERIALS AND METHODS
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Herbage mass was measured on 13 dates in 1997 (10, 16, 20, 25, and 27 June; 14 and 15 July; and 5, 6, 8, 11, 19, and 20 August) in dairy pastures under short-duration rotational stocking on three different farms in Tompkins and Cortland Counties, NY. Harvests were made just before the start of a grazing period so that the farmer's management pattern determined the paddock to be sampled. Occasionally, a previously sampled paddock was sampled again during a later grazing cycle. At each sampling date in the sampled paddock, 30 locations for clipping were selected approximately at random by tossing a tennis ball into the paddock. Sampled locations ranged from being tightly grazed to ungrazed in the previous cycle. An area was rejected only if the tennis ball landed in a manure pad or an area with tall, stiff weeds such as Canada thistle (Cirsium arvense L.), as recommended for making measurements with a rising-plate meter (Scrivner et al., 1986).
The harvest quadrats were 0.1 m2 in area. They were clipped with powered shears, either a 6.5-cm-wide Oster Stewart Shearmaster Clipper (Model EW 3114, Oster Professional Products, McMinnville, TN) or 10.2-cm-wide Disston Cordless Electric Extra Heavy Duty Shears (Model HD4, Disston, South Deerfield, MA). The shears were guided along a metal track that set the stubble height at 2 cm. The track made it possible to reproduce a repeatable cutting height throughout the experiment, avoiding the subjective decisions about where to cut partially buried stolons and stem bases. Smaller clippings that fell into the stubble were then vacuumed (Model 800M, Shop Vac, Williamsport, PA) to retrieve all unattached herbage within the quadrat. The unvacuumed and vacuumed plant material were bagged separately and dried for 48 h in a 70°C forced-draft oven. To reduce variability associated with the operator, the same person clipped all plots, and the same assistant vacuumed all plots.
After drying, the oven dry weights (DW) were recorded to the nearest gram, and then the unvacuumed and vacuumed samples were individually ground to pass a 2-mm screen in a laboratory mill (Serial no. 39017, Christy and Norris LTD, Chelmsford, England). A subsample was taken from each ground sample, further dried at 105°C for 24 h to determine the dry matter (DM) fraction, and then ashed at 500°C for 6 h to determine the ash fraction (Goering and Van Soest, 1970). Mass of unvacuumed and vacuumed samples was calculated as organic matter (OM), free of ash and soil contamination, and added to give a total OM mass of the quadrat. For comparison, DW yields excluding the vacuumed material were also calculated. These DW yields correspond to common practice without vacuuming or ashing. We did not attempt to use the alternative procedure of washing to remove soil contamination because it would have required additional labor and may not have been as accurate as ashing (Frame, 1993).
Six additional locations were similarly selected in the same paddock at the same time to estimate botanical composition. They were approximately 0.025 m2 in area and harvested as close to the soil surface as possible. Those areas were not vacuumed because visual inspection of the material collected by vacuuming showed such an interwoven mass of clipped herbage fragments and dead OM from the stubble that a tweezers and dissecting scope would have been necessary for any quantitative separation. Botanical components for each sampled paddock are shown in Table 1.
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Table 1. Botanical composition as a percentage of the oven dry weight based on six randomly selected plots in each sampled paddock.
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RESULTS AND DISCUSSION
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Additional Accuracy
The reason for vacuuming the stubble is to collect all of the available herbage, including small fragments that fall into the stubble after they are severed in the process of clipping a quadrat. Vacuuming should improve the accuracy of measurements if the extraneous material vacuumed from the stubble, including soil, dead OM, and manure, can somehow be quantitatively distinguished from clipped fragments. The average amount of vacuumed OM for the sampled paddocks ranged from 0.8 to 1.8 Mg/ha. The fraction of that mass that was herbage lost into the stubble can be deduced from Fig. 1
where the total OM mass, which includes material vacuumed from the stubble, is plotted as a function of the DW yield, which is unashed and does not include the material collected from the stubble. The intercept (0.75 Mg/ha) represents the average amount of OM that would have been in material retrieved by vacuuming had no herbage fragments been lost into the stubble. That is approximately 25% of the mass typically measured without vacuuming. The variability of vacuumed mass is also high, with a standard deviation estimated by root mean squared deviation (RMSD) of about 0.40 Mg/ha.
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Fig. 1. The linear relationship between total organic matter (OM) mass and the dry weight (DW) yield determined by conventional harvesting of small plots. Total OM includes vacuumed stubble and harvested DW does not.
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A possible explanation for the large vacuumed stubble mass is the collection of dead material on or near the soil surface in the vacuuming process. We found a highly significant relationship between the mass of dead OM (estimated from botanical composition data, Table 1) and the mass of all OM vacuumed from the stubble (Fig. 2)
. We excluded the point for 5 August because of its unusually high dead-matter fraction (Table 1), which gave 1.2 Mg/ha dead material for that date. The nonlinear pattern of Fig. 2 shows that vacuumed OM mass increased from about 0.7 to 1.5 Mg/ha only up to a dead OM mass of about 0.25 Mg/ha. Including the data point for 5 August did not affect that relationship (data not shown). A possible explanation for this pattern is that the stubble below the 2-cm cutting height becomes saturated with dead material at about 0.25 Mg/ha. Higher amounts of dead material must be vertically positioned above the cutting height, and thus harvested with the unvacuumed herbage sample.
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Fig. 2. The nonlinear relationship between the amount of organic matter (OM) collected by vacuuming the stubble and the mass of dead OM in the unvacuumed herbage.
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We used our botanical separations (Table 1) to estimate the amount of dead material in the unvacuumed samples (Fig. 2). When the mass of dead material from the unvacuumed samples increased by 0.2 Mg/ha (from 0.05–0.25 Mg/ha on the x-axis), the total mass of the vacuumed OM increased by about 0.8 Mg/ha (y-axis). Visual inspection showed that the bulk of the increased vacuumed OM was dead material, so our botanical samples must have greatly under-represented dead OM on the soil surface. Much of this was decomposing herbage from ungrazed patches in the previous grazing cycle, but some of it would also be soil OM and finely fragmented livestock manure on the soil surface that was not visually detectable. Our procedure would include some soil and manure OM in the vacuumed stubble but not in the botanical samples nor in the samples of unvacuumed harvested herbage. The regression procedure described above corrects for this error, but the patchiness of the vacuumed OM increased the variability in our data.
The average amount of herbage that was fragmented and lost into the stubble can be estimated by the slopes shown in Fig. 1 and 3
. Two relationships are presented because herbage mass might be estimated either by DW, which is the common practice, or by OM, which is recommended for quadrats harvested at 2 cm or closer to the soil surface (Frame, 1993). In the first case (Fig. 1), the regression shows that there was 0.045 Mg/ha OM in clippings recovered by vacuuming for each Mg/ha unvacuumed herbage DW. The same analysis based on OM instead of DW (Fig. 3) shows that there was about 0.286 Mg/ha OM in clippings recovered by vacuuming for each Mg/ha unvacuumed herbage OM. For our study, one can correct the herbage mass for clippings lost into the stubble by multiplying the unvacuumed herbage mass by the appropriate slope from Fig. 1 (if herbage mass is in DW) or Fig. 3 (if herbage mass is in OM). In either case, the resulting herbage mass estimate is in Mg/ha OM. There is uncertainty about the appropriate ash content to use for DW corrections (see discussion below), but if one assumes that DM/DW is 0.9 and that OM/DM is 0.92, then for our data, there would be 0.054 Mg/ha DW in vacuumed clippings for each Mg/ha in harvested DW without vacuuming.
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Fig. 3. The linear relationship between the total organic matter (OM) mass, including vacuumed stubble, and the OM mass determined by conventional harvesting of small plots. Harvested OM does not include vacuumed stubble.
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There was a sizable standard deviation associated with the data (about 0.4 Mg/ha, RMSD from Fig. 1 and 3). Thus, the corrections should be applied to means and not to individual measurements so that the precision of results not be compromised. A preliminary examination did show that vacuuming decreased the precision of treatment comparisons when applied to individual measurements (Unruh, 1998).
Additional Equipment and Labor
Although vacuuming the stubble improved the accuracy of herbage mass estimation in our experiment, the improvement came with considerable increase in costs for equipment, labor, and time. In a relatively small (0.5 ha) paddock, it took about 5 min per quadrat to vacuum and bag the clippings. It also took a vacuum cleaner, power generator, appropriate cart for moving the generator, and with our procedure, an extra person to do all of the vacuuming. In addition, vacuuming resulted in considerable soil contamination of the collected samples so that correction of weights by ashing was necessary.
Ashing of all pasture herbage samples cut close to the soil surface is the recommended procedure (Frame, 1993) because of soil contamination caused by rain splash and livestock treading in the previous grazing cycle. However, soil contamination of the vacuumed material from the stubble and of the unvacuumed herbage samples was quite different. In our study, the mean ash content of the vacuumed stubble was 34% of DM, with a range of 9 to 59%. The corresponding number for the unvacuumed harvested herbage was 12%, with a range of 7 to 29%. Muller and Fales (1998) reported that the herbage of cool-season pasture grasses averages 8 to 9% ash. Thus, our samples exhibit occasional soil contamination of unvacuumed herbage and occasional lack of soil contamination in the vacuumed samples from the stubble. However, the large difference in means indicates that mixing vacuumed stubble and unvacuumed herbage before grinding might make it more difficult to select a representative subsample for ashing. Thus, if vacuuming of stubble is practiced, it should also double the number of samples to be ashed. With limited equipment for ashing, this could be the most time-consuming aspect of the whole process.
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CONCLUSIONS AND RECOMMENDATIONS
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The conclusion of this study is that clipping losses into the stubble are proportional to harvested herbage mass. The procedures described here can be used to estimate the mass of OM lost into the stubble while harvesting small quadrats. However, considering both the cost and the small correction associated with stubble vacuuming, the procedure is probably not warranted in most experiments where pasture mass is estimated by the harvested DW of small plots. In some ecological studies where very accurate estimates of herbage OM are required, the procedure described here may be useful. The slope of the regression of total OM mass (with vacuuming) on unvacuumed harvested mass can be used to determine an average correction. The intercept of that regression measures the contamination of the vacuumed sample by soil, manure, and dead OM. We doubt that the corrections estimated in this study would apply to other kinds of pastures or in other years. However, the procedure to determine the correction should be applicable to many studies of herbage mass in pastures.
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NOTES
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This research was supported in part by Hatch funding allocated to the Cornell Agric. Exp. Stn. and to Regional Research Project NE-132.
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REFERENCES
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- Becker, C.F. 1959. Equipment for harvesting short-grass rangeland plots. Agron. J. 51:430.[Free Full Text]
- Davies, A., R.D. Baker, S.A. Grant, and A.S. Laidlaw (ed.) 1993. Sward measurement handbook. 2nd ed. Br. Grassl. Soc., Reading, England.
- Frame, J. 1993. Herbage mass. p. 39–67. In A. Davies et al. (ed.) Sward measurement handbook. 2nd ed. Br. Grassl. Soc., Reading, England.
- Goering, H.K., and P.J. Van Soest. 1970. Forage fiber analysis (apparatus, reagents, procedures, and some applications). Agric. Handb. 379. U.S. Gov. Print. Office, Washington, DC.
- Harmoney, K.R., K.J. Moore, J.R. George, E.C. Brummer, and J.R. Russell. 1997. Determination of pasture biomass using four indirect methods. Agron. J. 89:665–672.[Abstract/Free Full Text]
- Milner, C., and R.E. Hughes (ed.) 1968. Methods for the measurement of primary production of grassland, IPB handbook no. 6. Blackwell Sci. Publ., London.
- Muller, L.D., and S.L. Fales. 1998. Supplementation of cool-season grass pastures for dairy cattle. p. 335–350. In J.H. Cherney and D.J.R. Cherney (ed.) Grass for dairy cattle. CABI Publ., New York.
- Murphy, W.M., J.P. Silman, and A.D. Mena Barreto. 1995. A comparison of quadrat, capacitance meter, HFRO sward stick, and rising plate for estimating herbage mass in a smooth-stalked meadowgrass dominant white clover sward. Grass Forage Sci. 50:452–455.
- Rayburn, E.B., and S.B. Rayburn. 1998. A standardized plate meter for estimating pasture mass in on-farm research trials. Agron. J. 90:238–241.[Abstract/Free Full Text]
- Scrivner, J.H., D.M. Center, and M.B. Jones. 1986. A rising plate meter for estimating production and utilization. J. Range Manage. 39:475–477.
- Unruh, L.J. 1998. Calibration of a commercial rising plate meter for pasture yield estimation in New York State. M.S. thesis. Cornell Univ., Ithaca, NY.
- Van Dyne, G.M. 1966. Use of a vacuum-clipper for harvesting herbage. Ecology 47:624–626.
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