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a 126 Jessie Dunn, Northwestern Oklahoma State Univ., Alva, OK 73717
b Dep. of Anim. Sci., Univ. of Arkansas Div. of Agric., Fayetteville, AR 72701
c Dep. of Crop, Soil, and Environ. Sci., Univ. of Arkansas Div. of Agric., Fayetteville, AR 72701
d Usda-Ars, Ames, Ia 50011-4420
e Coop. Ext. Serv., Anim. Sci. Section, Univ. of Arkansas Div. of Agric., Little Rock, AR 72203
f North Carolina State Univ. Mountain Res. Stn., Waynesville, NC 28786
* Corresponding author (coblentz{at}uark.edu)
Received for publication July 20, 2004.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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0.048) in each case; however, responses were not consistent across these response variables. A second study was conducted with bermudagrass using similar techniques, except that the forage contained 761 (WET), 400 (MID), and 130 (IDEAL) g kg–1 of moisture when simulated rainfall was applied. For IDEAL bermudagrass forage, DM losses increased in linear (P = 0.001) and quadratic (P = 0.003) relationships with simulated rainfall, but the maximum DM loss was only 21 g kg–1. For both forages, DM loss and deleterious changes in nutritive value generally increased with rainfall amount, but these responses appeared to be much greater for orchardgrass.
Abbreviations: ADF, acid detergent fiber • AIRDRY, forages air-dried in wire cages for 48 h after application of simulated rainfall • DM, dry matter • DRY, orchardgrass wilted to 41 g kg–1 of moisture • IDEAL, forage wilted to an ideal moisture concentration for baling (153 and 130 g kg–1 of moisture for orchardgrass and bermudagrass, respectively) • ISDMD, 48-h ruminal in situ disappearance • MID, bermudagrass wilted to the approximate midpoint of dehydration (400 g kg–1 of moisture) • NDF, neutral detergent fiber • OVENDRY, forages oven-dried at 55°C following application of simulated rainfall • WET, forage evaluated immediately after mowing (674 and 761 g kg–1 of moisture for orchardgrass and bermudagrass, respectively)
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Several reports have outlined reductions in the nutritive value of hay crops in response to natural (Gordon et al., 1969; Rotz and Abrams, 1988) and simulated (Collins, 1982, 1983) rainfall events. Reductions in forage nutritive value are primarily associated with losses of soluble, nonstructural carbohydrates that are leached from plant tissues by rain (Collins, 1982). In addition to leaching losses, forages damaged by rainfall may exhibit losses associated with continued or reactivated respiration of available carbohydrates by plant enzymes and/or microorganisms (Rotz and Muck, 1994). Respiratory processes in plants are greatly inhibited when the concentration of moisture within plant tissues decreases to 300 to 350 g kg–1 (Wood and Parker, 1971; Wolf and Carson, 1973). However, rehydration of dry forages by rainfall during the wilting period may reactivate some enzyme systems and extend the period of time that respiration occurs (Rotz, 1995). Because both leaching and respiration processes occur in response to rainfall, it is difficult to quantify losses of DM associated with each individual process (Collins, 1983; Rotz and Abrams, 1988).
Most of the studies designed to assess the effects of rainfall on wilting forage crops have focused on alfalfa (Medicago sativa L.) and other legume hay crops, primarily because of their high potential feed and cash value. In addition, rainfall treatments generally have been limited to irregular and unpredictable natural events or rather simplistic artificial application techniques coupled with natural rainfall (Collins, 1982, 1983, 1985). Recently, some scientists have adapted various types of rainfall simulation systems to address these research needs (Rotz et al., 1991; Smith and Brown, 1994); however, these efforts have been oriented specifically toward alfalfa. Regardless of the forage type, these systems offer unique opportunities to apply graded levels of simulated rainfall to wilting forages and therefore more precisely identify variables that may affect the nutritive value of the damaged hay crop. Throughout the southeastern USA, a combination of warm- and cool-season grasses are grown and utilized commonly by producers. Specific knowledge about the effects of graded increments of rainfall on these wilting grasses would be useful to producers evaluating their hay-making options during less-than-ideal weather conditions. Therefore, the objectives of this study were to investigate the effects of simulated rainfall and two postrainfall drying methods on losses of DM, concentrations of fibrous components and N, and ruminal in situ DM disappearance (ISDMD) for wilting orchardgrass and bermudagrass hays. In addition, simulated rainfall also was applied when each forage was wilted to several different concentrations of moisture to assess the impact of forage moisture concentration at the time the rainfall event occurred on subsequent susceptibility to DM loss and other deleterious changes in forage nutritive value.
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MATERIALS AND METHODS |
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Immediately after mowing, three 0.25-m2 frames were placed directly on the hay swaths at random locations throughout the field. The mowed forage within each frame was clipped with handshears to estimate the density of the swath. The freshly swathed orchardgrass forage (674 g kg–1 moisture; WET) was then collected from throughout the experimental area, placed onto a tarp, and moved under a barn to minimize desiccation of plant tissues. Once under the barn, orchardgrass forage was weighed into 42 galvanized wire baskets (15-cm height by 31-cm width by 76-cm length; mesh size = 1.3 cm); baskets were filled with forage (485 ± 11 g DM m–2) so that the forage density within the wire baskets was comparable to that calculated from frames clipped from swaths in the field. As much as possible, care was taken to orient the orchardgrass forage within the basket in a manner consistent with that in the windrow. Thirty-six baskets were placed on a raised wire platform under a rainfall simulator (1.5- by 6.1-m coverage area); baskets were separated into three experimental blocks designated on the basis of location underneath the simulator. The remaining six baskets served as controls and did not receive simulated rainfall (0 mm).
Simulated rainfall was applied to experimental forages at a constant rate of 76 mm h–1. Therefore, to apply treatments that represented graded amounts of rainfall, two baskets from each block were removed at random from under the simulator at 10-min increments. This resulted in six treatments that received either 12, 25, 38, 51, 64, or 76 mm of total simulated rainfall. After the baskets were removed from under the simulator, each basket was allowed to drip-dry for approximately 0.5 h. Following the drip-dry period, baskets from each rainfall level were either: (i) weighed and the forage contents immediately transferred to a 30- by 43-cm paper bag and dried to a constant weight under forced air at 55°C (OVENDRY) or (ii) placed outside on grass stubble (5-cm height) and allowed to air-dry for 48 h before forages were collected from each tray (AIRDRY). All the AIRDRY forage in each basket was then transferred into a 30- by 43-cm paper bag and dried to a constant weight as described previously. Although the weight of (wet) forage placed into each paper bag varied widely depending on the rainfall increment that was applied and whether the tray had been designated as OVENDRY or AIRDRY, bags contained 90.4 ± 13.8 g of forage DM after removal from the drier. Control forages receiving no (0 mm) rainfall were randomly assigned to either OVENDRY or AIRDRY treatments. For this study, the OVENDRY method was used to assess DM losses that occurred in response to artificial rainfall while the AIRDRY procedure was used to assess these losses plus any additional losses that may have occurred from reactivated plant respiration (Rotz and Muck, 1994). Drying conditions were excellent while orchardgrass forages were evaluated; between 18 and 20 June, the daily high temperature ranged from 28 to 31°C with an average wind velocity of 8.5 to 11.3 km h–1. There was no evidence of significant leaf shatter or other loss of forage from the baskets during the application of simulated rainfall or during handling.
After completing this initial experiment for WET orchardgrass forage, identical experiments were conducted for orchardgrass at two additional concentrations of forage moisture. Our initial research plan was to evaluate orchardgrass forage at an approximate midpoint (
400 g kg–1 moisture) of the wilting process and when the forage was dry enough to bale (<200 g kg–1). However, wilting conditions were excellent, and the swathed orchardgrass forage dried very quickly in the field. By the time simulated rainfall was applied to WET orchardgrass and the samples processed, the swathed orchardgrass in the field was already dry enough to bale. Therefore, the procedures used to evaluate the effects of simulated rainfall on WET orchardgrass also were used subsequently for orchardgrass forage that was dehydrated to 153 and 41 g kg–1 of moisture in the field. These concentrations of moisture are nearly ideal (IDEAL) and excessively dry (DRY) for baling, respectively. Forage densities in galvanized wire baskets were 406 ± 22 g DM m–2 for IDEAL and 349 ± 21 g DM m–2 for DRY orchardgrass forages. Simulated rainfall was applied to all three forages (WET, IDEAL, and DRY) within one 24-h time interval, and no natural rainfall fell during the 48-h period that AIRDRY orchardgrass forages were placed outside on grass stubble. It should be noted that the decision to use second-cutting orchardgrass was intentional; this forage was primarily vegetative regrowth that dried quickly and allowed for application of simulated rainfall to orchardgrass at three diverse concentrations of moisture with limited risk from inclement weather.
Bermudagrass Study
A well-established stand of common bermudagrass was selected from another location at the University of Arkansas Forage Research Area. Common bermudagrass was selected for this project because many popular hybrids, including ‘Tifton 44’, ‘Tifton 85’, and ‘Coastal’, are not grown commonly in the Fayetteville area, in part because of potential for winterkill. Producers in the Fayetteville area most frequently utilize ‘Greenfield’ or common bermudagrasses of unknown origin that are well adapted to the colder climate in northwest Arkansas. On 2 August 2001, the second cutting of bermudagrass forage was harvested as described previously for orchardgrass, except that the mowing height was set at 5.0 cm. Drying conditions also were excellent for the bermudagrass studies; the daily high temperature between 2 and 4 August ranged from 33 to 34°C with an average wind velocity of 4.5 to 5.5 km h–1. All other experimental procedures associated with the allocation and transfer of swathed forage into wire baskets, application of simulated rainfall, and drying methodology were identical to those described previously. Simulated rainfall was applied when moisture concentrations of the experimental bermudagrass forage reached 761, 400, or 130 g kg–1, which corresponded to forage that was freshly mowed (WET), at about the midpoint of the drying process (MID), and ideal for baling (IDEAL), respectively. Densities of these forages weighed into galvanized wire baskets were 652 ± 5, 666 ± 10, and 1530 ± 14 g DM m–2, respectively. The density of the bermudagrass windrows was substantially greater than observed for the mowed orchardgrass forage that was described previously. To complete the applications of simulated rainfall on WET, MID, and IDEAL bermudagrass forages within a 24-h period, it was necessary to invert windrows of IDEAL bermudagrass with a side-delivery rake to facilitate drying. Adjacent windrows of bermudagrass forage were rolled against each other, thereby exposing the bottom of each windrow to the air. This practice is used commonly by producers throughout the region, particularly before packaging in large round bales. The density of IDEAL bermudagrass placed into the galvanized wire baskets reflects both the amount of forage found in these double windrows as well as the narrowing of the windrow as a result of raking. Simulated rainfall was applied to all three bermudagrass forages (WET, MID, and IDEAL) within a 24-h time interval, and no natural rainfall fell during the 48-h period that AIRDRY forages were outside on grass stubble.
Description of Rainfall Simulator
The rainfall simulation system used in these studies was modified from the design of Miller (1987) and consisted of eight TeeJet nozzles (Model 1/2 HH-SS50WSQ, Spraying Systems Co., Wheaton, IL) that were threaded directly into the body of an electrically operated solenoid valve. Solenoids were connected directly to a water supply pipe and were controlled by a custom-built electronic timing system. Water was supplied to the water supply pipe from a hydrant that connected to a water main at the University of Arkansas Forage Research Area. Solenoids operated on a rapid cycle in which they remained open for 1.0 s and were closed for 0.7 s. This cycle resulted in an intermittent simulated rainfall pattern that produced a constant rainfall intensity of 76.2 mm h–1. This intermittent water flow rate also allowed the rainfall to be applied over a longer period of time than could have been expected for a continuous flow rate (Rotz et al., 1991).
The water supply pipe, solenoids, and nozzles were supported by an aluminum, square-pipe frame that allowed artificial rainfall to be applied from approximately 3 m above the ground surface (Miller, 1987). This provided a uniform spray pattern over an area of 1.5 by 6.1 m. Water pressure was regulated at the nozzle (28 kPa) to produce artificial drops with sizes, velocities, and impact energies similar to those of natural rainfall (Shelton et al., 1985). In addition, vinyl curtains were used to cover three sides of the simulator to prevent the disruption of droplet paths by air currents.
For these studies, a 1.5- by 6.1-m platform was constructed from a wooden frame covered with wire mesh. This platform was placed within the coverage area of the simulator. Wire baskets containing experimental forages were placed on top of the wire mesh (approximately 9 cm above the soil surface) to prevent contact with the soil surface, eliminate puddling from runoff water, and to ensure the free movement of water through plant tissues.
Measurement of Dry Matter Losses
Losses of forage DM from wire-mesh baskets were calculated using concentrations of neutral detergent fiber (NDF) as an internal marker (Scarbrough et al., 2004). Previously, estimates of DM losses in experiments with rain-damaged forages often have been based on gravimetric techniques, but these techniques have been problematic. Rotz et al. (1991) and Rotz and Abrams (1988) have noted numerous problems, including negative estimates of DM loss, when DM losses in wilting alfalfa were measured by gravimetric techniques. One potential source of these errors has been associated with small fluctuations in the estimates of the initial DM concentration of each experimental forage (Rotz et al., 1991). For this study, the actual calculations of DM loss were made from the equation of Fonnesbeck et al. (1986) that was modified to include NDF as the internal marker:
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Chemical Analysis of Forage
Dry forage samples were ground through a Wiley mill (Arthur H. Thomas, Philadelphia, PA) to pass through either a 1- or 2-mm screen. Subsamples ground through a 1-mm screen were analyzed subsequently for NDF, acid detergent fiber (ADF), hemicellulose, cellulose, lignin, and total N. Analyses for NDF, ADF, hemicellulose, cellulose, and lignin were conducted sequentially using the batch procedures outlined by ANKOM Technology Corporation (Fairport, NY). Sodium sulfite and heat-stable 
-amylase were not included in the NDF solution. The ANKOM methods used in this study have been described and subsequently compared with conventional methods and found to give comparable results (Komarek, 1993; Komarek et al., 1994; Vogel et al., 1999). Concentrations of N were quantified by a rapid combustion procedure [Official Method 990.03, AOAC, Gaithersburg, MD (AOAC, 1998); LECO Model FP-428; LECO, St. Joseph, MI].
Ruminal Dry Matter Degradability
Animals and Diets
Two ruminally cannulated crossbred steers (mean body weight = 268 ± 21.2 kg) were used to determine ISDMD of rain-damaged orchardgrass forages during a 48-h ruminal incubation utilizing the nylon-bag method (Lowery, 1970). The University of Arkansas Institutional Animal Care and Use Committee approved surgical procedures for cannulations and the subsequent care of the fistulated steers. Steers were housed under an open-air pole barn in individual pens (3.4 by 4.9 m) with concrete floors that were cleaned regularly. A basal diet consisted of orchardgrass hay (159 g kg–1 crude protein, 590 g kg–1 NDF, and 340 g kg–1 ADF) and a concentrate formulation that contained (as-is basis) cracked corn (553 g kg–1), soybean hulls (337 g kg–1), limestone (49 g kg–1), trace-mineral salt (42 g kg–1), molasses (17 g kg–1), and a vitamin A, D, and E premix (2 g kg–1). On an as-fed basis, the ratio of forage to concentrate was 80:20. Steers were offered the basal diet in two equal portions (0730 and 1730 h) at a combined rate of 20.5 g kg–1 of body weight daily and adapted to this diet for 10 d before initiating the trial.
For bermudagrass, procedures were identical to those used to evaluate orchardgrass forages, except for the following: (i) mean body weight of the steers was 265 ± 40.1 kg, (ii) the basal diet contained bermudagrass hay harvested during the 2001 growing season (132 g kg–1 crude protein, 678 g kg–1 NDF, and 364 g kg–1 ADF), and (iii) there was a slightly different concentrate formulation. The concentrate mix included (as-is basis) cracked corn (474 g kg–1), soybean hulls (443 g kg–1), trace-mineral salt (51 g kg–1), molasses (25 g kg–1), limestone (5 g kg–1), and a vitamin A, D, and E premix (2 g kg–1). On an as-fed basis, the ratio of forage to concentrate was 83:17. Steers were offered the basal diet twice daily (0730 and 1730 h) at 19.2 g kg–1 of body weight, allowed ad libitum access to fresh water, and were adapted to the basal diet for 10 d before trial initiation. The basal diet was altered when evaluating rain-damaged bermudagrass forages based on the recommendations of Vanzant et al. (1998), who suggested that in situ incubations should be conducted in rumens of fistulated research animals consuming a basal diet similar to the feedstuffs being evaluated. Specifically, this was recommended if the research goal is to directly apply in situ values to a given production situation. Because orchardgrass and bermudagrass differ markedly in seasonal growth patterns, plant anatomy, mechanisms of C fixation, etc., rain-damaged forages of each type were incubated ruminally within animals consuming basal diets consistent with the forage type being evaluated.
Dacron Bag Procedures
Five-gram samples of forage ground through a 2-mm screen were weighed into dacron bags (10 by 20 cm, 53-µm pore size; ANKOM Technology, Fairport, NY) and sealed with an impulse heat sealer (Model CD-200, National Instrument Co., Baltimore, MD). Single bags of each forage sample were placed randomly into 36- by 50-cm mesh bags (21 dacron bags per mesh bag), inserted into the ventral rumen of each steer before feeding, and incubated for 48 h. Upon removal from the rumen, all bags were immediately rinsed 10 times in 47 L of cold water (1-min agitation, 2-min spin per rinse; Coblentz et al., 1997) in a top-loading washing machine (Model LXR7144EQ1, Whirlpool Corp., Benton Harbor, MI). Following rinsing, all bags were dried to a constant weight under forced air (55°C), allowed to equilibrate with the atmosphere (Vanzant et al., 1996), and weighed to determine residual DM. Ruminal disappearance was calculated as the proportion of DM disappearing from the dacron bags during the 48-h ruminal incubation period. Each forage was evaluated in both steers, and these values were averaged before conducting statistical analysis.
Statistical Analysis
Within forage type (orchardgrass or bermudagrass), each of the three test forages (WET, IDEAL, or DRY orchardgrass and WET, MID, or IDEAL bermudagrass) had to be wilted different lengths of time before applying simulated rainfall; therefore, simulated rainfall could not be applied to each simultaneously. For this reason, an independent analysis of variance was conducted for each combination of forage and initial moisture concentration. Within each analysis of variance, data were analyzed as a randomized complete block design with a 7 x 2 factorial arrangement of treatments. Treatments included seven levels of rainfall (0, 13, 25, 38, 51, 64, or 76 mm) and two postrainfall drying methods (OVENDRY or AIRDRY). There were three replications of each interactive treatment. With very few exceptions, there was no interaction of these main effects (P > 0.05); therefore, only main effects are reported and discussed throughout. Single degree-of-freedom orthogonal contrasts (PROC GLM; SAS Inst., 1989) were used to test for linear, quadratic, cubic, and quartic effects due to rainfall amount. Statistical significance was declared at P 0.05, unless otherwise noted.
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RESULTS |
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50 g kg–1) at all rainfall amounts, and a maximum of 88 g kg–1 was reached when 76 mm of simulated rainfall was applied. These losses increased in linear, quadratic, cubic, and quartic patterns with simulated rainfall. Drying method also affected DM loss; overall, OVENDRY forages lost 19 g kg–1 more DM than did AIRDRY.
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Drying method had no effect on concentrations of total N or hemicellulose, but DM loss and concentrations of NDF, ADF, cellulose, and lignin were greater for OVENDRY than for AIRDRY. The greater concentrations of most fiber components in OVENDRY forages also was reflected in estimates of ISDMD, which were smaller by 20 g kg–1 for OVENDRY forages.
DRY Forage
Orchardgrass forage that was wilted to excessively dry concentrations of moisture (41 g kg–1) lost a maximum of 107 g kg–1 of DM (Table 3), which was numerically greater by 19 g kg–1 than the maximum loss observed for IDEAL forage. As observed for IDEAL forage, a large loss (58 g kg–1) was associated with the first 12 mm of applied rainfall, which was greater than losses observed over the next 64 mm of simulated rainfall. Losses of DM were explained by linear, quadratic, and cubic terms.
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Bermudagrass Study
WET Forage
Although a quadratic effect was detected, there was no practical DM loss when simulated rainfall was applied to bermudagrass forage with 761 g kg–1 of moisture (Table 4). Similarly, rainfall amount did not affect concentrations of total N. Responses for fiber components were complex, with multiple polynomial effects. However, these responses for ADF, cellulose, and lignin were relatively static through the 64-mm application level, followed by large increases (53, 32, and 20.1 g kg–1, respectively) in association with the final rainfall increment. In contrast, a large concomitant decrease in the concentration of hemicellulose (50 g kg–1) was observed over this same, final rainfall increment. Reasons for these responses remain unclear. Similarly, ISDMD exhibited linear, quadratic, and quartic responses to rainfall; however, the range was very small (631 to 662 g kg–1) for all application levels, excluding the 38-mm rainfall increment (591 g kg–1). Drying method affected DM loss, NDF, and hemicellulose; however, the difference between AIRDRY and OVENDRY was very small (721 vs. 704 g kg–1) for NDF.
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DISCUSSION |
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19 and 1 g kg–1, respectively) at all rainfall application amounts. Despite the minimal response, a linear relationship was observed between DM loss and rainfall amount for WET orchardgrass, and a quadratic effect was observed for WET bermudagrass.
This overall trend is consistent with previous research work. McGechan (1989) summarized several models that indicate leaching losses increase as the forage becomes drier at the time rain damage occurs. Rotz et al. (1991) found that alfalfa forage damaged by rainfall when crop moisture concentrations were 400 g kg–1 had slightly greater estimates of DM loss than alfalfa damaged by rainfall when crop moisture was 650 g kg–1. Similarly, Smith and Brown (1994) measured increased leaching losses in alfalfa damaged by rainfall at 200 g kg–1 of moisture compared with forage damaged at 700 g kg–1 of moisture and suggested that the leaf cuticle becomes cracked as forage tissues become drier, thereby allowing water to penetrate deeper into plant leaves. Greater penetration of rain water facilitates leaching of nutrients from plant tissues. Leaching losses are enhanced further as the plasmalemma loses integrity (Butler and Simon, 1971), resulting in the accumulation of cell solubles in the intercellular spaces where they are more likely to be solubilized by penetrating rain water and subsequently leached from the plant.
A second major trend observed in these studies was the positive relationship between DM loss and rainfall amount. Losses of DM for WET, IDEAL, and DRY orchardgrass forages all increased with rainfall, exhibiting polynomial or multiple polynomial effects, but these effects were not consistent over the three orchardgrass forages. For bermudagrass, DM losses increased linearly for MID forage, and in linear and quadratic patterns for IDEAL forage, but there was essentially no DM loss for WET bermudagrass forage (maximum loss = 1 g kg–1). These results are consistent with other work; a positive relationship between DM loss and rainfall amount has been observed previously for various cool-season grasses and legumes (Sundberg and Thylén, 1994; Smith and Brown, 1994). Rotz et al. (1991) reported that DM was lost from alfalfa at a rate of 10 g kg–1 mm–1 when the rainfall rate was held constant at 18 mm h–1. Likewise, alfalfa hay that received 5 and 20 mm of artificial rainfall during the wilting period exhibited DM losses of 46 and 97 g kg–1, respectively, when cell wall content was used as an internal marker (Fonnesbeck et al., 1986).
Although not tested statistically, it also is clear from these results that orchardgrass was far more susceptible to DM loss than bermudagrass. This is illustrated in Fig. 1, where quadratic regressions of DM loss on rainfall amount for IDEAL bermudagrass and orchardgrass forages are presented concurrently. Theoretically, DM losses should be smaller for bermudagrass than for orchardgrass forage when the moisture concentration and rainfall amount is equivalent for both forages. Perennial warm-season grasses contain substantially smaller concentrations of soluble nonstructural carbohydrates than perennial cool-season grasses (Van Soest, 1982; Moore and Hatfield, 1994), which should greatly reduce the potential for losses via leaching and reactivated or continued respiration.
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Fiber Components
Plant cell wall constituents are not soluble in water (Van Soest, 1982; Fonnesbeck et al., 1986); therefore, concentrations of these components should theoretically increase as soluble carbohydrates and other compounds are leached from the forage. Any reactivation of respiratory processes by plant cells and/or microorganisms associated with the forage after rainfall events also should increase concentrations of structural carbohydrates. This premise has been verified by numerous researchers (Collins, 1982, 1983, 1985; Fonnesbeck et al., 1986; Rotz et al., 1991; Smith and Brown, 1994), and the results of the present study generally corroborate these efforts. Of all the fiber components evaluated, hemicellulose exhibited the least consistent responses, increasing with rainfall amount for only two of the six forage–moisture combinations. Generally, the greatest increases in concentrations of fiber components were observed for IDEAL and DRY orchardgrass; by comparison, increases observed for WET orchardgrass and all bermudagrass forages tended to be somewhat erratic and relatively small in magnitude.
Digestibility
Concentrations of ISDMD decreased in various linear and/or polynomial patterns with rainfall for all six forages. This response was expected based on the increased concentrations of fiber components and the known losses of highly digestible soluble components (primarily sugars; Collins et al., 1982) that are leached or respired from the plant. Reduced forage digestibility in response to rain damage has been reported previously (Collins, 1982, 1983, 1985; Rotz et al., 1991; Turner et al., 2003). Generally, the digestibility of WET forages was less affected by simulated rainfall than that of forages that were wilted before the simulated rainfall event.
Drying Method
While the AIRDRY and OVENDRY methods were designed generally in an attempt to assess reactivated and/or prolonged respiration in field-dried hay crops damaged by rain, our results were inconclusive. Differences between drying methods were observed for all individual response variables except total N, but these differences were not consistent across the six individual forage–moisture combinations. When differences between drying methods were observed for IDEAL and DRY orchardgrass and MID and IDEAL bermudagrass, more DM loss, greater concentrations of fiber components, and smaller concentrations of ISDMD were exhibited in OVENDRY compared with AIRDRY forages. For WET orchardgrass and bermudagrass, the opposite trend was generally observed. Normally, it would be expected that the OVENDRY technique would suspend reactivated and/or prolonged respiration sooner than the AIRDRY method, thereby preserving nonstructural carbohydrates. Theoretically, this should result in smaller DM losses, smaller concentrations of fiber components, and greater ISDMD. However, field-drying conditions were exceptional during these studies, and it is possible that AIRDRY conditions for IDEAL and DRY orchardgrass and MID and IDEAL bermudagrass resulted in comparable or faster desiccation and suspension of respiration than OVENDRY, thereby contributing to the inconclusive nature of these results.
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NOTES |
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This article has been cited by other articles:
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  W. K. Coblentz and W. E. Jokela Estimating Losses of Dry Matter from Wetted Alfalfa-Orchardgrass Mixtures Using Cell Wall Components as Internal Markers Crop Sci., November 24, 2008; 48(6): 2481 - 2489. [Abstract] [Full Text] [PDF]
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 D. A. Scarbrough, W. K. Coblentz, R. K. Ogden, J. E. Turner, J. B. Humphry, K. P. Coffey, T. C. Daniel, T. J. Sauer, J. A. Jennings, and D. W. Kellogg Nitrogen Partitioning and Estimates of Degradable Intake Protein in Wilting Orchardgrass and Bermudagrass Hays Damaged by Simulated Rainfall Agron. J., January 3, 2006; 98(1): 85 - 93. [Abstract] [Full Text] [PDF]
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