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III. Factors Influencing Water Loss during Field Drying of Hay

III. Factors Influencing Water Loss during Field Drying of Hay

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Hay crops are typically cut at various times during the season, and required drying time to achieve safe baling moisture varies during the season.

Dyer and Brown (1977)calculated that the probability of making dry hay

(23%) varied among the three harvest intervals during the season at each of

four locations in Ontario. At Guelph (43"Nlatitude), for example, the probability of making dry hay in 4 days or less varied from 50% in early June,

to 60% in late July, down to 5% in early September. They concluded that to

have at least a 50% probability of reaching safe baling moisture in the

Guelph area would require 4 drying days for May, June, and July, 5 days

for August, and 7 days for the first half of September, values which correspond fairly well to observed practice.

Relative humidity (RH) is one environmental factor which warrants

special emphasis because its influence of hay drying is often

underestimated. The RH which creates the gradient which influences water

loss from drying hay is that within the windrow adjacent to the plant tissue

(Clark and McDonald, 1977; Thompson, 1981). Under controlled drying

conditions, alfalfa arrayed in a thin layer reached 20% moisture in 25 hr at

45% RH, but required 47 hr, or almost twice as long, at 70% RH (Crump,

1985). When treated with K2C03,the influence of RH was comparable,

such that attaining 20% moisture required 6.2versus 11.2 hr at 45 and 70%

RH, respectively.

Because hay is hygroscopic or water absorbing, RH also influences the

equilibrium moisture content of hay approaching safe moisture storage

levels. Nash (1978)calculated that air of 80% RH will be in equilibrium

with hay at 22% moisture. Similarly, 60% RH air sets the lower limit of hay

moisture at 13YO.Diurnal trends in RH partition the day into periods which

are favorable and unfavorable for water loss (Clark and McDonald, 1977).

Dew condensation onto the surface of drying hay increases as the hay dries

(Tullberg and Minson, 1978), such that, particularly under cool, fall conditions, daily drying rate becomes a balance between nighttime rewetting and

daytime evaporation (Crump, 1985). Treatments which cause fastest drying

also allow fastest rewetting, thus eliminating any advantage gained during

the daytime (Feldman and Lievers, 1978). Wilman and Owen (1982)noted

that rewetting is greater in an immature than in a mature grass hay.


Detection and quantification of the various components of plant

resistance to water loss typically require drying under single-stem or thinlayer conditions, to avoid boundary layer effects. In the field, however, the

relevance of plant factors in retarding overall drying varies with the

magnitude of other factors, especially windrow resistance.



41 1

Stomatal Resistance

When fully open and intact, a plant leaf acts as a freely evaporating surface, allowing water to move readily through the system (Williams and h e r ,

1957). Estimates of stomatal density for the upper and lower surfaces of

alfalfa leaves have varied from 1090 and 880 stomata/mm2, respectively

(cited in Pedersen and Buchele, 1960) to 169 and 188 stomata/mm2 (cited in

Harris and Tullberg, 1980). Stomatal density on grass leaves appears to be an

order of magnitude less than that on legume leaves (various, cited in Harris

and Tullberg, 1980). The resistance of open stomata varies from 50 to 500

sec/m for mesophytic plants (Nobel, 1983). When hay is placed in a windrow,

progressive water loss and shading within the windrow block encourages

stomatal closure, which increases resistance to water loss by one to two orders

of magnitude (Harris and Tullberg, 1980).

Although stomata have been reported to close as early as 1 hr after cutting in grasses (Murdock, 1980), or at a moisture content of 2.3 kg water/kg

DM (70% FW basis) (Savoie et a/., 1984), approximately 20-30070 of water

loss occurs prior to stomatal closure (Harris and Tullberg, 1980).

Because stomatal closure applies a significant brake to transpirational

water loss, efforts have been made to keep stomata open artificially, using

applications of fusicoccin, a toxin produced by the fungus Fusicoccum

amygdali Del., which wilts plants by preventing stomatal closure (Morris,

1972; Turner, 1970), and other chemicals (Harris and Tullberg, 1980).

Under controlled drying conditions, alfalfa to which fusicoccin had been applied 3 hr prior to cutting reached 40% moisture in half the time required by

unsprayed control shoots (Turner, 1970). In field-dried trays, fusicoccin

reduced drying time to reach 22% moisture from 54 to 46 hr, or in effect, to

3 days instead of 4. Thus, stomatal closure, while significant, was less apparent under field than under controlled drying conditions.


Cuticular Resistance

The cuticle consists of cutin, an extracellular, insoluble polymer, together

with associated soluble waxes. The cuticle coats the outer surface of epidermal cells, forming a continuous layer which inhibits moisture loss, as well as

protecting against pests.

Cuticular resistance is high, ranging from 2500 to 10,OOO sec/m in crop

plants (Thompson, 1981: Nobel, 1983), and as such, the intact cuticle

presents a formidable barrier to water loss. The hydrophobic nature of the

cuticle comes from fatty acid groups, while that of the waxes is due to

long-chain fatty acids esterified with long-chain monohydric alcohols



(Kolattukudy, 1981). Because of the hydrophobic nature of the cuticle,

most biocide, growth regulator, or desiccant formulations contain a

detergent or wetting agent to facilitate uptake.

The location and chemistry of cuticular waxes have been examined for

white clover (Trifolium repens L.) and red clover (T. pratense L.) (Hall,

1967), and for perennial ryegrass (Lolium perenne L.) and orchardgrass

(Dactylis glomerata L.) (Harris et al., 1974). Species-specific differences in

wax composition arise from variations in the alcohol, fatty acid, and

straight-chain hydrocarbon constituents (Chibnall et al., 1933; Hamilton

and Power, 1969).

Removing or modifying the cuticle to reduce its effectiveness as a

moisture barrier increases drying rate (Dernedde, 1980; Harris and Shanmugalingam, 1982). Altering the surface waxes without modifying the wax

in the internal layers of the cuticle increased the drying rate of red clover

leaves, suggesting that external waxes may exert an overriding influence on

drying rate (Hall and Jones, 1961). Physical removal of the epidermis and

adhering cuticle increased drying rate of red clover leaflets, petioles, and

thick stems by as much as 13-, 11-, and 4-fold, respectively, compared to

that of intact tissues (Harris and Shanmugalingam, 1982).

In red clover, Harris and Shanmugalingam (1982) found that younger,

thinner stems (2-mm diameter) dried more slowly than older, thicker stems

(4-mm diameter), but when the epidermis was removed, drying rate of thin

stems was twice that of thick stems. They concluded that the permeability of

the cuticle to water increases with age in spite of the fact that the thickness

of the cuticle also increased with age. Harris and Tullberg (1980) and

Meidner (1986) cited evidence that the thickness of the cuticle per se does

not appear to relate to its effectiveness in water retention.

Wieghart et al. (1983) inferred that weathering would have depleted the

protective cuticle to a greater extent in older than in younger alfalfa leaves,

thus accounting for faster drying in more mature plants. Meidner (1986)

demonstrated great variations in cuticular conductance between and within

species, depending on age and growing environment. Significant genetic

and environmental influences on epicuticular wax content of alfalfa

cultivars and clones were reported by Galeano et al. (1986).

As a methodological note, the influence of the growing environment on

cuticular formation is of interest. Meidner (1986) reviewed literature indicating that cuticular conductance varies with age, with temperature and

irradiance during growth, and with degree of hydration of the leaf. Hull

(1958) reported that greenhouse-grown plants can have a much thinner cuticle than do field-grown plants, although, as noted, thickness is not

necessarily predictive of resistance. Potassium carbonate, which reportedly

enhances drying by reducing cuticular resistance, reduced drying time required to reach 20% moisture by 77% on field-grown alfalfa (Crump,



1985). However, the same chemical, applied to the same greenhouse-grown

cultivar, which was then dried under an identical temperature, irradiance,

and RH regime, reduces drying time by only 47%. Quantitative and/or

qualitative changes in the cuticle might be expected to influence responsiveness to conditioning (see Section. IV,B,3) and hence, applicability of

results derived from greenhouse-grown plants to the field (Crump, 1985).

3. Intra- and Extracellular Resistance

Water retention via osmotic and matric forces increases during the drying

cycle (Jarvis and Slatyer, 1970; Firth and Leshem, 1976; Murdock, 1980).

Increasing the osmotic potential of leaves of several nonforage species

either by direct introduction of glucose or by high irradiance reduced cell

permeability to water and decreased transpiration rates (Boon-Long, 1941).

While quantitative evidence is limited, it appears that intra- and extracellular factors, apart from stomata1 and cuticular resistance, retard drying primarily in the final drying phase (Jones and Harris, 1980).




Windrow Resistance

Freshly cut hay is often piled directly into a central pile, or windrow, rather

than being left in a swath where it was cut. Windrowing serves to limit surface

area exposed to weathering damage and reduces losses when hay is picked up

by the baler. However, concentrating forage into windrows also reduces surface area available for interception of incident irradiance and increases the

density of the cut crop to about five times that of the standing crop (Larsen

and Rider, 1985). For a 3-ton/ha (18%) crop with an initial moisture content

of 8O%, initial windrow density would be 1.25 kg DM and 5 kg water/m2 of

windrow surface. In addition, the physical structure of the windrow or swath

imposes a constraint on air circulation adjacent to the drying tissues. The frictional drag or aerodynamic constraint is termed boundary layer resistance, as

detailed in Clark and McDonald (1977) and Jones and Harris (1980). Thus,

practices designed to reduce weathering and enhance quality of conserved

feed may, in fact, prolong drying.

Jones and Harris (1980) stated that windrow resistances are most limiting

early in the drying cycle, while plant resistances become paramount later

on. Clark and McDonald (1977) demonstrated differential drying of vertically stratified layers within a grass windrow. The rate of water loss from

the surface layer was initially greater than that of the internal layers, due to

both greater incident irradiance and a shorter diffusive pathlength. When



the surface layer dried to the point where plant resistances became overriding, moisture began to move out of the underlying layers, but at a much

slower rate. Slower interior drying is caused by the limited penetration of incident energy, the smaller VPD, and the longer diffusive path length within

the windrow. Jones and Harris (1980) reported that incident irradiance 2 cm

deep in a windrow was only half that at the surface, while only 10% of

available energy penetrated to the base. Heterogeneity in drying within a

windrow can result in nonuniformity of moisture content within a windrow

and, ultimately, within a bale.

Using a mixture of perennial ryegrass, timothy (Phleum pratense L.), and

white clover, Wilman and Owen (1982) observed faster drying in a wide,

thin windrow than in a narrow, thick windrow (220 versus 728 g DM/m2,

respectively), but only down to 70% moisture. In later stages of drying, the

thinner windrow was disadvantageous, because the broader exposed surface

incurred a greater uptake of moisture from the soil, stubble, rain, and dew.

They concluded that forage should be swathed and then windrowed at 40%

moisture rather that windrowing just prior to baling.

2. Agronomic Factors

Practical methods of facilitating water loss from cut hay include growing

grasses and legumes in mixture to lessen the impact of the slower drying

legumes. In monoculture, Ciotti and Cavallero (1980) found that drying

rate of orchardgrass was faster and haymaking losses were lower than in

alfalfa. In mixture, drying rate was faster and haymaking losses were

significantly lower than in monocrop alfalfa. In addition to conditioning

(see Sections IV,B,2 and 3), other agronomic approaches include (1) leaving

a tall stubble to lessen soil contact and promote air movement under and

within the windrow; and (2) cutting small fields on each of several days,

rather than all at once, to lessen the risk of rainfall damage, as well as prevent overdrying losses. Other alternatives include baling at 3040%

moisture for subsequent barn drying (beyond the scope of this paper), or

baling at 20-30% moisture in conjunction with chemical preservatives (see

Section V).




Rate and pattern of water loss from plant tissues vary with tissue type,

starting moisture content, maturity, and nitrogen (N) fertilization. Due

partly to greater evaporative surface per unit tissue volume, leaf blades dry



faster than stems (Firth and Leshem, 1976). In annual ryegrass (Lolium

multiflorum Lam.), Jones (1979) reported that rate of water loss was three

time greater in leaves than in stems during the rapid, initial drying phase

and seven times greater during the slower, protracted drying phase. By the

time the leaves had dried to safe storage moisture, stem water content remained four to eight times greater than that of the leaves, depending on the

stage of maturity when harvested.

Murdock (1980) concluded that grass leaves dry 10-15 times faster than

stems, and further, that as much as 30% of stem water is actually lost

through the leaves. Jones (1975) used the analogy of grass leaves as wicks

drawing moisture out of the stems until some limiting level of leaf dryness

was reached.

Using thin-layer drying under controlled environmental conditions, Jones

(1979) found that vegetative tillers, in which leaves constituted 80% of total

dry weight, dried in less than one-third the time required by tillers at ear

emergence, when leaves represented only 40% of dry weight. After ear

emergence, however, drying time again decreased due to lower plant water

content and increased exposure of the stem to the drying environment (Harris et a/., 1974). Thus, Jones and Harris (1980) concluded that drying rate in

vegetative tillers was comparable to that in post-ear emergence tillers.

Starting moisture content, or that prevailing when forage is first cut,

directly affects both initial drying rate and required drying duration to

reach safe storage moisture (Hart and Burton, 1976; Savoie and Mailhot,

1986). As grasses mature, moisture content gradually declines from a high

of 85% or more at the leafy, vegetative stage to less than 65% moisture at

the seed-set stage (Nash, 1978; various cited in Harris and Tullberg, 1980).

A comparable range for alfalfa was reported to be 85-74%, with some

variations among tissue types (Harris and Tullberg, 1980).

Moisture content varies among species and with time during the season.

Starting moisture content in 6-week regrowths varied significantly among

five grass species harvested at intervals throughout the growing season

(Morris, 1972). Starting moisture content and initial rate of water loss were

generally lowest in the fine-leaved sheep fescue (Festuca ovina L.), while

starting moisture content was generally highest in perennial ryegrass, particularly in midseason.

While young, leafy forage dries quickly when arrayed in a thin layer, it

dries more slowly than mature forage when windrowed (Dexter, 1947).

Wilman and Owen (1982) concluded that difficulties in drying a young and

heavy grass crop have discouraged British farmers from cutting grass early,

when it is most nutritious, and, further, have resulted in recommended N

levels for grass hay being half that for grass silage. Increasing N from 112 to

637 kg/ha increased starting moisture content of Coastal bermudagrass

(Cynodon ductylon (L.) Pers.) by 8% (Hart and Burton, 1976), while 150



kg/ha N applied to perennial ryegrass increased starting moisture by only

3% over that of the control (Wilman and Owen, 1982).

Genetic variation in drying rate both within (Klinner and Shepperson,

1975; Murdock, 1980; Owen and Wilman, 1983) and between species (Harris

et al., 1974; Jones and Prickett, 1981; Owen and Wilman, 1983) suggest that

rate of grass drying may be enhanced by breeding. Using individual cultivars

of each species, Jones and Prickett (1981) found that tall fescue (Festuca

arundinacea Schreb.) dried faster than timothy, perennial ryegrass, and annual ryegrass, under lab conditions. The deeper root system of tall fescue was

interpreted as lessening the need for a heavy cuticle to act as a barrier to water

loss, thus indirectly enhancing drying rate. In comparisons involving 140

grass cultivars from seven grass species, harvested at hayable maturity for up

to 3 years, Owen and Wilman (1983) also found that tall fescue dried fastest,

followed in order by annual ryegrass = meadow fescue (FestucapratensisL.)

> timothy = orchardgrass > perennial ryegrass = hybrid ryegrass (L.

perenne x L. multiforum). Perennial and hybrid ryegrass required twice as

long as tall fescue to reach hayable moisture.

In comparisons involving a wide range of leaf types on vegetative tillers,

Morris (1972) noted that drying rate increased with increase in the surface

area to dry weight ratio, or for redtop (Agrostis giganteu Roth.), with the

presence of stolons. Grass species with broad leaves were considered easier

to dry than perennial ryegrass or sheep fescue.

2. Legumes

Morphological differences between grasses and legumes, including a

higher initial moisture content and a larger proportion of stem to leaf dry

weight, typically cause legumes to dry more slowly than grasses (Jones and

Harris, 1980). Because of their greater perceived nutritional value and the

high cost of chemical N, legumes are often managed more intensively than

grasses in North America, although the converse is true in Europe. Legumes

may be sown and fertilized with greater care, resulting in greater stand density and yield, and further, are typically harvested at a younger stage,

resulting in hay of greater nutritional potential. However, managing for

higher yields and lesser maturity at harvest may also cause higher initial

moisture content and slower drying.

Thomas et al. (1981) reported that grasses dried faster than alfalfa and

mature alfalfa dried faster than alfalfa cut at early bloom stage, as confirmed by Wieghart et al. (1983). Differences in initial moisture content

among legume species influenced treatment comparisons in a study by

Crump (1985). Correcting for such differences using initial DM content as a

covariate revealed treatment effects not apparent in the original data.



Species-specific differences in drying rate affect the feasibility of successful haymaking. For example, alfalfa dries relatively quickly, and is thus

the species of choice for haymaking, while the much slower drying of red

clover encourages its conservation as silage rather than hay. In a comparison

involving alfalfa, sweetclover (Melilotus alba Desr.), sainfoin (Onobrychis

viciaefolia Scop.), alsike clover (Trifolium hybridum L.), and red clover,

Clark et al. (1985) reported that drying time to 20% moisture varied from

44 to 86 hr among species. Drying was fastest in alfalfa and slowest in red

clover. Stem diameter accounted for 47% of the variation in drying rate.

Qualitative differences between the cuticles of grasses and legumes were

inferred by Chung and Verma (1986), who reported that chemicals which

accelerated drying in leguminous crops had no effect on grass crops.

Harris and Tullberg (1980) reviewed the literature on stem-to-leaf

transfer of water and concluded that leaf transpiration substantially affects

stem moisture loss. Tullberg (1975) demonstrated that at least 35% of stem

water was lost through the leaves in alfalfa, and that the stem-to-leaf

pathway remained viable until plant moisture content approached 40%.

Stem moisture loss through leaves was also inferred by Clark et al. (1985),

who contrasted water loss in the initial and final phases of drying (see Section I1,B) of internode sections of red clover. In intact sections, rate of

water loss was 153% faster in the initial than in the final phase of drying.

When blades were removed, however, rate of drying did not differ between

the two phases, an effect which was independent of internode age.




Carter (1960) summarized much of the earlier work detailing DM and

nutrient losses in haymaking. Based on nine published trials comparing

field- with barn-cured hay, total losses between cutting and feeding averaged

25 and 15%, respectively. In eight of these trials for which data was

available, average loss of crude protein (CP) in field-dried hay was 34%, or

25% greater than DM loss. Wilkinson (1981) summarized results of nine

trials showing that in vivo DM digestibility of field-dried hay was 8.1

percentage units lower than that at cutting (64.4 versus 72.5'7'0,

respectively). In addition, based on 26 comparisons, intake (of digestible

DM) of field-cured hays was 18% lower than that of the same grass at cutting. Dulphy (1980) also reported an 18% reduction in intake of field-dried

hay, relative to that of fresh material. Reduced intake was associated with

decreased soluble constituents and increased cell wall contents, which were attributed to respiration, leaf loss, and weathering damage occurring in the field.



The magnitude of these losses in yield and quality of forage during drying

and haymaking underscores the inherent conflict between managing for loss

of water while striving to retain yield and quality. Losses of dry matter and

nutrients in the conservation process may be considered in three phases:

respiratory and weathering losses during drying; harvest losses associated

with cutting, conditioning, raking, tedding, and baling; and storage and

handling losses.




Plant and microbial respiration throughout field drying can reduce

harvested yield by 2-8% under good drying conditions and by 16% under

poor conditions (Klinner and Shepperson, 1975). When drying is delayed by

extremely wet and humid conditions, as much as 30% of initial DM can be

lost due to respiration (Rees, 1982). Tullberg (1975) suggested that respiration losses in bulk samples of alfalfa can reach a maximum of 4% DM loss

per day, independent of plant maturity. However, photosynthesis may also

continue after cutting, partially offsetting respiratory losses (Greenhill,


Following cutting, plant respiration continues, but at a declining rate

(Wood and Parker, 1971) -until plant moisture content reaches about

3 0 4 % (May-Brown and Harris, 1974; Martin, 1980; and others cited in

Klinner and Shepperson, 1975). Even when whole plant moisture content

has reached 30-40%, the slower-drying parts of the plant will continue to

respire until they too reach that point (Rees, 1982). Rewetting due to dew or

rainfall prolongs respiration and increases overall loss. Using thin-layer drying, Simpson (1961) found that crushing forage stems stimulated respiration, but as crushing also accelerated drying and caused respiration to cease

earlier, overall respiration losses were reduced.

Protracted field drying exposes hay to potential leaching and weathering

losses, which can significantly reduce not simply DM but also digestibility

of the conserved forage. Leaching of soluble nutrients from the cut plant

material is the principal component of weathering damage, followed by leaf

and bloom loss and molding (Hill, 1976). Timing of rainfall is perhaps more

critical than amount, as tolerance to rainfall declines with drying time. The

relative integrity of the cuticle and cellular membranes in fresh-cut forage

prevents loss of soluble nutrients such as nonstructural carbohydrates and

potassium (Murdock and Bare, 1963).

Loss of digestibility, amounting to 5 percentage units or more of digestible organic matter (Nash, 1978), and decreased voluntary intake can accompany even modest rainfall. In 21 comparisons between grass hays which

were field-cured with and without rainfall damage, Wilkinson (1981)

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