Predicting the natural drying of hay

Agricultural Meteorology, 17(1976)195--204 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PREDICFING

THE NATURAL

DRYING

OF HAY

JERRY D. HILL

National Weather Service Office for Agriculture, University of Kentuclzy, Lexington, Kentucky (U.S.A.) (Received August 20, 1975; accepted March 30, 1976)

ABSTRACT Hill, J. D., 1976. Predicting the natural drying of hay. Agric. Meteorol., 17:195--204. The drying time required for hay to reach a safe moisture content for baling may often exceed the persistence of fair weather and rain may develop before a particular cutting can be put into storage. The decision as to whether the drying quality of the air over a given rain-free period is sufficient to remove the necessary amount of water has always been confounded by other factors such as the plant species being harvested, stage of plant growth, and the method of harvesting. Recent work in the hay drying area has shown that useful methods are available to predict hay drying time. The exponential drying rate exhibited by most hay material can be approximated by an exponential function of saturation vapor pressure deficit or accumulated latent evaporation. Comparisons of the various techniques of forecasting drying time were made with data gathered from fields of alfalfa--fescue, timothy--alfalfa, orchardgrass, bromegrass, and fescue. The tests showed the method of accumulated latent evaporation using appropriate crop constants provided the most accurate method of predicting drying time. INTRODUCTION H a y is a n i m p o r t a n t c o m p o n e n t in t h e p r o d u c t i o n o f l i v e s t o c k f e e d throughout the United States. It provides a method by which the excess amounts of forage which accumulate whenever supply exceeds demand can be stored, then fed, to animals whenever grasses and legumes become dormant T o t a l h a y p r o d u c t i o n in t h e U n i t e d S t a t e s d u r i n g 1 9 7 4 w a s 1 1 3 , 0 0 0 , 0 0 0 metric tons which was valued at $5.7 billion. The most important component o f h a y p r o d u c t i o n is a l f a l f a o r m i x t u r e s o f a l f a l f a a n d grass w h i c h a c c o u n t e d for 66,120,000 metric tons of the U.S. total. W e a t h e r c o n t i n u e s t o p l a y an i m p o r t a n t r o l e in t h e p r o d u c t i o n o f h a y s i n c e m o s t o f it is still c u r e d t o d a y as i t w a s a h u n d r e d y e a r s a g o b y l e a v i n g i t t o the natural drying process. This may require from 2 to 7 days to reduce the m o i s t u r e c o n t e n t t o n e a r 1 5 % d u r i n g w h i c h t i m e t h e h a y is s u s c e p t i b l e t o several factors causing a loss of quality. Estimates of harvest losses (USDA, 1954) range up to 21% of the hay produced and represent a value of over one billion dollars. While artificial drying has been used to reduce weather-related

196 d a m a g e to hay in some instances, the cost o f energy required is b e c o m i n g m o r e expensive and it. appears t h a t c o n v e n t i o n a l curing will remain a widely a c c e p t e d practice. T h e high value of this c r o p and its e x t r e m e sensitivity to w e a t h e r - i n d u c e d d a m a g e provides the m e t e o r o l o g i s t a u n i q u e o p p o r t u n i t y to serve the f a r m e r with an i n t e r p r e t i v e forecast. T h e time required to r e d u c e fresh-cut h a y from 70 or 80% m o i s t u r e d o w n to the 15% m o i s t u r e desired f o r baling d e p e n d s to an e x t e n t u p o n the crop, harvest m e t h o d , and e q u i p m e n t used b u t primarily u p o n the w e a t h e r c o n d i t i o n s prevailing during the curing period. S o m e t y p e o f relationship b e t w e e n d r y i n g time and forecast w e a t h e r factors w o u l d enable a j u d g e m e n t to be m a d e as t o w h e t h e r a h a y c r o p could be cut, cured, and baled prior to t h e n e x t e x p e c t e d rainfall. This p a p e r is i n t e n d e d to describe some of the c u r r e n t w o r k u n d e r t a k e n to d e t e r m i n e the relationship b e t w e e n w e a t h e r factors and d r y i n g time. WEATHER DAMAGE TO FORAGE CROPS T h e principal cause o f w e a t h e r damage to h a y is the leaching of soluble n u t r i e n t s b y the passage o f w a t e r t h r o u g h the plant material. Losses can also be caused by m o l d if t h e hay is c u t and remains w e t for an e x t e n d e d period. T h e leaching losses can be up t o 40% o f the total feed value. T h e o n l y t i m e w h e n leaching losses are m i n i m i z e d is i m m e d i a t e l y a f t e r cutting, w h e n the cells o f the p l a n t are still turgid and will resist infiltration of p r e c i p i t a t i o n . U n d e r c o n d i t i o n s of very light p r e c i p i t a t i o n a small a m o u n t o f w a t e r m a y a c c u m u l a t e u p o n the c r o p surface w i t h o u t p e r m e a t i n g the m a t e r i a l , b u t the limiting a m o u n t o f rainfall is a b o u t 1 m m (Van E i m e r n and Spatz, 1968.) During the d r y i n g process excessive w a t e r losses f r o m the e x p o s e d material on t o p o f the swath can cause the leaves o f t h e p l a n t to b e c o m e brittle and d r o p o f f w h e n mechanical d i s t u r b a n c e s occur. Since the leaves and small stems in e a r l y - b l o o m alfalfa contain up to 70% o f the protein and u p to 90% o f the vitamins, shattering losses are particularly costly. Studies o f these leaf losses d u r i n g handling indicate t h a t a small p o r t i o n is u n a v o i d a b l e b u t in e x t r e m e cases it m a y range u p to 65%. In a d d i t i o n to shattering, excessive d r y i n g can cause a loss of green c o l o r as well as c a r o t e n e , riboflavin, and o t h e r vitamins. In a s t u d y of alfalfa crops {Shepherd et al., 1 9 5 4 ) w h i c h had b o t h rain d a m a g e and no-rain d a m a g e t h e factors s h o w n in Table I were f o u n d . THEORETICAL CONSIDERATIONS In o r d e r to c o n s i d e r w h e t h e r a h a y c r o p can be cut, cured, and baled b e f o r e the n e x t rain e v e n t is e x p e c t e d , it is necessary t o d e t e r m i n e h o w the plant material will react t o the a m b i e n t w e a t h e r conditions. During curing, w a t e r m u s t be lost t h r o u g h the plant's o u t e r surfaces b y d i f f u s i o n and evaporated into the free a t m o s p h e r e . T h e plant w h i c h is cut c o n t a i n s w a t e r w i t h i n

197 TABLE I Effect of rain on cured hay Crop description

Leaves retained (%)

Protein retained (%)

Total digestible nutrients (%)

Decline in milk production (%)

Standing crop Field cured, no rain Field cured, rain*

100 62 38

100 72 55

100 59 52

0 6.7 13.6

* Rainfall amounts up to 25 ram. the vascular bundles and the cells themselves as well as w a t e r v a p o r in t h e intercellular spaces o f the leaves. T h e a m o u n t o f w a t e r c o n t a i n e d initially in t h e p l a n t varies with plant species and also w i t h the stage of g r o w t h . Increasing m a t u r i t y reduces the m o i s t u r e c o n t e n t as s h o w n in Table II. To r e d u c e an a m o u n t of fresh cut h a y at 80% m o i s t u r e t o a t o n o f cured h a y at 15% m o i s t u r e requires the r e m o v a l o f over 2 , 7 0 0 kg o f water. The p a t h w a y for the loss of this w a t e r is primarily t h r o u g h s t o m a t e s b y w a y o f the vascular s y s t e m and intercellular space. When the plant is initially cut, w a t e r v a p o r is freely available in the intercellular spaces and its rate o f d i f f u s i o n into t h e free a t m o s p h e r e d e p e n d s o n l y u p o n the m o i s t u r e c o n c e n t r a t i o n gradient, t h e resistance of the s t o m a t e s and the resistance o f the air. The m o i s t u r e c o n t e n t d r o p s rapidly during the first few h o u r s of drying until the intercellular w a t e r v a p o r is d i m i n i s h e d and m o s t of the remaining w a t e r is w i t h i n the cells. The higher resistance o f the cell walls as w a t e r m o v e s f r o m the interior of the cells into the intercellular space reduces the flux o f w a t e r v a p o r thus r e d u c i n g the d r y i n g rate. A t y p i c a l l y observed d r y i n g curve u n d e r c o n s t a n t , c o n t r o l l e d c o n d i t i o n s is s h o w n in Fig.1. The early drying, c h a r a c t e r i z e d b y a rapid, a l m o s t - c o n s t a n t rate, changes to a decreasing rate as the resistances t o d i f f u s i o n increase. The d r y i n g curves are o f t e n expressed as a m o i s t u r e ratio r a t h e r t h a n m o i s t u r e c o n t e n t w h e r e the m o i s t u r e ratio after any time t: M R t is: MR

t -

m o i s t u r e r e m a i n i n g to be lost after t i m e t t o t a l m o i s t u r e to be lost w h e n e q u i l i b r i u m is reached

and can be calculated as: Mt - Me MR

t Mo

- Me

w h e r e M t = observed m o i s t u r e c o n t e n t at a n y time, t; Me = equilibrium m o i s t u r e c o n t e n t at the given d r y i n g c o n d i t i o n (a f u n c t i o n o f the t e m p e r a t u r e

195

T A B L E II E f f e c t o f age o n initial m o i s t u r e c o n t e n t o f cut alfalfa

Initial m o i s t u r e c o n t e n t ( w e t basis)

May (very early b l o o m )

Early J u n e (late b l o o m )

Mid-June (early seed)

82%

78%

70%

and humidity); M o = initial moisture content. (Note: all moisture c o n t e n t s are calculated on a wet basis,) In lab o r ato r y drying studies of many materials it has been found that the moisture ratio curve exhibits an exponential shape as shown in Fig.2 and can be closely a p p r o x i m a t e d with an exponential function. Under actual field conditions, the drying of the material depends upon not only the resistances within the plant but also how the hay swath is handled. The resistance of the air to diffusion of moisture into the b o u n d a r y tayer is inversely p r o p o r t i ona l to the wind speed such t h a t higher speeds provide lower resistances. An open, loose swath would then lend itself to the maxim u m benefit of the wind in drying but may not favor diffusion from the leaf. 100

90

80

70

60 Moisture Content

(%) Wet Basis

50

4O

30 20 10 0 0

2

4

6

8

10

12

14

Hours after cutting

Fig.1. Typical curve o f m o i s t u r e c o n t e n t vs t i m e .

16

18

20

22

24

26

28

199 1.0 .9 .8 .7 .6 Moisture Ratio .5 .4 .3 .2 .1 0

|

i

!

=

i

=

i

2

4

6

8

10

12

14

16

18

20

22

24

26

Hours after cutting

Fig.2. T y p i c a l curve o f m o i s t u r e ratio vs time.

80 MOWED 70

6050,

~RAKED

Moisture Content (%) Wet Basis

uNCP,US~4ED ~ ~ ~ ~

~

~

"-"-

40

20

12 noon

4

8

12 mid

4

August 9

Fig.3. D r y i n g curves for crushed and uncrushed alfalfa.

8

12 noon August 10

4

i

8

200

Research in Mississippi (Jones, 1939) found that windrowing alfalfa about 2--3 h after cutting produced the most rapid drying rate. T em perat ures were 5°--8°C cooler and the relative hum i di t y about 10% higher in the windrow. The guard cells bordering the air spaces became turgid again thus partially reopening the stomates to allow more rapid drying. The drying of a hay crop is controlled closely by the n u m b e r of stomates unique to the plant species, the leaf--stalk ratio, and the behavior of the stomates during the drying process. In order to provide an additional avenue beside the stomates for the m o v e m e n t of water out of the plant, machinery has been developed in recent years to crush the stems during the mowing process. This crimping or conditioning process then allows the direct movement of water from the interior p o r t i o n of the stems to the air thus overcoming the resistance of the cuticle layer to the diffusion of vapor. During good hay drying conditions, hay not cut with this type of e q u i p m e n t has required an additional 50% drying time to reach a safe moisture c o n t e n t for baling. A comparison of the drying time (Hall, 1957) for the t w o harvesting techniques is shown in Fig.3. METHODS AND MATERIALS

Predicting the drying time of hay -- saturation vapor pressure deficit

The exponential decay curve of ~he moisture ratio can be expressed as: MRt = e -kt

where k is a function o f t he material and the drying condition; and t is time in hours to reach the given moisture ratio. Studies of the drying of f odde r in G e r m a n y (Agena et al., 1968) used the saturation vapor pressure deficit to integrate all environmental factors affecting the drying rate. A similar approach was taken to use k in the exponential drying relationship as a single parameter which integrated all factors influencing drying rates of alfalfa, t he n relate k to the accumulated daily saturation vapor pressure deficit. Alfalfa samples were gathered at the University of K e n t u c k y Agricultural E x p e r i m e n t Station and dried in thin layers on wire baskets under 9 different controlled conditions of t em pe r at ur e , humidity, and air flow. A non-linear regression program was used to fit the observed data and det erm i ne the coefficient k for each c o m b i n a t i o n o f drying conditions. A 2-parameter model of t h e f o r m M R t = e -kt~ was also tested which provided a b e t t e r fit to the data than the single parameter model. The parameter n was nearly constant for the e x p e r i m e n t a l material and had an average value o f 0.80. In similar studies (K emp et al., 1972) n was f ound t o be a constant and k to be a function of latent evaporation. In this study, k was found to have a simple linear relationship with the

201 s a t u r a t i o n v a p o r p r e s s u r e deficit. T h e c o r r e l a t i o n c o e f f i c i e n t was 0.82. T h e m o i s t u r e ratio o f d r y i n g alfalfa ( u n c r u s h e d ) c o u l d be r e p r e s e n t e d as: MRt

= e

r~to.8

w h e r e k = 0 . 0 0 7 9 ( V P D ) + 0 . 0 9 8 ; V P D = m e a n s a t u r a t i o n v a p o r pressure d e f i c i t since c u t t i n g e x p r e s s e d in millibars. Previous studies o f t h e e f f e c t of c o n d i t i o n i n g on d r y i n g rates ( K e m p et al., 1972) allowed a p p r o p r i a t e c o r r e c t i o n s to be m a d e t o t h e r e l a t i o n s h i p w h i c h e x p r e s s e d the m o r e r a p i d loss o f m o i s t u r e . In t h a t case, t h e value o f k is increased b y 40% and the d r y i n g e q u a t i o n b e c o m e s : MR t

=

e-l.4kto.8

T o t e s t t h e e q u a t i o n , actual d r y i n g d a t a was c o l l e c t e d f r o m a field of 80% alfalfa and 20% fescue w h i c h h a d been cut w i t h a c o n d i t i o n e r . T e m p e r a t u r e s d u r i n g t h e d r y i n g p e r i o d ranged f r o m 19 ° t o 31°C and the relative h u m i d i t y f r o m 50 to 93%. Sunlight was limited o n l y b y s c a t t e r e d c u m u l u s c l o u d s during m i d d a y . T h e o b s e r v e d m o i s t u r e r a t i o and t h e r a t i o p r e d i c t e d b y the 2-param e t e r e q u a t i o n are s h o w n in T a b l e III.

TABLE III Observed and predicted moisture ratios Drying time after cutting (h)

Mean saturation vapor pressure deficit (mbar)

Observed moisture ratio

Predicted moisture ratio

0 2 4 6 9 11 13

-12.19 12.26 12.53 9.48 9.14 9.82

1.00 0.63 0.35 0.22 0.17 0.05 0

1.00 0.58 0.39 0.27 0.20 0.15 0.11

T h e t w o d r y i n g curves are p l o t t e d in Fig.4 and it is a p p a r e n t t h a t t h e pred i c t e d curve o v e r e x t e n d s the a n t i c i p a t e d d r y i n g t i m e . A possible cause of this e r r o r is the use of air t e m p e r a t u r e and relative h u m i d i t y o b s e r v a t i o n s in a s t a n d a r d s h e l t e r t o c o m p u t e the v a p o r p r e s s u r e deficit. T h e r a d i a n t e n e r g y load on the d r y i n g c r o p surface elevates t h e surface t e m p e r a t u r e a n d creates an actual v a p o r pressure deficit g r e a t e r t h a n i n d i c a t e d b y c a l c u l a t i o n s based o n a s s u m p t i o n o f u n i f o r m t e m p e r a t u r e . Unless surface t e m p e r a t u r e s are m e a s u r e d o r e s t i m a t e d , it is likely t h a t d r y i n g rates will b e u n d e r - e s t i m a t e d b y this t e c h n i q u e on all e x c e p t c l o u d y days.

202 1.0

.9

.8,

.7,

.6,

Moisture Ratio .5,

,4.

!

i

2

4

•

6

8

10

t2

14

Hours after cutting

Fig.4. Observed d r y i n g rate and predicted rate calculated f r o m mean vapor pressure

deficit. Predicting the drying time o f hay -- accumulated potential evaporation The drying quality of the air is of t e n measured by potential evaporation in o r d er to integrate the m a n y factors affecting t he process. The use of potential evaporation has been proposed ( H a yhoe and Jackson, 1974) for use in a slightly d if f er en t e xpone nt i a l relationship to predict the moisture c o n t e n t of the hay at the end of each day it remains on t he ground. The relationship can be expressed as:

where Mn = moisture c o n t e n t at the end of the nt h day; Mo = original moisture c o n t e n t at time of cutting; a = a weighting factor characteristic of the hay material; and PEi = potential evaporation on the ith day after cutting (mm). The weighting f a c t or a has been determined from actual field drying e x p e r i m e n t s and various values are shown in Table IV. These values are appropriate for hay which has been cut with a conventional mower. The comparisons of drying time for crushed and uncrushed hay which show tha~ the time requirements can be reduced by one-third imply t hat the vatue o f a should be increased by 50% whenever a c o m b i n a t i o n m o w e r - c o n d i t i o n e r is used in the field.

203 TABLE IV Weighting factors used in the drying equation Plant material

a

Alfalfa (25%) and timothy (75%) mixture Fescue 100% Orchardgrass 100% Bromegrass 100% Alfalfa (80%) and fescue (20%) mixture

0.071 0.075 0.075 0.070 0.063

T h e p o t e n t i a l e v a p o r a t i o n can be a c t u a l l y m e a s u r e d as p a n e v a p o r a t i o n or e s t i m a t e d f r o m o t h e r w e a t h e r p a r a m e t e r s . A regression e q u a t i o n has b e e n d e v e l o p e d (Hill, 1 9 7 4 ) w h i c h uses m a x i m u m daily s a t u r a t i o n v a p o r pressure deficit, average d a y t i m e w i n d speed, and t o t a l daily solar r a d i a t i o n to p r e d i c t daily p o t e n t i a l e v a p o r a t i o n . T h e e q u a t i o n has the f o r m : EVAP

= 0.1125(VPD)

+ 0.1784(WIND)

+ 0.004923(SUN)

- 2.032

w h e r e E V A P = t o t a l daily p a n e v a p o r a t i o n in m m ; V P D = s a t u r a t i o n v a p o r pressure deficit in m b a r ; W I N D = w i n d s p e e d in k m / h ; and S U N = solar r a d i a t i o n in L y / d a y . A n o m o g r a m has b e e n p r e p a r e d t o solve the regression e q u a t i o n f r o m a n t i c i p a t e d values of w i n d speed, r a d i a t i o n , m a x i m u m t e m p e r a t u r e , a n d m i n i m u m relative h u m i d i t y . Using t h e a c c u m u l a t e d p o t e n t i a l e v a p o r a t i o n e q u a t i o n w i t h a value o f a e q u a l to 0.07, an initial m o i s t u r e c o n t e n t of 0.80 and a final m o i s t u r e cont e n t o f 0.15, we find t h a t a b o u t 24 m m of p o t e n t i a l e v a p o r a t i o n are r e q u i r e d t o satisfy the r e l a t i o n s h i p . I f t h e c r o p was cut w i t h a c o n d i t i o n e r , it w o u l d reach 15% m o i s t u r e in a b o u t t h e t i m e r e q u i r e d to e v a p o r a t e 16 m m o f water. A s s u m i n g daily e v a p o r a t i o n a m o u n t s are 7 m m , the c o n d i t i o n e d h a y w o u l d be r e a d y t o bale a f t e r a little o v e r 2 d a y s of d r y i n g as c o m p a r e d to m o r e t h a n 3 d a y s r e q u i r e d for c o n v e n t i o n a l l y m o w e d h a y . SUMMARY AND CONCLUSIONS Viable r e l a t i o n s h i p s are available w h i c h allow e s t i m a t e s t o be m a d e of t h e t i m e r e q u i r e d f o r h a y to d r y to a safe m o i s t u r e c o n t e n t f o r storage. T h e m e t h o d w h i c h uses m e a n s a t u r a t i o n v a p o r p r e s s u r e deficit as a m e a s u r e of t h e d r y i n g p o w e r of t h e air e x h i b i t s a t e n d e n c y t o o v e r - p r e d i c t t h e d r y i n g t i m e . This m a y n o t be a c o n s i s t e n t e r r o r b u t one w h i c h c o u l d be e x p e c t e d w h e n e v e r significant a m o u n t s of solar r a d i a t i o n increase the t e m p e r a t u r e o f t h e c r o p surface. T h e m e t h o d using a c c u m u l a t e d p o t e n t i a l e v a p o r a t i o n is s t r a i g h t f o r w a r d b u t m a y require s i m p l i f y i n g graphs f o r easier use. A n y m e t h o d available f o r

204 p r e d i c t i n g e v a p o r a t i o n d u r i n g t h e f o r e c a s t p e r i o d several d a y s in a d v a n c e a l l o w s a j u d g e m e n t to be m a d e w h e t h e r h a y c o u l d be c u t a n d d r i e d b e f o r e t h e n e x t r a i n occurs. T h e s e r e l a t i o n s h i p s b e t w e e n t h e d r y i n g rate of h a y a n d t h e e n v i r o n m e n t a l c o n d i t i o n s have c o n s i d e r e d o n l y t h e m e t e o r o l o g i c a l f a c t o r s i n v o l v e d . An e n t i r e field o f r e s e a r c h is o p e n to a n y o n e wishing t o s t u d y t h e a d d i t i o n a l f a c t o r s w h i c h i n f l u e n c e t h e d r y i n g r a t e . T h e s e involve i n t r i n s i c v a r i a b l e s such as t h e e f f e c t of f o r a g e c r o p , v a r i e t y , a n d stage o f g r o w t h on d r y i n g . E x t r i n s i c f a c t o r s r e q u i r i n g a d d i t i o n a l s t u d y are: (a) t h e e f f e c t of soil m o i s t u r e c o n t e n t on d r y i n g rate; (b) t h e e f f e c t o f p o r o s i t y o f w i n d r o w a n d w i n d s p e e d intera c t i o n s on d r y i n g rate; and (c) t h e e f f e c t of d e w on r e t a r d i n g t h e d r y i n g of hay. REFERENCES Agena, M. U., Batjer, D. and Wessels, G., 1968. Wie viel "Einfuhrtage" stehen im nordwestdeutschen Raum fiir die Bergung yon Winterfutter zur Verfiigung? (How many "carting days" are available in the northwestern region of Germany to bring in the winter fodder?) Meteorol, Rundsch., 21(6): 169--175. Hall, C. W., 1957. Drying Farm Crops. Agricultural Consulting Associates, Inc., Reynoldsburg, Ohio, 336 pp. Hayhoe, H. N. and Jackson, L. P., 1974. Weather effects on hay drying rates. Can. J. Plant Sci., 54: 479--484. Hill, J. D., 1974. The Prediction of Daily Drying Rates. National Weather Service Central Region Tech. Mem., 56. Jones, T. N., 1939. Natural drying of forage crops. Agric. Eng., 20: 115--116. Kemp, J. G., Misener, G. C. and Roach, W. S., 1972. Development of empirical formulae for drying of hay. Trans. ASAE, 15: 723--725. Shepherd, J. B. et al., 1954. Experiments in harvesting and preserving alfalfa for dairy cattle feed. USDA Tech. Bull., 1079. USDA, 1954. Losses in Agriculture. ARS -- 20 -- 1. Van Eimern, J. and Spatz, G., 1968. Das Problem der verfi~gbaren Tage fi]r den Wiesenschnitt. (The problem of available days for grass cutting.) Bayer. Landwirtsch. Jahrb., 45: 350--363.