Maintaining and Enhancing Forage Quality During Harvest and Storage

C. Alan Rotz

US Dairy Forage Research Center,
USDA/Agricultural Research Service,
Agricultural Engineering Department,
Michigan State University, East Lansing, Michigan, USA 48824. E-mail: rotz@msu.edu

Take Home Message

To maintain forage quality during harvest, rapid field drying is essential. Mechanical and chemical treatments can provide effective tools for speeding drying, but neither process can compensate for poor drying weather and/or thick dense swaths. Swath manipulation with a tedder or inverter can speed drying, but increased costs and losses caused by the operation may be greater than the average benefit received. Baling of moist hay can also reduce field curing time and harvest losses, but an effective treatment is needed to prevent excessive storage losses. The best preservation of moist hay is obtained through a low-cost drying system. Silage systems reduce harvest losses, but storage losses and quality degradation can be high. Silage inoculants and ammonia treatments can at times reduce these losses a small amount. A new mat drying process is being developed where forage is cut, shredded and pressed into a mat for very rapid field curing. The quality of the forage produced is very high, perhaps superior to that produced by any other method.

Introduction

The awareness of forage harvest and storage losses and their effect on animal performance has improved considerably in the past 50 years. This knowledge is being used to improve equipment and management strategies used in forage production.

Harvest losses are often categorized as: 1) respiration loss, 2) rain damage and 3) mechanical or shatter loss (6). Loss from plant respiration is relatively small when the crop is dried quickly under good drying conditions, but under poor drying conditions it can be over 10% of the crop dry matter (DM). When rain damage occurs, the resulting losses can be as much as 30% of the crop DM, and of course, at times the whole crop is lost. Mechanical losses increase as more machinery operations are used. For example, a typical raking operation can reduce the crop yield 5 to 10%. All of these losses, but especially respiration and leaching losses, reduce the content of digestible nutrients and increase the fiber content of the forage (Table 1). These quality changes can reduce the production of animals consuming the forage.

Forage is stored as hay or silage. Losses and quality changes are small when dry hay is stored in a shed (6). When stored without protection, losses can be much greater with 15% or more DM loss. Lost DM is protein and other highly digestible nutrients. Silage DM losses generally range from 5 to 15% depending on the type of silo used and other management practices (Table 1). This loss again comes from the most nutritious portions of the forage and much of the protein is converted to non protein nitrogen (NPN) which has less value to the animal.

Forage growers are presented with a variety of products and strategies promoted to reduce losses and/or improve forage quality. New products are introduced each year with much promise, yet few survive for the long term. The many choices tend to create confusion. How much do these products or management strategies effect losses and quality? Given the technology available today, what is the best way to harvest and store forage? These are the questions to be addressed. Beginning with mowing, major options available will be discussed with an emphasis on their effect on forage quality.

Mowing and Conditioning

Reciprocating cutterbars have been used for many years in mowing. Their major disadvantage is limited capacity or field speed. The rotary disk mower is a viable option for increasing mowing capacity. In fact, field speed is often limited only by the operator's ability to control the machine. Mowing loss is similar to that of a cutterbar mower. The disadvantage is a higher initial cost and a likely greater repair and maintenance cost. Disk mowers require about four times more power to operate. Therefore, a larger tractor may be required and more fuel is used per hour. With faster field speeds, less labour and tractor time are required. All things considered, neither type of mower has a strong advantage. If mowing capacity is limiting harvest capacity, a disk mower can improve forage quality by providing a more timely harvest.

Mower types with more serious disadvantages are the rotary drum and flail mowers (9). These require much more power. Drum mowers also tend to leave a less uniform swath which may cause uneven drying. Flail mowers provide faster drying but they double mowing losses. With dull knives or improper adjustment, flail mowers (and sometimes rotary mowers) cause a ragged cut or damaged crown which retards alfalfa regrowth and reduces stand persistence.

Mechanical conditioning and mowing are normally combined in the same machine. Many types of conditioning devices are used; these devices can be categorized as either roll or flail conditioners. Rolls are used to smash and/or break the plant stems, and flails abrade the waxy surface of the plant and break the stems. Both processes can improve drying, but for alfalfa, roll devices are more effective. Some roll designs are promoted as providing faster drying, but field and laboratory drying studies consistently show little or no difference in the drying of alfalfa or grass treated with commonly-used crushing roll designs (10).

The intermeshing, rubber roll is the most common type of conditioner. In Michigan, we found this type of conditioning to be very effective on first cutting alfalfa, but less effective on later cuttings (2). For first cutting alfalfa, the stem is normally relatively thick, but the stem size decreases in the regrowth of second and later cuttings. Since thick stems are more difficult to dry, the smashing and breaking action has more potential for improving drying. The finer stems of later cuttings also tend to flow through the rolls with less crushing. When the rolls run the full width of the machine, a thinner mat is created for more effective conditioning. This effect is small and difficult to detect in field drying studies.

Flail conditioners, developed in Europe for grass crops, use rotating brushes or tines to scratch or abrade the plant surface. Since the stems are not crushed or ruptured, they remain more stiff creating a less dense swath or windrow. This type of conditioner has a greater throughput capacity when harvesting high yielding or entangled crops. For alfalfa, flail conditioning is generally less effective than roll conditioning and it can cause greater loss (9).

Dry matter losses and the associated nutrient changes caused or promoted by conditioning increase with crop maturity and the severity of conditioning. Although more severe conditioning can provide faster field curing, harvest losses are generally greater. Normally less severe conditioning is recommended to obtain adequate drying with relatively low loss (1 to 2% of yield).

When compared to a cutterbar mower, a mower-conditioner increases production costs due to the higher initial cost and slightly higher power requirement. In the midwest, the benefits of faster drying and reduced field losses return up to four times the amount spent on increased costs. With the drier weather and heavier swaths commonly encountered in western Canada, the economic value may be less, but it is still likely to be beneficial for most situations.

Chemical conditioning is a newer process for speeding the drying of alfalfa. A chemical, referred to as a conditioner or drying agent, is sprayed on the crop at mowing (2). The chemical affects the waxy surface of the plant to allow easier moisture removal from the plant. Several chemicals are known to speed drying, but potassium and sodium carbonates are most commonly used. Chemical conditioning is most effective on cuttings harvested in the summer months. The treatment can double the drying rate of the crop when used under good drying conditions with the crop dried in a relatively thin swath. On the average, this increase reduces field curing time about half a day. With a treatment cost near $7(Can.)/t DM of hay, the technique returns the cost of the treatment through improved hay quality, and may provide a small economic gain for the producer.

Swath Manipulation

As alfalfa dries in the field, the top of the swath dries more rapidly than the bottom. Manipulation of the swath can speed the drying process by moving the wetter material to the upper surface where it dries more quickly. Swath manipulation can also improve drying by spreading the hay over more of the field surface. Spreading can increase the drying rate by increasing the exposure of the crop to the radiant solar energy and drying air. There are three operations used in haymaking which manipulate the swath: tedding, swath inversion and raking.

Interest in tedding has grown in recent years following the introduction of the European style tedder. This device uses rotating tines to stir, spread and fluff the swath. This operation can be used anytime during field curing, but it is best to do so before the crop is too dry (above 40% moisture content). Faster drying is the primary benefit of tedding. The stirring or fluffing aspect typically reduces field curing time up to half a day. Tedders are sometimes used to spread a narrow swath formed by the mower-conditioner over the entire field surface. When done soon after mowing, the average field curing time in the midwest is reduced up to two days compared to drying in a narrow swath. In addition to speeding the drying process, tedding also tends to create more uniform drying, so wet spots in the swath are reduced.

Disadvantages of the tedding process include increased losses and increased fuel, labour and machinery costs. When tedding is done on a relatively wet crop (above 50% moisture), the resulting loss is less than 3%; however, applied late in the drying process, the loss can be more than 10%. Tedding will also increase raking loss. When a light crop (less than 3.5 t/ha) is spread over the field surface, raking loss can be more than double that when raking narrow swaths. Spreading the hay may promote bleaching of hay colour. Bleaching does not normally affect the nutritive value of hay, but it often affects the market value. The machinery, fuel and labour costs of tedding increase production costs up to $50/ha (8).

Swath inversion machines are available which gently lift and turn the swath placing it back on the field surface inverted from its original position. Exposing the wet bottom layer of the swath speeds drying enough to reduce the average field curing time a few hours. Swath inversion is not as effective in improving drying rate as tedding, but shatter loss is reduced. In general, the gentler crop handling by the swath inverter causes little loss, but the added operation increases labour, fuel and machinery costs. Although swath inversion causes less loss, it also has less drying benefit and thus less potential to reduce rain damage and respiration losses. The cost of the operation is generally greater than the economic benefit received (8).

Raking is another form of swath manipulation. Raking tends to roll the wetter hay from the bottom of the swath to the outer surface of the windrow which improves drying. Following the initial improvement, the increase in swath density can reduce drying rate, so the moisture content of the crop when raked is important. If the crop is too wet, the wet material rolled into the center of the windrow dries slowly. Raking also causes loss, and the loss is related to crop moisture (2% when wet to 15% in very dry crops). The best moisture content to rake for low loss and good drying is between 30 and 40%. In dry climates, hay should be raked at night or in early morning when leaves are moist and less prone to shatter. Raking at the proper time can reduce field curing time a few hours to allow an earlier start at baling.

Another option in field curing is to use a narrow swath with no manipulation. Hay dries more slowly requiring about two more days in the field. With slower drying, more loss will occur from plant respiration and perhaps rain damage offsetting losses saved by not raking. When harvesting silage, it is best to avoid raking unless raking will improve harvest capacity. A substantial economic benefit sometimes can be obtained by rolling a couple swaths together to allow larger balers or forage harvesters to operate more efficiently.

Baling and Chopping

Balers are available which produce bales of a wide variety of shapes and sizes. The traditional small rectangular bale is a viable option, but handling bales of this size tends to require much manual labour. Large, high density bales are becoming more popular, particularly for hay transported long distances. Balers producing these large packages also offer greater baling capacity with the ability of harvesting up to twice as much hay per hour as the small package balers. A popular option for hay producers in some areas is the large round bale system. Round balers are priced higher than conventional square balers, but they have a little greater harvest capacity and a lower labour requirement. Generally, use of a large bale system reduces production costs a few dollars per tonne compared to small bales.

Typical DM losses during hay baling vary between 2 and 5% of the yield with the loss equally divided between pickup and chamber losses (6). Pickup losses are high when the machine is pulled at a faster speed than the rotating speed of the pickup device. The machine tends to overrun the swath causing excessive loss, as high as 5%.

Chamber loss is largely influenced by baler design and crop moisture content (6). Chamber losses are generally 1 to 3% in small rectangular balers and 0.5 to 2% in large rectangular balers. In large round balers, losses vary among baler designs. For a variable chamber baler, chamber loss is similar to that in a small rectangular baler, but the loss can be three times as much with a fixed chamber baler. When hay is baled at night, leaf moisture is higher, and chamber loss can be cut in half. Chamber loss is very sensitive to the feed rate of hay. Excessive loss occurs at low feed rates because the bale is rolled or compressed in the chamber too much per unit of hay baled. Chamber loss is mostly high quality leaf material, so chamber loss has more effect on the quality of the remaining forage than most other machine losses. By maintaining the chamber loss below 3%, the effect on forage quality is relatively small.

Losses in forage chopping vary from 2 to 6% with similar amounts lost from the pickup and by drift (6). Drift losses occur as the chopped material exits the spout of the harvester and travels toward the trailing wagon or truck. Drift losses are influenced by the crop moisture content, the wind conditions, machine adjustment, and operator skill. The quality of the lost material is similar to that harvested, so the loss has little effect on the quality of the remaining forage.

Hay Storage

Respiration of microorganisms (bacteria, fungi, and yeasts) on the hay causes heating of the hay and further DM and nutrient loss during storage (6). Similar loss occurs in all sizes and types of bales stored in a shed. Greater heating occurs as the hay density in bales is increased, particularly in large bales. Dry matter loss during the first month of storage varies from 1 to 8% increasing with hay moisture content. For hay containing more than 30% moisture, excessive loss and even spontaneous combustion can occur. Although a major portion of the loss occurs in the first month, a small loss of about 0.5% DM per month continues in hay stored in a shed.

Unprotected hay stored outside experiences the same loss as hay stored inside plus an additional loss from weathering on the exposed bale surface (outer 10 to 20 cm). Losses in large round bales stored outside vary widely, ranging from 3 to 40% (6). Of the factors affecting this loss, weather, length of storage, and storage method have the greatest impact. The loss is again primarily caused by microorganisms in the hay, and the biological activity is greatest when the hay is moist and warm. Loss is less in hay stored over winter in northern climates or in more arid climates where the hay remains relatively dry. Storage method affects loss by providing different levels of protection from rain. When set on damp soil, high moisture levels develop in the bottom of the bale causing considerable deterioration and loss in that portion of the bale.

Dry matter loss and heating of hay affect the concentration of most nutrients. Much of the lost DM is nonstructural carbohydrate respired to carbon dioxide and water. Some CP is also lost. The lost protein is the more soluble nitrogen (N) components which causes small increases in the water insoluble N and acid detergent insoluble N (ADIN) concentrations. In addition, the heating in high-moisture hay causes the formation of further ADIN (unavailable protein) through Maillard reactions. Fiber concentrations increase during storage due to the loss of non-fiber constituents. Because the loss is primarily highly digestible nutrients, the digestibility of forage DM decreases during storage (Table 1).

Hay Preservation Treatments

In hay making systems, field losses can be reduced by baling hay at a moisture content near 25%. Baling moist hay reduces baler chamber losses providing a small increase in harvested yield (up to 2%) and harvested quality. Raking and pickup losses also may be reduced slightly. Field curing time on the average is reduced one day which reduces the potential for rain damage. With all of these factors combined, harvested yield is increased an average of 7%. However, the moist hay deteriorates rapidly in storage, offsetting the benefit of reduced field losses unless treated to enhance preservation. Additives used for the preservation of high-moisture hay include propionic acid, organic acid mixtures, buffered acid mixtures, anhydrous ammonia and microbial inoculants.

Propionic acid (or an effective organic acid mixture) when applied at rates of 1 to 2% of hay weight, normally reduces mold growth and heating. To reduce corrosion of equipment, buffered acid products have largely replaced the straight acids. Acid treatment reduces storage loss in damp hay during the first few months of storage, but the loss is higher than that in dry hay. Acid-treated hay maintains a higher moisture content throughout storage, and thus a little higher level of microbial activity. Over a six month storage period, the loss in acid-treated hay catches up, providing little difference in DM and nutrient losses between treated and untreated high-moisture hays. When compared to dry hay, acid-treated damp hay is often higher in fiber content and less green in colour (3). Overall, storage losses offset the reduced harvest losses providing little real improvement in nutritional value.

Anhydrous ammonia is perhaps the most effective hay preservative. Mold development, heating and DM loss are reduced or eliminated in hay of up to 35% moisture when wrapped in plastic and treated with ammonia at 1 to 2% of hay weight. Ammonia treatment increases CP concentration by adding NPN and it may further enhance quality by increasing the digestibility and available energy in the hay. Although anhydrous ammonia is very effective, animal and human safety concerns deter its use. Ammonia treatment of forage has caused toxicity to animals when high application rates (greater than 3% of hay weight) were used on alfalfa hay. Direct exposure to anhydrous ammonia can cause severe burns, blindness, and death.

Bacterial inoculants are sometimes applied to hay. Inoculation with a few strains of Lactobacillus have shown little effect on mold, colour, heating, DM loss, and quality change in high-moisture hay. Another bacterial product is available which uses a different bacteria called bacillus. Bacillus bacteria are better suited to the aerobic hay environment. Although this type of product shows more potential, there is still little scientific evidence that it can provide substantial improvement in preserving moist hay.

In the past few years, interest has grown in the use of bale ventilation to enhance hay preservation. Bale ventilation is a process that creates a hole about 4 cm in diameter through the center of small rectangular bales. The hole is supposed to promote moisture loss from the center of the bale during storage and thus enhance the preservation of hay at moisture contents up to 25%. Research does not show any benefit in hay preservation with the use of bale ventilation (4).

An alternative to preservation treatments is the use of a low-cost drying system during storage. Hay is stacked on pallets with a plenum under the center of the stack. A fan is used to push ambient air through the stack during the first month of storage. The air movement dries the hay and prevents heating and mold development. Losses are maintained at a level similar to dry hay (7). The economic value of this low-cost drying system is better than that of chemical preservatives due to less storage loss and normally a lower treatment cost.

Silo Storage

A wet forage is best preserved by ensiling in an anaerobic environment. Creating an environment without oxygen is essential for stopping plant respiration, preventing aerobic microbial growth, and stimulating the growth of lactic acid bacteria (6). The lactic acid bacteria ferment sugars producing lactic and acetic acids which lower silage pH. A low pH inhibits plant enzyme activity and prevents the growth of undesirable anaerobic microorganisms.

During silo filling, the dominant process affecting forage quality is plant respiration. Respiration during filling causes a small DM loss, and it may increase silage temperature which influences the rates of many other ensiling processes. Once the silo is filled, remaining oxygen is rapidly removed allowing anaerobic fermentation to decrease silage pH. After anaerobic microbial activity has ceased because of low pH or lack of substrate, the stable phase begins. Even if silos are well sealed, slow diffusion of air occurs through the silo walls or cover. This oxygen again is used in respiration contributing to further loss. When the silo is opened, much more oxygen is present at the open face, and this oxygen penetrates into the silage mass. With greater oxygen availability, aerobic microbial growth and respiration increase substantially, causing heating as well as DM and nutrient loss. Silage of high moisture can create effluent. Effluent contains many soluble compounds (sugars, fermentation products, soluble protein, NPN, ash, and minerals) which causes further DM and nutrient loss. Total silo losses range from 6% in sealed structures to over 15% in bunkers (Table 1) (6).

Digestible carbohydrates are largely lost through respiration, increasing the concentration of other forage components (6). For example, little loss of N occurs in the silo unless effluent losses and/or nitrate in the incoming crop are present. Consequently, a 1 to 2% DM increase in CP should occur dependent on the CP of the crop entering the silo and the DM lost during silo storage. Changes in cell wall or neutral detergent fiber (NDF) content are also dependent on the amount of enzymatic and acid hydrolysis of structural carbohydrates during storage. The change in NDF levels during silo storage may range from a 1% DM decline to a 4% DM increase, dependent on the respiration loss relative to the amount of cell wall hydrolysis. With good silo management, the concentration of ADF increases 2 to 5% DM with the loss of other carbohydrates. Carbohydrates lost are highly digestible, so DM digestibility or total digestible nutrient (TDN) concentration declines by 2 to 7% DM.

Silage Treatments

A wide variety of additives is used in silage making. The principal additives include bacterial inoculants, enzymes, anhydrous ammonia and urea (6). The most common silage additives are bacterial inoculants which augment the natural lactic acid bacteria of the crop. Inoculant bacteria have been selected from forages for a fast growth rate of the most beneficial organisms. When the added bacteria dominate fermentation, the resulting silage has less acetic acid and ethanol, more lactic acid, and a lower pH than expected from the unaided natural fermentation. This shift in fermentation should improve DM recovery a small amount. An additional benefit from inoculant use is a small reduction in proteolysis and the resulting breakdown of true protein to NPN. An inoculant is most beneficial when the natural lactic acid bacterial population is low.

Enzyme treatments are targeted for reducing the fiber content of silage. These additives are more effective on grass silage than alfalfa silage. Typical reductions in fiber contents range from 1 to 5% DM. These products also appear to improve DM recovery slightly through less aerobic loss. The cause of reduced aerobic loss during silo storage is uncertain.

Both ammonia and urea are common additives to corn silage. These additives boost the CP content and make silages more stable by killing aerobic microorganisms. With lower volatile losses, urea is more efficient in increasing CP, whereas ammonia is more effective in improving aerobic stability. These additives increase crop pH at ensiling and thereby cause more fermentation and fermentation products, particularly acetic acid. Both compounds, but especially ammonia, improve DM and fiber digestibility and reduce proteolysis. In spite of these benefits, animal performance is normally not improved. Also, DM recoveries with these additives are often reduced, presumably due to increased fermentation losses by either lactic acid bacteria or clostridia.

Maceration and Mat Drying

A new harvest process called maceration and mat drying is under development which can greatly speed drying, reduce losses and enhance forage quality. Maceration shreds the plant stems, fully exposing the internal moisture. Macerated alfalfa can dry to a moisture content suitable for baling in 8 h of favorable drying conditions. Macerated material is very susceptible to field loss unless maceration is combined with a mat forming process. The mat system requires several new machines. A mat maker mows, shreds and presses the crop into a mat that is placed back on the field for rapid drying. When dry, the mat can be picked up with a baler or chopper modified to handle the mat with minimal loss. More research and development is needed to work out the details of this new system for harvesting, handling and storage. Commercial equipment is under development with limited availability over the next couple years.

Fast drying is a major advantage of the mat process. The drying rate of alfalfa mats is two to three times greater than the rate of conventional swaths. With an appropriately designed mat system, hay can be made with one day of field curing. Another potential advantage of the mat process is the quality of forage produced. With reduced harvest loss, forage quality is improved. In addition, the maceration process increases the digestibility of the forage. Improved digestibility allows the animal to obtain more energy from the crop, and it may increase the animal's feed intake, both of which lead to higher production. The mat process, therefore, may produce a forage that is superior in nutritive value to any currently produced.

A disadvantage of the mat system is expensive equipment. The projected price for the mat maker is over $40,000 for a trailing machine with a 2.7 m cutting width and over $110,000 for a self-propelled model. The power requirement for this operation is high, requiring at least 90 kW. The field capacity is likely up to 50% less than that of a comparable sized mower-conditioner. The added equipment and fuel cost is high, but the long-term benefits appear to outweigh the cost. The gain in feed value obtained with the mat process could return up to four times the amount spent on the increased costs (5).

References

1. Rotz, C.A. 1985. Economics of chemically conditioned alfalfa on Michigan dairy farms. Trans. ASAE 28(4):1024-1030.
2. Rotz, C.A., S.M. Abrams and R.J. Davis. 1987. Alfalfa drying, loss and quality as influenced by mechanical and chemical conditioning. Trans. ASAE 30(3):630-635.
3. Rotz, C.A., R.J. Davis, D.R. Buckmaster and M.S. Allen. 1991. Preservation of alfalfa hay with propionic acid. Appl. Eng. Agric. 7(1):33-40.
4. Rotz, C.A., T.M. Harrigan and R.J. Tillotson. 1993. Hay preservation in ventilated bales. 1993 Proc. Am. Forage and Grassl. Conf., Am. Forage and Grassl. Counc., Georgetown, TX. pp. 112-116.
5. Rotz, C.A., R.G. Koegel, K.J. Shinners and R.J. Straub. 1990. Economics of maceration and mat drying of alfalfa on dairy farms. Appl. Eng. Agric. 6(3):248-256.
6. Rotz, C.A. and R.E. Muck. 1994. Changes in forage quality during harvest and storage. pp. 828-868. In G.C. Fahey, Jr. et al. Forage Quality, Evaluation, and Utilization. Am. Soc. Agron., Madison, WI.
7. Rotz, C.A. and H.A. Muhtar. 1992. Ambient air drying of baled hay. 1992 Proc. Am. Forage and Grassl. Conf., Am. Forage and Grassl. Counc., Georgetown, TX. pp. 103-107.
8. Rotz, C.A. and P. Savoie. 1991. Economics of swath manipulation during field curing of alfalfa. Appl. Eng. Agric. 7(3):316-323.
9. Rotz, C.A. and D.J. Sprott. 1984. Drying rates, losses and fuel requirements for mowing and conditioning alfalfa. Trans. ASAE 27(3):715-720.
10. Shinners, K.J., R.G. Koegel and R.J. Straub. 1991. Leaf loss and drying rate of alfalfa as affected by conditioning roll type. Appl. Eng. Agric. 7(1):46-49.