Application of Manure to Established Stands of Alfalfa and Alfalfa-Grass

Quirine M. Ketterings, Associate Professor, Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853; Erica Frenay, Support Staff, Department of Crop and Soil Sciences (currently Project Coordinator, Small Farms program, Department of Horticulture), Cornell University, Ithaca NY 14853; Jerry H. Cherney, Professor, Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853; Karl J. Czymmek, Senior Extension Associate, PRODAIRY, Cornell University, Ithaca, NY 14853; Stuart D. Klausner, Senior Extension Associate (retired), Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853; Larry E. Chase, Professor, Department of Animal Science, Cornell University, Ithaca, NY 14853; and Ynte H. Schukken, Professor, Department of Population Medicine and Diagnostic Sciences, Cornell University, Ithaca, NY 14850

Abstract

Nutrient management plans require that manure application to corn and forage grasses be limited to crop N needs. This increases the likelihood that manure will need to be applied to other cropland such as alfalfa fields. In this review we summarize scientific literature on the potential agronomic and environmental impacts of manure application to alfalfa and mixed alfalfa-grass stands.

An estimated 3,290,000 acres of new alfalfa (Medicago sativa L.) and alfalfa-grass seedings were established in the USA in 2005 (52). Wisconsin, Minnesota, South Dakota, and Nebraska combined represented almost 40% of this acreage but also, in non-Mississippi River Basin states such as California, Idaho, Michigan, and Washington, alfalfa is an important forage crop. This combined acreage in new seedings equated to 20% of the total acreage in alfalfa dry hay and haylage production in 2005 (52) indicating the importance of alfalfa production in the USA.

From a nutrient-use efficiency standpoint, corn (Zea mays L.) and forage grass fields tend to be the preferred fields for manure application on dairy farms. Alfalfa typically meets its N requirement through biological N fixation so N from other sources is unnecessary if conditions for N fixation are satisfactory. However, nutrient management plans in many dairy regions of the US require manure application to corn and forage grasses be limited to crop N needs, increasing the likelihood that manure will need to be applied to alfalfa fields. In addition, newly-harvested alfalfa fields may at some times (e.g., during the summer cropping season) be the only fields accessible for manure application. This raises questions about optimum manure application rates, methods, and timing for production (both dry matter yield and forage quality), stand survival and longevity, and environmental impact. In this review we summarize scientific literature on the potential agronomic and environmental benefits and disadvantages of manure application to alfalfa and mixed alfalfa-grass stands.

N Fixation

The 2005 forage analysis database of the Dairy One Forage Laboratory showed an average of 2.7% N (DM basis) for nearly 9,000 alfalfa-grass samples mostly from the Northeast and Mid-Atlantic regions. For established stands in the northeastern USA, an average annual yield of 4.5 ton/acre is not uncommon. This would imply an annual N removal of about 250 lb/acre, while a high yielding stand with a 6-ton/acre crop would remove a little more than 300 lb of N per acre.

In a large-scale assessment across 787 watersheds in the Mississippi River Basin, Russelle and Birr (57) estimated N derived from N fixation to range from 40 to 420 lbs N/acre, showing great spatial variability of N fixation across a large region. This variability is also reflected in study-to-study and within-study variability in the contribution of N fixation to the total amount of N in the harvested crop. Cherney et al. (15) reported that 55 to 85% of the total crop N (above ground) over a two-year study (no N added) was derived from N fixation. In a study by Heichel et al. (30), two alfalfa populations averaged 43% of crop N uptake from N fixation in the seeding year while additional work (31) estimated 58% of the total crop N in the seeding year from N fixation versus 77% in the 4th and last year of the study. It is well established that N addition suppresses N fixation (31,54,58,59,71). If manure addition results in a decline in N fixation similar to the amount of N added with the manure, alfalfa fields could be an excellent option for manure addition. However, estimates of the percentage reduction in N fixation upon N addition (either manure or fertilizer) are variable. Shuler and Hannaway (66) cite studies that suggest the presence of readily available soil nitrate completely inhibits biological N fixation. Others conclude that although biological N fixation decreased with increasing levels of N fertilization, a high rate of fixation continued at the highest N application rates (13,15,41). In a study by Cherney et al. (15) the proportion of plant N from N fixation reduced from 55 to 85% of total crop N uptake to 13 to 27% at high N fertilization rates. Similarly, Lamb et al. (41) reported that even when 750 lb of N per acre (split over 4 applications following harvest) were applied, biological N fixation still supplied 20 to 25% of the total N uptake. This may reflect differences in spatial distribution of inorganic N and alfalfa roots but additional research is needed to investigate this further. These estimates suggest manure application rates should not exceed the equivalent of 75 to 85% of N removed in harvest to minimize annual manure N leaching losses.

The amount of N fixed varies among varieties. For example, non-fixing alfalfa varieties can remove similar amounts of N as N-fixing alfalfa varieties but obtain the N from the soil (5,41) suggesting non-fixing alfalfa varieties or mixed alfalfa-grass stands might be better alternatives for manure application than monocultures of N-fixing alfalfa cultivars. However, germplasm and plant-to-plant variability can be high (13), indicating opportunities for plant breeding and/or genetic engineering for selection/development of germplasms most effective in reducing N fixation in high-N situations without compromising yield and/or quality.

Yield

If soil fertility levels are optimal and N fixation is not compromised, a pure alfalfa stand is not likely to respond to manure application. Manure topdressing of a pure alfalfa stand can lead to a yield response if a response to P, K, and/or secondary macro- and/or micronutrients is expected.

Mixed alfalfa-grass stands are likely to respond to N in the manure if there is more than 60% grass in the stand (27) although N needs differ depending on sod composition (Fig. 1). Bock (6) found that application of 50 lb of N per acre to alfalfa-timothy (Phleum pratense L.) and alfalfa-smooth bromegrass (Bromus inermis Leyss) stands on five different soil types in Pennsylvania produced 25 to 36% higher yields than in the control treatment across soil types. More recently, Jacobsen and Surber (33) studied mixed alfalfa-orchardgrass (Dactylis glomerata L.) in Montana and reported a response to N with a fall application of N at 44 lb/acre broadcast urea. Research conducted in New York from 1994-1998 showed higher alfalfa-orchardgrass yields with N applications of 60 lb/acre or greater (15).

Fig. 1. Established stands with non-fixing alfalfa varieties or mixed alfalfa-grass stands with more than 60% grass are better alternatives for manure application than newly established monocultures of N-fixing alfalfa cultivars.

N Losses (Nitrogen Use Efficiency)

A disadvantage of topdressing established stands (grasses or alfalfa) with manure is the potential for large runoff and N volatilization losses as compared to runoff and/or volatilization losses typically seen when manure can be incorporated into the soil during or shortly after application (38). Staff at the Minnesota Soil and Water Conservation District used an Aerway aerator/tillage tool to incorporate manure on established alfalfa in an attempt to reduce runoff losses (24). Their data suggest this type of tool is effective in reducing nutrient runoff flow as compared to surface-applied manure, and such partial incorporation will reduce ammonia volatilization losses as well (4,67), but additional work is needed to quantify the impacts.

In addition to runoff and volatilization, direct flow of manure to subsurface drains can cause significant N loss and environmental degradation. Hoorman and Shipitalo (32) suggested monitoring tile drain outlets when applying liquid manure to perennial crops in tiled fields as deep cracks in the soil, root channels, and earthworm burrows promote preferential flow of manure to tile drains, especially when applying liquid manure (versus semi-solid manure).

Alfalfa has a deep rooting zone that facilitates utilization of nitrates lower in the soil profile (46,62). Yet, nitrate leaching can be an issue at high manure application rates. Daliparthy et al. (20) studied the impact of various manure application rates on stands of pure alfalfa at two sites in Massachusetts. They showed that 2 years of manure application at relatively low N application rates (5,000 gal/acre, 0.33% total N and 0.15% ammonium-N) had no adverse effects on quality, yield, and soil nitrate levels, but significant leaching occurred if manure was applied at 15,000 gal/acre after the first cutting (20).

The practice of applying manure shortly before plow-down and rotation to corn is a common and convenient method of manure application, since alfalfa injury is not a concern, and there is a wider window of time during which manure can be spread. However, except at very low rates, this practice should be discouraged because it will likely overload the field with N, both from the manure and from the alfalfa, leading to increased risk of N leaching (54). Alfalfa plow-down alone (without addition of fertilizer) has been shown to supply sufficient N for the following corn crop (7,23,50,61). If manure is being applied to address P and K levels, it is recommended to apply the manure while the crop is still actively growing to enhance N uptake (during summer or early fall) and to kill the sod in the following spring (rather than the previous fall) to prevent large N fluxes prior to establishment of the following corn crop. Nitrogen mineralization can also be reduced by removing regrowth before tillage (65) or by using no-till (49).

Phosphorus Buildup

When manure is applied to meet crop N requirements, P is almost always applied at rates exceeding crop removal. The 2005 Dairy One Forage Laboratory database showed an average hay crop forage P concentration of 0.29% suggesting an alfalfa-grass stand removes about 13 lb of P₂O₅ per ton DM. Therefore, an alfalfa-grass mixture will remove about 80 lb of P₂O₅ per acre annually with a 6 ton/acre yield. These estimates can vary depending on the level of luxury consumption of P (8). Phosphorus concentrations in dairy manure can vary as well depending in the level of P in the ration (9) but assuming an average P₂O₅ equivalent content in manure of 0.17% [~14 lb of P₂O₅ per 1,000 gal; based on 500 samples submitted to the Dairy One Forage Laboratory in 2003 (Paul Sirois, 2004, personal communication)] the annual manure application rate to fields with the highest DM production (6 ton/acre) should not be higher than about 5,700 gal/year over each year of the stand if P replacement is the target. Given average yields for established stands in the northeastern USA (4.5 ton/acre), annual application rates should not exceed 4,000 gal/acre. At higher annual rates, soil test P levels will increase and although this might not impact crop yields, this can lead to greater P runoff risk over time (18).

Stand Composition and Stand Persistence

Several studies show that N fertilization of a mixed alfalfa-grass stand tends to favor the grass component over the N-fixing legume, resulting in increasing N needs with age of the stand. A recent example is a 5-year study of grass versus grass-alfalfa in the semi-arid Great Plains (3) where six varieties of grass were sown in monoculture or in mixtures with ‘Rangelander’ alfalfa, in control plots receiving no fertilization and in treatments receiving 45 lb of N per acre. The data from the first 3 years of this study showed supplemental N did not increase the total yield of the mixture, but favored the grasses; with no N addition, the ratio of grass to alfalfa DM yield was 1:19 and with 45 lb of N per acre the ratio was 1:10. Similar shifts towards grass were obtained by others (10,27,53,76). Longer-term impacts of N fertilization on the percentage grass in the stand vary depending on actual amount of N applied. In the same Great Plains study (3) grass species dominated the mixtures during the first 3 years after seeding but alfalfa dominated by the 5th year after seeding possibly due an inadequate supply of soil N for the grass component in the stand, impacting the grasses more than the alfalfa. Griffeth et al. (27) did not find the same shift in dominance after the first 3 years, instead observing that the legume population continued to shrink.

Grasses and legumes also differ in their tolerance to low K levels. Joern and Volenec (35) pointed out that grass can effectively tolerate lower soil test K levels than alfalfa, indicating that soil test K and relative amounts of K and N in applied manure are critical factors. Work by Cherney and Cherney (12) in New York shows less drastic results; only a very modest reduction in perennial grass yields occurred under very low soil K availability. As manure contains substantial amounts of K in addition to N, manured fields are generally sufficiently high in K to not favor grasses over legumes from this perspective.

In situations with a recurring manure surplus (i.e., more manure than can be applied to corn and grass at optimum agronomic N rates), a manure-induced shift in stand composition might not impact yields as legume-grass stands can yield forages that can be as good as or better than either pure alfalfa or pure grass for lactating dairy cows (14,47). However, if the goal is to apply manure and maintain a substantial legume component in the stand over time, the choice of a grass species that is not unduly competitive with alfalfa is crucial. Chan and MacKenzie (10) concluded in their study in Quebec that grass with upright leaves would allow the grass to be more competitive. For producers who harvest forage frequently with the goal of retaining a high alfalfa component in mixtures, reed canarygrass (Phalaris arundinacea L.) sown with alfalfa may present a more desirable option than bromegrass or orchardgrass (64).

Weed Pressure

Manure application tends to increase weed pressure in the seeding year (19,20,21,38,39,62). Weed infestation severity tends to correspond with soil fertility levels (Fig. 2) so growers need to be prepared to handle the extra weed pressure with herbicides or timely clippings on manured fields (21,38,62). However, weed management is generally a concern in the establishment year only (62).

Fig. 2. Growers need to be prepared to handle the extra weed pressure with herbicides or timely clippings as weed infestation severity tends to increase with manure application. In this situation, weed control measures failed due to an extremely wet spring resulting in heavy weed infestation where large amounts of manure (20,000 gal/acre annually) had been applied to the previous corn crops.

Compaction

When manure is applied to established alfalfa or alfalfa-grass stands, a combination of high soil moisture levels and heavy application equipment can result in severe compaction resulting in up to 100% plant mortality in the compacted areas (29,42,63,73). Wheel traffic damage can be minimized by planting traffic-tolerant varieties, using small tractors for cutting, raking, and harvest if possible, avoiding unnecessary trips across the field, using larger harvesting equipment, avoiding tractors with dual wheels, and driving on fields as soon after cutting as possible (73).

Burn, Salt Damage, and/or Smothering

Direct burn, salt damage, and physical smothering of the stand are other concerns when manure is applied. Research at the Northern Research and Demonstration Farm of Iowa State University showed that alfalfa recovered successfully from manure applications up to 4,500 gal/acre, but suffered from reduced vegetative regrowth at 6,000 and 9,000 gal/acre (1). A recent study by Lamb et al. (42) showed no smothering effect of hog manure application if applied within 4 days after cutting and if rates were limited to no more than 2,950 lb/acre organic solids (the equivalent of about 2,900 gal/acre of manure with 12% solids). To minimize plant damage in established stands, Kelling and Schmitt (38) recommended no more than 3,000 to 5,000 gal/acre liquid manure (or 10 tons of solid manure) applied immediately after cutting. Undersander et al. (72) recommend to limit manure applications to 3,000 gal/acre and similar suggestions were given by Lory et al. (44). Delaying the manure application after cutting will increase burn risk as new leaves are most sensitive to the ammonium and salts in the manure (38). Additional research is needed to determine if manure inter-row banding or injection could lower burn risk. Currently, manure injection is recommended for ammonia management in Europe and is a mandatory in the Netherlands [e.g., (22,74)].

Forage Quality

Manure application could increase forage protein levels in mixed stands, especially when a significant portion of the stand is grass (48). On the other hand, high N applications could cause nitrate toxicity (17,21,60) although work by Lee and Smith (43) suggested nitrate toxicity is not likely a concern if N applications do not exceed crop N removal.

Another forage quality-related benefit and concern is manure-induced elevated K levels. Potassium is important for winter survival and maintenance of an alfalfa stand. However, high K forages can cause metabolic health problem including ketosis, metritis, retained placenta, and displaced abomasums for non-lactating cows (2,11,25). It is recommended to only feed forage with less than 2.5% K (DM basis) to non-lactating cows (25). Grasses and alfalfa are luxury consumers of K and if sufficient N is available for growth, herbage K concentration will be directly related to soil-available K; uptake will greatly exceed plant requirements if there is excess available soil K (11). Kelling and Schmitt (38) reported that the 12,000 gal application rate used in their studies in Minnesota and Wisconsin added about 360 and 270 lb of K₂O per acre, respectively. By comparison, average manure K in 500 manure samples submitted to the Dairy One Forage Testing Laboratory in 2003 was 0.33% K, the equivalent of about 6.6 lb of K₂O per ton of manure or 26 lb of K₂O per 1,000 gal (Paul Sirois, 2004, personal communication). Schmitt et al. (63) found that herbage K levels increased in direct proportion to the rate of manure or commercial fertilizer applied at all sites in Rosemount and Waseca, MN. The same trend was observed by Lory et al. (44) in Missouri. Thus, repeated application of high rates of manure to forages may increase forage K to levels that are undesirable for non-lactating cows.

Special consideration should be made if alfalfa is to be put up for silage rather than baled as hay, as manure contamination of forage at the moment of harvest will likely result in poor silage fermentation (55). Wiederholt et al. (75) recommend the use of silage inoculants to ensure the presence of sufficient lactic acid bacteria in alfalfa forage but work by Kung et al. (40) showed that Lactobacillus addition had a greater effect on aerobic stability compared to the fermentation process in alfalfa silage. Additional work with alfalfa forage is needed but silage additives are not likely to improve the quality of silage that has significant manure contamination (55).

Manure application following harvest does not necessarily result in manure contamination of the next harvest. Research conducted at the Miner Institute over a 2-year period evaluated the effect of applying 4,200 gal of dairy manure slurry per acre after first cutting to alfalfa-grass stands (69,70). In these studies there were no differences between the control and manure treatments for forage analysis, vomitoxoin, or microbiological levels (minisilo study).

Human and Animal Pathogens

Aside from forage nutritional quality, the decision to apply manure to alfalfa should be considered from the perspective of disease and sanitation as well. Pathogens that are of potential concern include Escherichia coli O:157:H7, Listeria monocytogenes, Salmonella spp., and Mycobacterium paratuberculosis. Among these, M. paratuberculosis is the most likely to be persistent over longer periods of time (45,68). Also, concern about the spread of Johne’s disease caused by M. paratuberculosis is growing due to its increasing prevalence, resulting economic losses, and controversy over possible spread to humans as Crohn’s disease. Currently, 22% of dairy herds in the USA are estimated to be infected with the disease, though only 1 to 5% of the infected cows in a herd will show any clinical signs (16,34). Calves are most susceptible, only requiring ingestion of a small amount of manure or milk to become infected. Infected animals may show no sign of disease for 2 to 10 years, and may infect other animals in that time (34) so it is critical to monitor and continually test herds for the presence of Johne’s shedders. In the early stages of the disease, all animals will test negative, so Collins and Manning (16) suggest continuous testing utilizing two or more of the available methods. However, testing in the absence of changes to calving hygiene management will not be economical or effective in controlling the prevalence of Johne’s disease (28).

Application of manure from infected animals to pastures or forages is a potential method of spreading Johne’s disease to other animals because M. paratuberculosis can live in the soil for up to a year (68). Manure injection reduces the risk of contact between M. paratuberculosis and animals and/or forage crops as well as decreases the potential for surface water contamination due to runoff. However, injection can also favor bacterial survival because of reduced risk of drying and sunlight exposure. Additional research is needed, but it is currently advised to prevent exposure of young animals (< 1 year old) to potentially contaminated pastures or feed coming from manured fields. Ensiling with 60 to 75% moisture, low pH, and high formic acid, and/or addition of 3% ammonia to high-moisture forage has decreased counts of M. paratuberculosis in small-scale fermentation studies (36,37). In the USA, formic acid and ammonia are not generally used as alfalfa silage additives and it is unknown if the bacteria counts would be lowered without these treatments.

Hygienic farm management practices can reduce new infections in a herd. Prevention of Johne’s disease is much more economical than containment of the disease, with the added benefit that such measures tend to improve overall herd health. Although prevention programs are farm-specific, based on herd risk assessments, there are some general recommendations related to manure management, such as decreasing calf exposure to manure and preventing manure contamination of feed and water for the entire herd (34,68). In addition to testing the herd on the farm, new livestock should be purchased from herds certified to be at low risk from Johne’s disease in order to ensure that the bacterium is not present in the manure (28,51). Additionally, manure storage and composting techniques may aid in the reduction of pathogen to be spread on alfalfa (26).

Odor

Although from a water quality point of view, summer application of manure can have great advantages, many odor related complaints initiate from situations where manure is being spread in the summer months on recently harvested alfalfa or grass fields (56). This could in part be addressed by manure banding, using partial incorporation or injection techniques, and applying manure in cool weather. For example, work by Bittman et al. (4) showed great promise for aerators as a low-disturbance method to enhance manure filtration into the soil, conserving ammonia and addressing odor issues as well. Other companies have developed injection or partial incorporation equipment as well (see Cornell's Manure Management Program for a list of companies that supply manure injection equipment).

Summary

• Nutrient management plans require manure application to corn and forage grasses to be limited to crop N needs, possibly resulting in manure having to be applied to other cropland such as alfalfa fields.

• The deeper rooting system of alfalfa as compared to grasses and/or corn, its high P and K demands, and its ability to reduce N fixation upon availability of a readily available N form, make alfalfa a more appropriate alternative for manure application than corn or grass fields for which N needs have already be met.

• Established mixed alfalfa-grass stands with more than 60% grass are better alternatives for manure application than newly established monocultures of N-fixing alfalfa cultivars.

• Liquid manure application rates should not exceed 4,000 gallons/acre/year over the life of the stand to avoid soil P buildup over time, burn, smothering, and/or salt injury to the stand. Application of semi-solid manure is not recommended, but where done, should be limited to no more than 10 tons/acre. Manure should be applied shortly after cutting and fields should be checked for forage K content when the forage is being considered for feeding to non-lactating cows.

• Wheel traffic damage can be minimized by planting traffic tolerant varieties, using small tractors and tanks if possible, avoiding unnecessary trips across the field, using larger harvesting equipment, and driving on fields as soon after cutting as possible.

• Application of manure from animals infected with pathogens, particularly Johne’s disease is a potential method of spreading these infections. In the case of Johne’s disease, exposure of young animals (< 1 year old) to contaminated pastures or to feed coming from these fields should be prevented.

• Plant breeding and/or genetic engineering for selection/development of germplasms should focus on ways to effectively reduce N fixation in high N situations without compromising yield and/or quality.

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