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Production Papers

Influence of Nitrogen Fertilization on Multi-Cut Forage Sorghum–Sudangrass Yield and Nitrogen Use

^a Southern Crop Protection & Food Research Centre, 1391 Sandford Street, London, ON, Canada, N5T 4T3
^b P.O. Box 186, Delhi, ON, Canada, N4B 2W9

^* Corresponding author (beyaertr{at}agr.gc.ca)

Received for publication March 15, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Forage sorghum–sudangrass [Sorghum bicolor (L.) Moench] is a relatively new crop to eastern Canada and the effects of additions of fertilizer N on yield, N accumulation, and N use efficiency are not available for this region. In 1998, 1999, and 2000 the response of forage sorghum–sudangrass to additions of fertilizer N rates (0, 50, 100, 150, 200, and 250 kg N ha^–1) either applied as a single sidedress application or split into two sidedress applications was evaluated on a Fox loamy sand (Psammentic Hapludalf). Although timing of N application had little effect on DM production, splitting the N application into two equal applications may be of benefit by enhancing the NUE and ANR of individual cuts. Maximum yield was estimated at 5.95 Mg ha^–1 at an N rate of 125 kg N ha^–1 and the most economic rates of N ranged from 83 to 107 kg N ha^–1 dependent on the cost of N fertilizer and value of hay. Nitrogen concentration increased linearly with increasing N application and the maximum N accumulation was 161 kg N ha^–1 at an N rate of 196 kg N ha^–1. Total N use efficiency and apparent N recovery decreased with increasing N rates ranging from 36 to 11 kg DM kg^–1 and 90 to 24%, respectively. Optimum yield and N efficiency occurred when 100 kg N ha^–1 was applied as a split application. Producers in southern Ontario require N fertilizer additions to optimize sorghum–sudangrass yields but need to avoid overfertilization with N to maximize N use efficiency and apparent N recovery.

Abbreviations: ANR, apparent nitrogen recovery • DAP, days after planting • DM, dry matter • MERN, most economic rate of nitrogen • NUE, nitrogen use efficiency

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
FORAGE SORGHUM–SUDANGRASS is an important livestock feed in the USA, often used to produce silage, hay, or pasture during summers when there is inadequate moisture for the production of other crops (Fribourg, 1995). In 2003, 3.22 million Mg of silage sorghum–sudangrass from 139000 ha with an average yield of 23.3 Mg ha^–1 was produced in the USA (USDA-NASS, 2004). In contrast, forage sorghum–sudangrass is a relatively new crop to eastern Canada and has a limited production. In 2003, approximately 129000 ha of fodder corn (Zea mays L.) with an average yield of 6.4 Mg ha^–1 (OMAF, 2004a) and 940000 ha of hay with an average yield of 1.1 Mg ha^–1 (OMAF, 2004b) was produced in the province of Ontario. The recent development of sorghum–sudangrass hybrids with comparable yields to fodder corn make this crop a potential alternative forage crop for areas with sandy soils and shorter growing seasons.

Forage sorghum–sudangrass is usually managed with low N fertilizer inputs but has shown some growth and yield response to N application (Patel et al., 1992, 1994; Shukla and Sharma, 1994; Wanjari et al., 1996; Ram and Singh, 2001). Results obtained in New York state suggest that brown midrib (BMR) sorghum–sudangrass hybrids should be fertilized more like an intensively managed perennial grass than a corn crop with N fertilizer being applied at rates of 110 to 150 kg N ha^–1 before planting and after each cut in a multi-cut system (Ketterings et al., 2004). In recent studies, Kilcer et al. (2002) have concluded that BMR forage sorghum–sudangrass fertilized at these rates produces yields comparable with those obtained for silage corn in New York State. Current recommendations for producing sorghum–sudangrass hybrids for forage in Ontario suggest that a total of 50 to 100 kg N ha^–1 be applied in a split application with one-half of the N applied at planting and the remaining one-half applied after the first cut (OMAF, 2002).

Timing and placement of N fertilizers have a major effect on the efficiency of N management systems. Nitrogen should be applied to a crop at times that avoid periods of significant loss and provide adequate N when needed by the crop while placement of N fertilizers should aim at maximizing availability of N while minimizing potential losses. Studies with grain sorghum have shown that fertilizer knifed-in at planting has increased yields relative to broadcast applications (Lamond, 1987; Sweeney, 1989; Khosla et al., 2000). Time of N application studies have lead to the general conclusion that N should be applied nearest to the time of crop needs. For corn production, a single sidedress application several weeks after corn emergence has maximized fertilizer N efficiency in most situations (Pielielek and Fox, 1992; Fox et al., 1986; Aldrich, 1984; Olson and Kurtz, 1982), whereas multiple applications throughout the season have been suggested for optimum forage production of grass species (Lauriault et al., 2002). These results suggest that increased dry matter yields and N use efficiency may be realized if N is knifed into the soil at more than one time during the growing season.

Studies of the yield response of forage sorghum–sudangrass hybrids developed for areas with shorter growing seasons to additions of fertilizer N have not been previously reported. Little is known about the effects of the amount and timing of N additions on forage sorghum–sudangrass dry matter production, N accumulation, and N use efficiency. Our objective was to determine the response of forage sorghum–sudangrass to addition of fertilizer N rates and timing of N application and evaluate the influence of these fertilization practices on the dry matter (DM) yield, N accumulation, N concentration, N use efficiency (NUE), and apparent N recovery (ANR) of forage sorghum–sudangrass.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field experiments were conducted at the Southern Crop Protection and Food Research Centre, Delhi, ON, from 1998 to 2000. The experimental site was different each year, although the soil at each site was classified as a Psammentic Hapludalf with a surface horizon having a loamy sand texture with a soil pH of 6.2, 6.1, and 6.2; organic C content of 8.9, 9.1, and 8.8 mg C g soil^–1; total N content of 0.53, 0.57, and 0.52 mg N g soil^–1; 0.5 M sodium bicarbonate extractable P of 112, 110, and 109 mg kg soil^–1; and a 1.0 M ammonium acetate–extractable K content of 113, 115, and 112 mg kg soil^–1 for each of the three sites, respectively. At each site the previous crop was fall rye (Secale cereale L.) fertilized at 45 kg N ha^–1 with no additions of P and K. In each year, a total of 11 treatments were arranged in a randomized complete block design with four replications of each treatment. Individual plots measured 2.4 m in width and 7 m in length.

In all years, 50 kg K₂O ha^–1 (muriate of potash) was incorporated to a depth of 0.10 m before seeding sorghum–sudangrass. Soil tests determined that there was no requirement for P additions at these sites. Sorghum–sudangrass (cv. CFSH 17) was planted with a drill on 21 May 1998, 9 June 1999, and 5 June 2000. Each experimental plot consisted of four rows spaced at 0.6 m. All plots were over-planted and thinned following emergence to a population of 160000 plants ha^–1. Weed control was achieved by cultivation and hand hoeing as required. Fertilizer N was applied as a liquid urea–ammonium nitrate mixture (28% N w/w) injected 0.10 m deep, midway between the rows and metered through an orifice attached to a knife configured behind a rolling coulter. Injected N rates were established by varying the application speed and/or the orifice size. Two N application regimes were compared. The first involved sidedress application at approximately the six-leaf stage of growth and the second involved a split application with one-half of the N applied at the six-leaf stage of growth and the remainder of the N applied following the first cut. For each fertilizer application method, five N rates, ranging from 50 to 250 in 50 kg N ha^–1 increments, were compared with a control plot where no N fertilizer was applied.

Sorghum–sudangrass was harvested in three cuts on 15 July, 18 August, and 23 September in 1998; 28 July, 19 August, and 15 September in 1999; and 4 August, 28 August, and 25 September in 2000 with a forage chopper when the forage sorghum–sudangrass was at the preflowering to early flowering stage. Sorghum–sudangrass yield was determined by weighing the harvested material from 6 m of the center two rows of each plot and correcting for water content and area harvested. Water content of the plant material was estimated by determining the percentage loss in weight after drying at 50°C in forced air driers until weight loss ceased. Total yield was calculated as the sum of the individual harvests. Harvested yields are reported on a dry-matter basis.

The dry subsamples used to determine water content were ground to pass a 1-mm screen in a Wiley mill (Thomas Manufacturing, Philadelphia, PA). Dry samples were digested with a mixture of sulfuric and selenious acids using the procedure described by Issac and Johnson (1976) in preparation for total N determination. Total N of the plant material was determined with a Quickchem AE Lachat autoanalyzer (Lachat Instruments, Milwalkee, WI) using Lachat method 13-107-06-2-D. Means of the individual harvests were computed and weighted to reflect the contribution of each harvest to the accumulated N of the cumulative harvests. Total N accumulation (kg ha^–1) was calculated as dry matter yield x total N concentration. Apparent N recovery (ANR) was calculated by the difference method as {[(kg N_accumulated at N_x – kg N_accumulated at N₀)/(N fertilizer applied at N_x)] x 100} as described by Guillard et al. (1995) and Zemenchik and Albrecht (2002). Nitrogen use efficiency (NUE) was calculated as (yield at N_x – yield at N₀)/N fertilizer applied at N_x as described by Guillard et al. (1995) and Zemenchik and Albrecht (2002).

Statistical analyses utilized the MIXED procedure (SAS Institute, 1999) for a randomized complete block design. In all cases, the methods of N application were compared for those rates greater than zero and the N application rates were compared with the control treatment. Preliminary analysis of the full model, in which cuts were considered repeated within a year and modeled using a random statement, indicated that the error terms for the data sets were not homogeneous; therefore, separate analyses are reported for each cut. In the analyses of individual cuts, year, method of N application, and N rate were considered fixed affects while replication within year was considered a random effect for all data analyses. Yield, N concentration, N accumulation, NUE, and ANR data were subjected to regression analyses to determine the response of each variable to the N rates applied. Linear and quadratic models were fit to the individual data points from all years using the General Linear Model (GLM) procedure in SAS. Linear-plus-plateau and quadratic-plus-plateau models were fit to the same data points using the NLIN procedure in SAS. The model that was significant at the 5% probability level and with the lowest residual sum of squares was used to describe the response of the variable to the N rates applied. For the linear-plus-plateau and quadratic-plus-plateau model, plateau values were considered to be the maximum or minimum for each value (Cerrato and Blackmer, 1990). For the quadratic model, predicted maximum or minimum values were obtained by equating the first derivative of the regression equation to 0, solving for N, substituting the value of N into the response equation, and then solving for the variable in question. Predicted maximum N rates were calculated by equating the first derivative of the regression equation to 0 and solving for N. The economically optimum N fertilizer rate was derived from the yield response curve by considering the price ratio between the unit costs of fertilizer N inputs and the unit value of yield (Bullock et al., 1998). In these analyses the price for N fertilizer was varied from $0.60 Cdn to $0.90 Cdn kg^–1 N and the forage value varied from of $70 Cdn to $110 Cdn Mg^–1 based on the average price paid for hay in Ontario from 1998 to 2000 (OMAF, 2004c). The predicted economic optimum rates of N were calculated by equating the first derivative of the response curve to the price ratio.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Dry Matter Production
Precipitation totalled about 202, 434, and 725 mm from May through September in 1998, 1999, and 2000, respectively (Table 1). Extended periods of low rainfall occurred in both 1998 and 1999 while the 2000 growing season was marked with greater than mean precipitation throughout the growing season. During the 1998 season, dry periods occurred early in the season before the first cut (May and June) and during the latter portion of the growing season between the first and second cuts (August). In contrast, a dry period was experienced during the middle portion of the growing season between the first and second cuts (July and the first 22 d of August) during the 1999 growing season. Normal air temperatures were experienced throughout most of the 2000 growing season (Table 2). In contrast, mean monthly air temperatures during both the 1998 and 1999 growing seasons were above the long-term average. However, mean air temperatures during May of 1999 more closely resembled those measured during the 2000 season than those of the 1998 growing season. While mean daily (data not shown) temperatures were above average during the 1998 season, minimum temperatures at or below 0°C occurred at 13 to 15 d after planting (DAP).

Yields of the first cut increased with the addition of N but were not affected by increasing N rate (Fig. 1a) . Application of fertilizer N increased DM yields of the second and third cut and increasing N rate affected the DM yield of each of the second and third cuts (Fig. 1b and 1c). Dry matter yield increases with increasing N applications were best described by quadratic-plus-plateau response with maximum yields of 1.70, 2.45, and 1.84 Mg ha^–1 at rates of 45.8, 84.9, and 216.7 kg N ha^–1 for the first, second, and third cuts, respectively. Increasing amounts of N required for maximum DM for successive cuts were also found by Wanjari et al. (1996), suggesting that higher amounts of fertilizer N are required to obtain maximum DM for each successive cut as the time between N application and harvest increases. This may be attributed to reduced N availability as a result of crop removal and/or N losses to the environment as the growing season progresses.

View larger version (26K): [in this window] [in a new window]   Fig. 1. Relationship between dry matter yield and fertilizer N application for (a) first cut, (b) second cut, (c) third cut, and (d) cumulative harvests of sorghum–sudangrass. Data points are the mean ±SE of the number of experimental units indicated. Calculated variation of dry matter yield response for the model is expressed as the root mean square error (RMSE).

The most economic rate of nitrogen (MERN) at various costs of N and forage values determined from the yield data determined in this study are presented in Table 3. The MERN ranged from 83 kg N ha^–1 at high fertilizer costs and low crop values to 107 kg N ha^–1 at low fertilizer costs and high crop values. The MERN at an N cost of $0.70 Cdn kg^–1 and a forage value of $100 Cdn Mg^–1 was 102 kg N ha^–1. Ketterings et al. (2004) suggest that at similar N costs, forage value and fixed costs of $440 Cdn ha^–1, that the MERN was 150 kg N ha^–1 per cut or a total application of 300 kg N ha^–1. This is substantially higher than that determined with our results. Since yields were higher with lower N inputs and lower soil organic matter contents in our study compared with that of Ketterings et al. (2004), our data suggest that higher fertilizer use efficiencies were found with the methods of N application employed under the environmental conditions experienced in our study.

View larger version (31K): [in this window] [in a new window]   Fig. 2. Relationship between N concentration [N] and fertilizer N application for (a) first cut, (b) second cut, (c) third cut, and (d) cumulative harvests of sorghum–sudangrass. Data points are the mean ±SE of the number of experimental units indicated. Calculated variation of [N] response for the model is expressed as the root mean square error (RMSE).

Averaged over years, N concentrations in the second and third harvests and the cumulative harvests were best described by a linear response. Nitrogen concentration in the second cut increased with increasing N rates but higher N concentrations were found in the split N application than in the single sidedress application (Fig. 2b). Nitrogen concentration increased at a rate of 0.0213 g N kg^–1 for each kg N applied with the sidedress application and by a rate of 0.0258 g N kg^–1 for each kg N applied with the split N application. Averaged over methods and years, the third cut N concentration increased by a rate of 0.0279 g N kg^–1 for each kg N applied (Fig. 2c) and the N concentration of the cumulative harvest increased at a rate of 0.0225 g N kg^–1 for each kg N applied (Fig. 2d). Other studies have also shown that N concentrations of sorghum–sudangrass increase linearly with increasing N fertilization (Buxton et al., 1999; Ram and Singh, 2001; Ketterings et al., 2004).

Nitrogen Accumulation
Method of N application did not affect N accumulation by the aboveground material and no method x N rates interaction existed for the second and third cuts or for the cumulative harvest. Apparently, sorghum–sudangrass accumulates as much N when all of the N is knifed in at the six-leaf stage as when two separated applications, one at the six-leaf stage and one following the first cut, are applied in the same manner. Although N concentration of the second cut increased when N was applied as a split application, N accumulation by the second was similar for the two methods of N application. In contrast, Kilcer et al. (2002) found N accumulation to be greater when N fertilizer applications were split compared with a single application before seeding. Again, this difference in N accumulation between the two studies may be attributed to differences in N availability between the different methods and placement of N used in the two studies.

Total N accumulation increased for all individual cuts and the total crop as a result of N application and increasing N rates. As N rates were increased, N accumulation by the total crop increased primarily due to significant increases in N accumulation as a result of increasing N concentration and dry matter yield in the second and third cuts with increasing N application. Nitrogen accumulation in the first cut was best described by a quadratic-plus-plateau response to N rates with a maximum of 46.9 kg N ha^–1 at an N rate of 87 kg N ha^–1 (Fig. 3a) . Although N rate affected N accumulation by the second cut, a significant year x N application rate existed for the N accumulated due to a steeper increase in accumulated N with increasing N rates in 1998 than in either 1999 or 2000. In all years, N accumulation response to N rates was best described by a quadratic-plus-plateau model (Fig. 3b). The maximum second cut N accumulation was 75.8, 65.9, and 55.4 kg N ha^–1 at an N rate of 199, 133, and 163 kg N ha^–1 in 1998, 1999, and 2000, respectively. Averaged over years, N accumulation of the second cut was also best described by a quadratic-plus-plateau response to N rates, reaching a maximum of 65.5 kg N ha^–1 at an N rate of 170 kg N ha^–1. Nitrogen accumulation by the third cut was best described by a quadratic response indicting that additional N fertilization may have further increased N accumulation (Fig. 3c). The N accumulation of the cumulative harvests averaged over the two methods of N application had a quadratic-plus-plateau response to N rates, reaching a maximum of 161 kg N ha^–1 at an N rate of 196 kg N ha^–1 (Fig. 3d). This is comparable to the maximum N accumulation of 171 kg N ha^–1 at an N rate of 161 kg N ha^–1 reported for a silage corn crop at silking by Cox and Cherney (2001).

View larger version (31K): [in this window] [in a new window]   Fig. 3. Relationship between N accumulation (NA) and fertilizer N application for (a) first cut, (b) second cut, (c) third cut, and (d) cumulative harvests of sorghum-sudangrass. Data points are the mean ±SE of the number of experimental units indicated. Calculated variation of N accumulation response for the model is expressed as root mean square error (RMSE).

View larger version (37K): [in this window] [in a new window]   Fig. 4. Relationship between nitrogen use efficiency (NUE) and fertilizer N application for (a) first cut, (b) second cut, (c) third cut, and (d) cumulative harvests of sorghum–sudangrass. Data points are the mean ±SE of the number of experimental units indicated. Calculated variation of NUE response for the model is expressed as the root mean square error (RMSE).

Apparent N recovery of the first cut was affected by N rate and no interactions existed. First cut ANR decreased with increasing N rate and was best described by a quadratic-plus-plateau model. First cut ANR had a maximum of 59.2% at 25 kg N ha^–1 and reached a minimum of 13.7% at a rate of 90 kg N ha^–1 (Fig. 5a) . The ANR of the second and third cuts were not affected by the method of N application, suggesting that the split N application did not enhance fertilizer N recovery above the single sidedress application. However, as occurred with NUE, first cut ANR was increased when lower N rates were applied with a split N application and second cut ANR were not adversely affected with this method of N application. This also suggests that optimum first cut ANR can be obtained by applying one-half of the N fertilizer with a split N application rather than applying all of the N in a single sidedress application. The ANR of the second cut decreased with increasing N application rates but a significant year x N rate interaction existed. In 1998, second cut ANR response was best described by a quadratic model (Fig. 5b). In 1999, the response was best described by a linear decrease in ANR. In 2000, the ANR response to N rates was best described by a quadratic-plus-plateau model. Second cut ANR had a maximum of 59.2% at 50 kg N ha^–1 and declined with increasing N to 22.3% at 250 kg N ha^–1 in 1998. In contrast, second cut NUE decreased at a rate of 0.092% from a maximum of 27.2% at 50 kg N ha^–1 in 1999. Apparent N recovery had a maximum of 53.6% at 50 kg N ha^–1 but reached a minimum of 18.9% at a rate of 196 kg N ha^–1 in 2000. The difference in the decline in ANR with increasing N rate in the different years of the study can be attributed to the effects of rainfall and temperature on N uptake. In general, the drier, warmer conditions experienced between the first and second cut in 1999 reduced DM, N uptake, and, therefore, ANR. In contrast, the ample precipitation and cooler temperatures that occurred in 1998 and 2000 increased ANR and resulted in a significant decline in ANR with increasing N rates. Zemenchik and Albrecht (2002) also reported significant deviations in ANR and NUE for individual harvests of grasses due to reduced yield and N uptake during drought periods. Averaged over years, the second cut ANR was best described by the quadratic model ANR = 62.7 – 0.377N + 0.000773N² and declined from a maximum of 46.6% at 50 kg N ha^–1 to a minimum of 16.0% at 250 kg N ha^–1. The third cut ANR tended to decline linearly with increasing N rates (p = 0.08) at a rate of 0.028% from a maximum of 18.0% at 50 kg N ha^–1 (Fig. 5c).

View larger version (30K): [in this window] [in a new window]   Fig. 5. Relationship between apparent nitrogen recovery (ANR) and fertilizer N application for (a) first cut, (b) second cut, (c) third cut, and (d) cumulative harvests of sorghum–sudangrass. Data points are the mean ±SE of the number of experimental units indicated. Calculated variation of ANR response for the model is expressed as the root mean square error (RMSE).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
For the production of a two-cut forage sorghum–sudangrass system in Ontario, it is recommended that a total of 50 to 100 kg N ha^–1 be applied in a split application with one-half of the N applied at planting and the remaining N applied after the first cut. In our study, N applied as either a single sidedress application at the six-leaf stage of growth or as a split N application with N applied at the six-leaf stage and following the first cut resulted in sorghum–sudangrass total DM yield increases with N rates up to 125 kg N ha^–1 in a three-cut system. At fertilizer costs of $0.70 Cdn kg N and a forage value of $100 Cdn Mg^–1, the MERN was calculated to be approximately 100 kg N ha^–1, which was slightly less than the N rate for maximum yield. Forage N concentration of all harvests and the cumulative harvest increased with increasing N rates but was higher in the second cut when N was applied in a split application rather than a single sidedress application. Both NUE and ANR decreased with increasing N application rates. Models fitted for NUE, suggest that at low fertilization rates (50 kg N ha^–1), NUE of sorghum–sudangrass was approximately 36 kg DM kg^–1 but dropped to 11 kg DM kg^–1 at N rates greater than 221 kg N ha^–1. Similarly, ANR decreased from a maximum of 93% at 50 kg N ha^–1 to 35% at 250 kg N ha^–1. At the MERN, NUE was estimated at 24 kg DM kg N^–1 and ANR was estimated at 68%, approximately 66 and 73% of the NUE and ANR values obtained at the lowest N rate. However, both the first cut NUE and ANR were higher at the lower N rates applied with the split application but were not different from the single sidedress application at similar total N rates in the second cut. This suggests that a split N application does not adversely affect DM production but may enhance NUE and ANR of individual cuts. Furthermore, there is a potential for variation in NUE and ANR due to soil moisture depletion related to reduced rainfall coupled with the low moisture-holding capacity of this soil. Dry periods in 1999 coincided with reduced NUE and ANR compared with 1998 and 2000 when ample precipitation was received, suggesting improved N utilization with favorable precipitation.

We conclude that producers in southern Ontario can produce newer sorghum–sudangrass hybrids economically by applying N rates at the higher end of the current recommendations. Nitrogen fertilizer applied at the MERN optimize yield, NUE, and ANR and minimizes N loss to the environment. Although splitting the N application into two separate applications with one application at the six-leaf stage of growth followed by another following the first cut had little impact on total DM yield, NUE, and ANR, a split application of N increased first cut NUE and ANR values and resulted in similar second cut DM yield, NUE, and ANR values as the single sidedress application. Therefore, producers will minimize the potential for environmental N loss by applying N fertilizer as a split N application. The slope of the fitted NUE and ANR models indicate that exceeding the MERN could result in an increased likelihood of N losses to the environment and cost to the producer, provided ample precipitation is received.

	ACKNOWLEDGMENTS

 
The authors gratefully acknowledge the technical and field assistance of Jacqueline Schott and Alex More. The authors are appreciative of the partial funding for this project provided by the Agriculture and Agri-Food Canada Matching Investment Initiative Program and by Agriculture Environmental Renewal Canada.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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