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Texas A&M Univ. Agric. Res. and Ext. Cent., P.O. Box 200, Overton, TX 75684
* Corresponding author (g-evers{at}tamu.edu)
Received for publication June 18, 2001.
| ABSTRACT |
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| INTRODUCTION |
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Advantages of broiler litter over commercial fertilizer are: (i) it contains plant nutrients other than N, P, and K, which may be limiting; (ii) with organic matter decay, there is slower release of the soluble nutrients N, S, and B, which reduces the risk of nutrient loss by leaching if animal manure is applied to coincide with crop needs; (iii) added organic matter improves soil structure and the water- and nutrient-holding capacities; (iv) Ca compounds in poultry litter help maintain soil pH; and (v) it can be more economical than commercial fertilizer (Edwards, 1996; Wood, 1992). One disadvantage of using broiler litter as a nutrient source is the nutrient ratio in broiler litter may not match crop requirements. This difference in nutrients applied vs. crop nutrient requirements results in the soil buildup of excess nutrients, especially P (Sims, 1995). Water quality problems can occur if P enters surface water in runoff (Sharpley et al., 1993).
Perennial pastures are an ideal recipient of animal manure (Edwards, 1996). Annual land preparation and planting are not required. The forage sod protects the soil from erosion and provides a firm surface for manure application equipment and grazing animals during wet weather. In the southeastern USA, mixtures of cool- and warm-season forages provide a year-long growing season and sink for N mineralized from animal manure throughout the year. Essentially, all of the P and K in broiler litter is available for plant uptake (Wilkinson, 1979).
Because most soils in the southeastern USA are sandy, acid, and have low nutrient-holding capacity (Ball et al., 1991), broiler litter is a desirable alternative to commercial fertilizer. Bermudagrass hybrids, such as Coastal, are well adapted to these soils and can take up large amounts of N. In an extensive review of the literature, Wilkinson and Langdale (1974) estimated maximum Coastal bermudagrass yields of 29 Mg ha-1 at 1230 kg N ha-1 under the best soil and climatic conditions. Response to N is linear up to 560 kg N ha-1 and then becomes quadratic. Soil type appears to have only a minor influence on N response. Because of the low nutrient-holding capacity of these sandy soils, fertilizer recommendations for hybrid bermudagrass are usually based on the production level desired by the producer and an estimate of the nutrients removed at that production level (Wilkinson and Langdale, 1974). Broiler litter compares well to commercial fertilizer for hybrid bermudagrass production (Evers, 1998).
In a review of nutrient utilization of southern forage crops, Robinson (1996) reported that the N, P, and K uptake of Coastal bermudagrass needed to reach 90% of maximum yield was 440, 48, and 300 kg ha-1, respectively, with an approximate ratio of 9:1:6. Annual ryegrass is the most widely used cool-season annual forage in the southeastern USA because it is easy to establish and very productive (Evers et al., 1997). Brink et al. (2001) have shown annual ryegrass to be one of the most effective temperate grasses to remove soil P because it is very productive and has a high P concentration. Reported N, P, and K uptake needed to produce 90% of maximum yield of annual ryegrass is 340, 34, and 280 kg ha-1, respectively (Robinson, 1996), for an approximate ratio of 10:1:8. Previous research has shown that both species are capable of taking up P in excess of plant requirements when grown on high-P soils (Evers and Doctorian, 1998). The average NPK ratio in broiler litter is 2.2:1:1.3 (Sims and Wolf, 1994).
This discrepancy between the NPK ratio for forage crop requirements and that of broiler litter nutrients results in a buildup of excess P in the soil over time if broiler litter is applied at rates to meet the N requirements of the forage crop (Sims, 1995). If broiler litter was applied at a lower rate where N became the limiting nutrient, the application of commercial N fertilizer should enhance crop growth to take up excess P. This practice would not only reduce environmental problems from high-P soils but also would increase the value of broiler litter because more of the nutrients in the broiler litter would be utilized by the forage crop.
One objective of this study was to test the hypothesis that combining commercial N fertilizer with broiler litter would increase P and K uptake from an annual ryegrassCoastal bermudagrass pasture. A second objective was to determine the number and timing of N applications that would result in the greatest P and K uptake.
| MATERIALS AND METHODS |
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TAM 90 annual ryegrass was overseeded on the Coastal bermudagrass at 33.6 kg ha-1 on 15 Oct. 1998 and 21 Oct. 1999. Poor stands resulted in 1999 because of drought; therefore, the study was replanted on 30 Nov. 1999. Ten N fertilizer treatments consisted of 0 and 56 kg N ha-1 NH4NO3 (34% N) at various times. A single N application was applied in December, March, May, or July. Two 56 kg N ha-1 applications per year were applied in December and March, May and July, or March and May. Other treatments were three N applications in March, May, and July for a total of 168 kg N ha-1 and four N applications in December, March, May, and July for a total of 224 kg N ha-1. In the second year, N fertilizer treatments were repeated on the same plots as the first year. Plots were 1.8 by 4.6 m arranged in a randomized complete block with four replications.
The study was harvested with a sickle bar mower to a 3-cm stubble height seven times from January to August in 1999 and six times from March to October in 2000. A subsample of the harvested forage from each plot was dried at 60°C for 48 h to determine percent dry matter and calculate yield. The harvest in May was a mixture of ryegrass and bermudagrass. The green dry matter sample was hand-separated before drying to determine botanical composition and for chemical analysis.
Nitrogen, P, and K concentrations were determined on the dry matter sample ground to pass a 40-mesh screen. Forage samples were digested (Nelson and Sommers, 1980) and N determined by Kjeldahl, P by vanadomolybdic acid method (Jackson, 1958), and K by atomic absorption spectrophotometry. Nutrient uptake was determined by multiplying nutrient concentration of the forage by dry matter yield for each harvest.
Data were analyzed as a split plot by analysis of variance using PC-SAS (SAS Inst., 1985). Years served as main plots and N fertilizer treatment as subplots. When a year x N fertilizer treatment interaction was significant at 0.05 level, statistical analysis was conducted within year. Mean separation was by Fisher's protected LSD at the 0.05 significance level.
| RESULTS AND DISCUSSION |
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Robinson (1996) reported that annual ryegrass removes approximately 360 kg N ha-1 to reach 90% of maximum yield, which is about the amount applied in the broiler litter each year (Table 2). However, it is estimated that only 60 to 65% of the N in surface-applied broiler litter is available the first year, with about 20 to 25% lost through volatilization and the remaining 10 to 15% available after the first year (Cabrera and Gordillo, 1995). Based on this estimate, only about 200 kg N ha-1 from the broiler litter was available, and the crop should require an additional 160 kg N ha-1 to reach 90% of maximum yield.
Maximum bermudagrass yield of about 6.8 Mg ha-1 was produced when N was applied in March, May, and July for a total of 168 kg N ha-1 (Table 3). Applying additional N in December did not increase bermudagrass yield. Bermudagrass yield was slightly less at 5.8 Mg ha-1 when N was applied in May and July, which implies that there was some N carryover from the March N application to the ryegrass. May, July, or March + May treatment yields were similar to the May + July treatment but less than those in plots receiving three or four N applications per year. Nitrogen applied only in December and/or March, when bermudagrass was dormant, resulted in the lowest bermudagrass yield, which was similar to the zero-N treatment. When not overseeded, pure stands of hybrid bermudagrass remove about 440 kg N ha-1 to reach 90% of a maximum yield of about 20 Mg ha-1 (Robinson, 1996). Competition from the ryegrass in late spring and poor moisture conditions during late summer and early autumn (Fig. 1) limited bermudagrass growth in this study.
Total yield peaked at 13.4 Mg ha-1 when N was applied in March, May, and July for a total of 168 kg N ha-1 (Table 3). Applying additional N in December did not increase total yield. The next highest-yielding treatments of about 12 Mg ha-1 occurred when N was applied in March and May or in December and March. The March application was common to both treatments and coincided with the beginning of the peak ryegrass growth period. Applying N in May and July or once in any month produced yields of only 10.5 Mg ha-1. The zero-N treatment produced the least forage, substantiating the hypothesis that the N in 9 Mg ha-1 of broiler litter did not meet the N requirements of a ryegrassbermudagrass system.
Nitrogen Uptake
Forage N concentration was related to time and frequency of N fertilizer application. Nitrogen concentration tended to be high in forage that had been fertilized with N since the last harvest and low if it had not. The concentration of N in ryegrass ranged from 12.7 to 33.3 g kg-1 in 1999 and from 15.1 to 32.3 g kg-1 in 2000 (data not shown). The critical N concentration for producing 90% of maximum ryegrass yield is 18 g kg-1 (Robinson, 1996). Lower N concentrations occurred at the last ryegrass harvest in late April or May when ryegrass was mature. Bermudagrass N concentrations ranged from 9.8 to 29.2 g kg-1 in 1999 and from 11.5 to 24.7 g kg-1 in 2000 (data not shown). The critical N concentration of bermudagrass to achieve 90% of maximum yield is 22 g kg-1 (Robinson, 1996). Broiler litter was applied in October when ryegrass was planted. Ryegrass removed much of the available N from the broiler litter, which left the bermudagrass N deficient and responsive to N fertilizer. Lower N concentrations occurred in late summer when high temperatures and poor moisture reduce the nutritive value of the bermudagrass.
Nitrogen uptake was affected by year and N treatment (Table 4). Nitrogen uptake followed the same pattern as yield. Uptake by ryegrass was greater in 1999 than 2000 while uptake by bermudagrass was greater in 2000 than in 1999, with no difference in total (ryegrass + bermudagrass) N uptake between years. The two treatments where N was applied in December and March during the ryegrass growing season resulted in the greatest N uptake of about 150 kg N ha-1. A December or March N application was a component of the next four highest treatments for N uptake. Treatments where no N was applied in December or March had the lowest N uptake of 85 to 95 kg N ha-1 and were not different from the zero-N treatment.
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There was a year x N treatment interaction (P = 0.022) for N uptake by bermudagrass (Table 4). Among the single N application treatments in 1999, the March treatment was not different from the zero-N treatment and removed less N than the May and July treatments (Fig. 2) . In 2000, the March treatment removed more N than the zero-N treatment and was not different from the May and July treatments. The MayJuly treatment removed as much N as the three and four N application treatments per year in 1999 but removed less in 2000.
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Phosphorus Uptake
Phosphorus concentration was inversely related to the amount of N applied each year (data not shown). Forage production increased as the N rate increased, and that had a dilution effect on the P concentration in the forage. Ryegrass P concentration ranged from 4.2 to 7.7 g kg-1 in 1999 and 3.8 to 6.4 g kg-1 in 2000. The critical P concentration for producing 90% of maximum ryegrass yield is 3.4 g kg-1 (Robinson, 1996), so P was not limiting in any treatment. The higher values observed in this study are twice that of the 3.4 g kg-1 critical value. This finding agrees with previous work indicating ryegrass is capable of removing excess soil P (Evers and Doctorian, 1998).
Bermudagrass P concentration ranged from 1.8 to 4.6 g kg-1 in 1999 and from 1.2 to 4.6 g kg-1 in 2000, which is about half of the P concentration reported for ryegrass. The critical P concentration for obtaining 90% of maximum bermudagrass yield is 2.4 g kg-1 (Robinson, 1996). Lower bermudagrass P concentrations from July through October harvests were probably due to poor moisture conditions and not low soil P values.
Phosphorus uptake was directly related to forage production. Year had a significant effect on P uptake for ryegrass and total forage production because of the higher ryegrass yields in 1999 than 2000 (Table 3). Nitrogen treatment affected P uptake of ryegrass, bermudagrass, and total P uptake. Ryegrass removed from 36 to 39 kg P ha-1 when N was applied at least twice during the year with one of the applications occurring in March (Table 5). March is the beginning of the peak ryegrass growing period in northeast Texas. Ryegrass removed from 32 to 34 kg P ha-1 when N was applied in December or March but only 26 to 29 kg P ha-1 when no N was applied during the ryegrass growing season.
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Total annual P removal was influenced more by ryegrass than bermudagrass because of the higher P concentration in ryegrass. The greatest P uptake of 52 to 55 kg ha-1 occurred when N was applied at least twice during the year and one of the applications was in March. Applying N in December and March or in March and May removed as much P as three or four N applications during the year. These were the same N fertilizer treatments that maximized ryegrass production (Table 3). A single N application in December or March removed 47 to 48 kg P ha-1, and one in May and/or July did not remove any more P than the control treatment without N.
Potassium Uptake
Potassium concentration of both ryegrass and bermudagrass was not affected by N treatment in 1999 (data not shown). There were small differences in K concentration in four of the six harvests in 2000, but there were no trends related to N rate or time of application. Ryegrass K concentration ranged from 21.8 to 43.2 g kg-1 in 1999 and from 27.2 to 48.1 g kg-1 in 2000. The critical value for producing 90% of maximum ryegrass yield is 28 g K kg-1 (Robinson, 1996). The low K concentrations occurred in the April or May harvest when ryegrass was mature. Bermudagrass K concentrations ranged from 11.9 to 21.7 g kg-1 in 1999 and from 12.6 to 23.6 g kg-1 in 2000 and were about half the K concentration in ryegrass. The critical K concentration for bermudagrass to produce 90% of maximum yield is 15 g K kg-1 (Robinson, 1996). Bermudagrass K concentrations below the critical level occurred under poor moisture conditions in late summer.
Year only affected K uptake by bermudagrass (Table 6) because bermudagrass yields were higher in 2000 than 1999 (Table 3). Nitrogen treatment influenced K uptake of ryegrass, bermudagrass, and total K uptake. Potassium uptake by ryegrass was directly related to yield because N treatments had minimal influence on K concentration within harvest dates. Maximum K uptake of about 225 kg ha-1 occurred in the two treatments where N was applied in December and March (Table 6). These treatments were followed by the March + May treatment and the March + May + July treatment that removed about 206 kg K ha-1. The single N application resulting in the most K removal was made in December. Applying N in May or July when ryegrass was not present removed no more K than the zero-N treatment.
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As with P, total K uptake was primarily influenced by the effect of N fertilizer treatments on ryegrass yield. Applying N in December and March or March and May removed as much K as applying 56 kg N ha-1 three or four times a year (Table 6). Moderate amounts of K were removed when N was applied in December, March, or May and July. Less K was removed when N was only applied in May or July during the bermudagrass growing season because of lower K concentrations in bermudagrass than ryegrass. However, all N fertilizer treatments removed more total K than when no N was applied.
| CONCLUSION |
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| NOTES |
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| REFERENCES |
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