title / Further N cycle studies on farmlets
/ MAFF
project code / NT1829
ministry of agriculture, fisheries and food CSG 15
Research and Development
Final Project Report
(Not to be used for LINK projects)
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Project title / Further N cycle studies on farmlets
MAFF project code / NT1829
Contractor organisation and location / IGER, North Wyke, Devon, UK. EX20 2SB
Total MAFF project costs / £ 824,831
Project start date / 04/01/98 / Project end date / 31/03/01
Executive summary (maximum 2 sides A4)
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CSG 15 (1/00) 3
Projecttitle / Further N cycle studies on farmlets
/ MAFF
project code / NT1829
A prime requirement of this project was to compare a conventional (CONV) fertilizer management system (based on RB209) with that of ‘best nitrogen management practice’ (BNMP). The results from this project have shown that a reduction in N fertilizer input of up to 25 % can be achieved without loss of yield. New strategies for utilising FYM in an all-grass farm situation were also explored. Composted FYM was incorporated into swards after 1st and 2nd cut silage (BNMP treatments only) without any obvious problems in sward re-growth, or grazing utilisation, provided that 3-4 weeks was allowed before grazing recommenced. However, silage made from all areas where FYM was previously applied had elevated counts of clostridial bacteria when the bales were opened after 12 weeks storage, which may point to cross-contamination in the baling process. The more common practice of applying uncomposted (i.e. stored) FYM in the autumn (CONV treatments only) resulted in increased leaching losses in over-winter drainage. However, overall leaching losses measured over the three winter periods (1998/9, 1999/00 and 2000/01) were lower than the losses previously recorded on this site, which may, in part, be attributed to a sequence of warm/wet summers preceeding the drainage periods in all three measurement years. In these circumstances, better utilisation of nutrients by the grass crop and increased losses from ammonia volatilization and denitrification would have reduced the excess N available in the soil N pool before winter drainage commenced. Even though leaching losses in the winter following spring ploughing were much reduced compared with autumn cultivation, ploughing out long-term grassland in the spring to establish new leys could not be fully justified. The sacrifice of a whole grazing season was only partially compensated for by the savings that could be made in fertilizer N from a silage crop produced without fertilizer by utilising the enhanced supply of soil N in the first growth period. In the newly established swards (spring and autumn), hergage yield was increased in the seasons following reseeding, especially in the spring sown crops, which also benefited from the improved varieties which were employed to make more efficient use of N (BNMP). Leaching losses, following autumn cultivation in 1998, exceeded all other losses recorded during the lifetime of this project, but losses from these and spring sown swards quickly returned to levels comparable with undisturbed swards, once the new swards had become established (viz. 3-6 months after sowing).
CSG 15 (1/00) 3
Projecttitle / Further N cycle studies on farmlets
/ MAFF
project code / NT1829
Scientific report (maximum 20 sides A4)
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CSG 15 (1/00) 3
Projecttitle / Further N cycle studies on farmlets
/ MAFF
project code / NT1829
Objectives
A summary of the main ojectives of the project are given below:
1. To develop farmlet systems to compare conventional practices with a best N management practice
2. To examine effects of re-seeding within a farmlet context
3. To examine effects of management on nitrate leaching
4. To determine effects on production
5. To incorporate effects of improved FYM management
6. To provide measurement/estimates of other losses
INTRODUCTION
The aim of this project was to investigate the processes involved in N cycling, using systems appropriate to beef production. The experimental programme was based on a layout of plots, each representing small farmlets, which made use of the facilities available at the Rowden Drainage Experiment to obtain leachate samples from individual plots. The concept of farmlets was initiated in a previous MAFF-funded contract (WA0504) which allowed for both grazing and cutting for conservation and inputs of organic and inorganic N to be investigated. Nitrogen balances can be determined in conventionally managed swards and compared with strategies aimed at minimising the environmental impact. All treatments are replicated and the plots are hydrologically isolated and of sufficient size (1 ha each) to enable drainage losses to be evaluated under realistic grazing mangement systems.
In NT 1829, the primary target was to achieve a 25 % reduction in fertilizer N inputs whilst maintaining herbage production and minimising losses to the environment. In addition, improved FYM management was investigated, so that an annual grassland cycle was completed with the return of manures from over-wintered animals. The particular difficulties of handling/disposing of FYM from straw-bedded cattle on an all-grassland farm severely limit the opportunities to exploit this valuable nutrient resource and composting was investigated as an alternative strategy. The important questions arising from the cultivation and resowing of new swards (in terms of N loss) were also addressed and new varieties of grasses with improved nitrogen-use-efficiences were exploited. Additional measurements were made to estimate gaseous N loss by denitrification and the role of soil N availability was examined in detail to try to avoid imbalances which might lead to greater losses, particularly after cultivation.
METHODOLOGY
General approaches
Self-contained farmlets at the Rowden Drainage Experiment at IGER, North Wyke, Devon, UK. consisted of 1 ha paddocks (plots) divided into 10 sectors (each 0.1 ha), some which were either only grazed (sectors 1-3), or only cut (sectors 9-10), or grazed and/or cut (sectors 4-8). The experiment looked at the grazing phase of an 18 month grazing system using autumn-born (Limousin x Friesian) calves. A cohort of 4 core animals (mean weight at turnout of 200 kg) was placed in each farmlet. The sward height was measured and the grazing area adjusted by moving an electric fence to maintain a mean sward height of > 75 mm. The cut area was managed on a conventional 3–cut silage system with FYM spread in the autumn.
The new treatments were imposed in 1998. The first was a ‘conventional’ (CONV) N management (MAFF: RB209 pattern of application) with an input of 280 kg N/ha, in five equal applications of 50 kg N/ha (March – July) with one late season (August/September) application of 30 kg N/ ha on the grazed area (sectors 1-3). The cut areas (sectors 4-10) received the same annual input, but with 100 kg N/ha in March and May and 80 kg N/ha in July. FYM (equivalent to the over-winter output of the animals from these treatments) was applied in the following autumn to part of the the cut areas (sectors 6-10). A second treatment was termed as a ‘best nitrogen management practice’ (BNMP), and attempted to make optimal use of the predicted soil N contribution (NFERT model) without loss of herbage production. The pattern of application, therefore, differed in the timing and rate from the CONV treatments with an overall target of reducing N input to 75% of CONV. Composted FYM (stored for c. 12 months since mucking out) was also applied to these treatments after 1st and 2nd silage cuts. In addition, four of the 1 ha plots were ploughed out and reseeded. In spring 1998, two plots (BNMP, undrained) were cultivated and sown with new varieties (all diploids) of perennial ryegrass: Aber Elan, Aber Silo (both bred for N use efficiency) and Guilford, to provide a well-tillered base to the sward. In autumn 1998, the other two plots were cultivated (CONV, drained) and sown with the same variety of perennial ryegrass (Melle) as the original undisturbed CONV pastures. Herbage and livestock data were collected at appropriate stages: animals were weighed every 2 weeks and before opening up new grazing sectors, sward measurements were made to assess the herbage on offer and the unconsumed herbage remaining after grazing. Leaching losses were determined from samples obtained from the outfalls of the field drains and in surface run-off collected from the perimeter interceptor channels. Nitrate and ammonium were determined on these samples and dissolved organic N (DON) was determined from the difference between the total N and inorganic N content of the leachates.
Additional measurements were made to provide a more complete picture of the N cycling in these treatments. The effects of cultivation were examined to determine the extent and duration of the disturbance on soil processes that could result in a flush of mineral N. Net mineralization was measured monthly (occasionally more frequently when conditions were conducive) in the permanent pastures and also in the recently cultivated swards, i.e. reseeded in spring (BNMP), or autumn (CONV) 1998. Soil characteristics and microbial content were also examined in both these swards to compare changes after cultivation with those in the undisturbed pastures. A similar field incubation technique was employed to estimate denitrification losses, targeted to coincide with conditions conducive to the process, e.g. wet/warm soils and following fertilizer applications.
Detailed methodology
1) Fertilizer management
Fertilizer applications were dependent on seasonal variations and ground conditions, but for the CONV treatments, a 1st application target date of early/mid March was used. However, the mild winter of 1998/9, followed by a particularly wet spring, resulted in more over-winter grass growth than normal and this tended to delay the soil from drying out. An early 1st silage cut was taken to remove the old swath and help bring the pastures under control more quickly, especially on swards that had been resown in the spring and autumn of the previous year (1998). The cattle were turned out on 18 May 1999, but in the subsequent years (1999 and 2000) fertilizer was applied in March and turnout was in April.
Fertilizer application patterns for the BNMP treatments were derived using NGAUGE, a decision support system (developed from NFERT) for optimising fertilizer applications for improved efficiency of N use in grassland. Use of NGAUGE enables efficiency of N use to be improved by:
(i) Matching the temporal pattern of N supply to the efficiency of the plant in recovering N, together with
(ii) taking account of the amount of soil N available to the plant (by modelling and measurement) and adjusting fertilizer applications accordingly.
Profiles are generated according to a specific target, which in this case was a 25% reduction in the use of N fertilizer, relative to the conventional treatment. Soil mineral N was assessed at the beginning of the year, and at strategic points in the growing season, such as after a period of drought. Where soil mineral N differed from that predicted, fertilizer N was adjusted. In the experimental years, growing conditions were generally good and, soil mineral N was seldom found to accumulate in the profile in excess of that predicted by NGAUGE. In cases where the replicates within a treatment differed widely in mineral N, different amounts of N fertilizer were applied to each replicate. Fertilizer applications were made using a full-width pneumatic boom spreader, calibrated and then verified by weighing the unused fertilizer remaining from each plot. This soil type tends to be low in phosphorus. An application of 0-24-24 PK fertilizer was made to all plots at the start of each growing season to supply 26.2 kg P and 49.8 kg K/ha and further application of K were made after 1st and 2nd cut silage (cut areas only) to replace K removed (50 kg K/ha). Lime is applied, as necessary, to maintain > pH 6.0 in all treatments.
2) Herbage offtake
Grass was cut for silage using a gang mower, wilted and then made into big bales. Yields were assessed from weigh-bridge readouts and dry matter (DM) contents were found from sub-samples of the standing crop taken with a reciprocationg mower before the main crop was cut. During grazing, regular (twice weekly) assessments were made of the herbage on offer using a sward height stick and the grazing area was adjusted by moving the electric fence to maintain a mean sward height of > 75 mm. Additional herbage samples were removed from within a quadrat (to the same mower cutting height of 3 cm) to confirm the amount of herbage on offer in new sectors, before opening them up to grazing. Grazed only areas were topped regularly to remove stemy material, to encourage better tillering and to control docks. Thistles were removed by hand-roguing as part of the regular sward inspections. Samples of silage were obtained after 90 days, from the bales made from CONV and BNMP treatments, packed in a cool box and sent for microbial analysis within 24h of sampling.