Sustainability of long-term organic ley/arable crop rotations in Northern Scotland.

Robin L Walker, Brian R Taylor, Christine A Watson, David Younie

Abstract - This paper describes aspects of sustainability linked to four long-term organic ley/arable rotations split between two contrasting sites in Northern Scotland. The two sites (one in Aberdeen and the other near Inverness) started their organic conversion in 1991, with one rotation being common to both sites (50% grass / clover), while the Aberdeen site also had a 67% grass / clover rotation and the Inverness site a 38% grass / clover rotation. By measuring physical and financial outputs, and changes in soil properties and weed populations, the trials described here aimed to compare the productivity and sustainability of different organically-managed crop rotations. This paper outlines some of the key findings of these experiments.   

Introduction
Crop rotation is the primary method of maintaining soil fertility and minimising weeds, pests and diseases in organic farming. Fertility maintenance depends on the balance of N-fixing legumes and N-depleting non-legumes in the rotation (Watson et al. , 2002), with crop protection achieved principally through restriction of suitable environments for weeds, pests and diseases. Traditionally, the fertility building phase lasts between 3 and 6 years and is largely supplied by clover in leys utilised by stock through grazing and conservation. In areas where arable cropping predominates, a lesser emphasis may be placed on crops for livestock feed, even considering livestock free systems.
In the long-term, sustainable farming systems are likely to be ones where soil fertility is maintained through the recycling of nutrients and organic matter, environmental degradation is minimised and producers receive adequate economic returns. By measuring physical and financial outputs, and changes in soil properties and weed populations, the trial reported here aimed to compare the productivity and sustainability of four different organically-managed crop rotations.

Robin L Walker is with the Scottish Agricultural College, Organic Systems Team, Aberdeen, UK (robin.walker@sac.ac.uk)
Brian R Taylor is recently retired from the Scottish Agricultural College, Aberdeen, UK
Christine A Watson is with the Scottish Agricultural College, Organic Systems Team, Aberdeen, UK (christine.watson@sac.ac.uk).
David Younie is with the Scottish Agricultural College, Organic Select, Aberdeen, UK (david.younie@sac.co.uk)
Materials and methods
Rotation trials at two sites with contrasting soil fertility and climate in the north east of Scotland (Tulloch, Aberdeen (long 02o15’W, lat 57o11’N; 125m above sea level; sandy loam soil; average annual rainfall 820 mm) and Woodside, Inverness (long 03o24’W, lat 57o38’N; 25m above sea level; loamy sand / sandy loam soil; average annual rainfall 730 mm)) were started in 1991, the first year of conversion status.
The rotations were designed to have different proportions of fertility-building crops with 6-year rotations at Tulloch containing 50% (T50) and 67% (T67) of grass clover / leys and the 8 and 6 year rotations at Woodside containing 38% (W38) and 50% (W50) grass / clover respectively (see Table 1). There were two replicates of each rotation at each site, with plot sizes of 27m x 30m, with these being fenced to contain grazing sheep. Plots cut for silage were grazed after the final cut, and undersown cereal plots were grazed after the cereal harvest. Sheep were grazed for as long as herbage was available. Total FYM applications for each rotation were applied based on calculations of the recommended stocking rates (Younie and Hermansen, 2000). Crop management followed recommended organic practice.

Table 1. Rotations at Tulloch and Woodside.
Site	Rotation	Cropping Year	
		1	2	3	4	5	6	7	8	
Tulloch	T50	G1 	G2 	G3 	C1  	S   	C2 			
	T67	G1 	G2 	G3 	G4 	C1 	C2 			
Woodside	W38	G1 	G2 	C1  	P    	C2 	GR 	S   	C3 	
	W50	G1 	G2 	G3 	C1  	P   	C2 			
(G1, G2, G3, G4 = 1st, 2nd, 3rd, 4th grass/white clover, C1, C2, C3 = 1st, 2nd, 3rd cereal spring oats (Avena sativa L.) respectively after the main fertility-building phase, S = Swedes (Brassica napus L.), P = potatoes (Solanum tuberosum L.), GR = grass/red clover. The 2nd cereals in T50 and T67 and the 3rd cereal in W38 were undersown with a grass/white clover mixture (24 kg/ha Lolium perenne L., 4 kg/ha Phleum pratense L., 4 kg/ha Trifolium repens L.); the 2nd cereal in W38 was undersown with a grass/red clover mixture (5 kg/ha Lolium perenne L., 15 kg/ha Trifolium pratense L.).

Grain and straw yields were estimated from two 20m strips per plot cut with a 2m wide combine harvester. Potato and swede yields were estimated from ten random 2m lengths of row (spaced 0.7m apart) per plot. Silage yields were estimated from four random 5m strips cut with a Haldrup harvester.
Bulked soil samples were taken to a depth of 15cm in each plot in January and were analysed for pH, P and K (Modified Morgan test) as well as organic matter by loss on ignition (OM).
Estimates of ground surface covered by crop, major weeds and other unsown grasses were estimated annually in May/June and October. All data was analysed statistically using ANOVA.

Results
Oat yields (the cereal used in all rotations) showed considerable year to year variation, but did not indicate any significant progressive change (data not shown). Yields were similar in the two rotations at Tulloch (both 6 course rotations), as they were in the two rotations at Woodside (despite one being an 8 course and the other a 6 course rotation). Yields of both grain and straw were significantly higher from the first cereal after the ley than from the second cereal at both sites (data not shown). Less vigorous growth in the second cereal than the first cereal was also indicated by significantly shorter straw length at Tulloch, but no significant differences in this characteristic were found at Woodside.
	Average outputs over each whole rotation are given per ha in Table 2, with previously published gross margins. Only minor differences in the gross margins from the two rotations at each site were apparent, although gross margins were higher at Tulloch than at Woodside. At Tulloch the value of the extra swede crop in T50 was balanced financially by additional grazing and notional livestock output in T67. At Woodside the extra crop of swedes in W38 did not outweigh extra grazing in W50.
	Changes in major weed groundcover were compared over two complete cycles of each rotation at Tulloch by taking means of 6 years data from the same crop in each rotation. Slight increases between the first and second cycle were found from the May/June assessments, particularly in the cereal crops, however, there was a small decrease in weed ground cover in the October assessment of the ley plots. At Woodside, there was an increase in the weed ground cover between the first and second cycles of both rotations at the May/June assessments, mainly in the cereals, but again, there was little difference in the October assesment of the ley plots.

Table 2. Average output and gross margin per ha over the whole rotation, 1992-2003.
	T50	T67	W38	W50	
Oats (t @ 850g d.m./kg)	1.39	1.41	1.35	1.22	
Swedes (t fresh swedes)	8.48		5.95		
Potatoes (t fresh potatoes)			4.52	4.43	
Silage (t d.m.)	2.50	2.54	1.90	1.53	
LUGD	111.9	182.8	70.0	97.8	
Gross margin (£/ha)*	805	828	716	672	
SED for comparing gross margins at each site	4.3 (p=0.12)	62.0 (p=0.61)	
*Gross margin (gross saleable output less production costs not including fixed costs, rent, depreciation and labour) and SED from Taylor et al.(2004). 
There was some indication that potash levels declined, (before potassium sulphate was applied in 2002), but trends were not significant.

Discussion
The two rotations at each site may be compared in terms of physical outputs, profitability and sustainability. Although the outputs from each rotation were not the same there was no indication that one rotation was more ‘productive’ than the other at either site. There were no indications that yields were declining over the period of the trial, and this was thought to be due to the relatively large proportion of the cropping area being devoted to N2 fixing crops and the nutrients being recycled through grazing animals and FYM application. Watson et al. (2000), in previously reported studies on T50 and W50, found that N was in surplus in both rotations and suggested this was typical of organic mixed farms.
Changes in soil properties were small, although there was some indication of a decline in soil K at Woodside, however, low soil K can, at present, be corrected using organically approved materials.
Although groundcover of major weeds approximately doubled in the arable crops, it fell in the leys, and the decline in the weed seedbanks (Younie et al., 2002) indicated that all rotations were sustainable in terms of weed growth.
Using the above indicators as a measure of sustainability, the four rotations described here appeared to be resilient in both agronomic and economic terms over the trial period. There was little to suggest any immediate decline in the sustainability of the these systems based on the interpretation of the data collated from this trial.

Acknowledgements
This project has been sponsored by SEERAD (SAC/257/00).

References
Taylor, B.R., Younie, D., Matheson, S., Coutts, M and Mayer, C. (2004) In "Organic Farming : Science and Practice for Profitable Livestock and Cropping, Occasional Symposium No.37", pp. 263-267, ed Hopkins, A. (British Grassland Society: Reading, UK).

Watson, C.A., Younie, D., Stockdale, E.A. and Cormack, W.F. (2000) Aspects of Applied Biology 62:261-268.

Watson, C.A., Atkinson, D., Gosling, P., Jackson, L.R. and Rayns, F.W. (2002) Soil Use & Management 18:239-247.

Younie, D. and Hermansen, J. (2000) Grassland Science in Europe 5:493-510.

Younie, D., Taylor, B.R., Coutts, M and Matheson, S. (2002) In "Proceedings of the UK Organic Research Conference, Research in Context", pp. 215-220, (Organic Centre Wales: Aberystwyth, UK).