Viable long-term continuous cropping is supported by the world’s longest running cereal cropping research.
Research assessing winter wheat production for 175 years (since 1843) on land cropped for many centuries prior to 1843, is an invaluable study, with data relevant to today.
The Broadbalk Wheat Experiment is one of the UK’s internationally famous Rothamsted Long-term Experiments, located around 52 degrees north of the equator in a latitude similar to a little south of New Zealand, with average rainfall of 704mm.
Continuously cropped wheat without fertiliser averages under 2.0 t/ha grain, fairly steady despite upgrades in varieties fungicides and herbicides.
Best yielding wheat are those in rotation since 1968 (oats, forage maize, wheat, wheat, wheat since 1996). On average the first wheat in the rotation with NPK (nitrogen, phosphorus, potassium) fertiliser (includes maximum of 288 kg/ha nitrogen) now yields just under 12 t/ha.
Wheat in rotation treated with farm yard manure (35 t/ha) plus added nitrogen (96-144 kg/ha) on average yields just under the best yielding NPK fertiliser treated crops. Wheat with farm yard manure plus N (FYM+N) and NPK fertiliser treatments in rotation (first wheat crop in rotation) yielded around 4.0 t/ha more grain compared to continuous wheat with FYM only and NPK (144 kg N/ha).
Since 1955 lime has been applied to maintain soil pH at a level where crop yield is not limited. Since 2000 phosphorus use has been excluded on some plots because soil test indicated it was well above crop needs in the NPK plots.
A long-term Continuous Spring Barely Experiment (Hoosfield), also comparing FYM with NPK fertiliser, was established in 1852 (166 years).
Nil fertiliser plots yield under 2.0 t/ha with FYM+N yielding best at around 9.0 t/ha in recent years. FYM+N is ahead of NPK treatments. Researchers believe this may be due to improved soil water-holding capacity and structure because of the improved OM in FYM plots.
Many agronomic parameters are monitored in these experiments. They include soil organic matter, soil biology, weeds, pests and diseases. Soil organic matter (SOM) in the plough layer (0-23cm), has more than doubled on the FYM treatments, with increases greatest in the early years.
Mineral nitrogen fertiliser has enhanced SOM a little, probably because of increased returns of organic matter in crop roots and above ground residues. SOM has remained relatively constant in the no fertiliser treatments.
Microbial biomass of FYM plots is approximately twice that of those given either NPK or no fertilisers. “Take-all” a common Australian disease, has an interesting story in these long-term experiments.
“Take-all” decline, a phenomenon still inadequately understood, has occurred when cereals are grown continuously. Since the introduction of rotations, “Take-all” decline can be demonstrated where severe symptoms are often noted in the short sequences of wheat but less commonly in the continuous wheat.
In the early years’ fields were ploughed by oxen (later horses) and crops were cut with scythes, sheaved, and carted to barns for threshing.
Trials are now ploughed by a tractor-mounted reversible plough and harvested by a plot combine harvester. Weeding were initially by hand-hoeing. When this became impracticable (labour shortage in war years), fallowing as part of a 5-year rotation was used with each phase present each year. Herbicides have been used since 1964, except on one section.
The Broadbalk and Hoosfield experiments are part of the Rothamsted Long-Term Experiments National Capability (LTE-NC) supported by the UK Biotechnology and Biological Sciences Research Council (BBSRC) and Lawes Agricultural Trust (LAT).