Why cell grazing beat set stocking in Devon trial
© Charlie Morten Cell grazing is a more sustainable and productive grazing method than set stocking, results from a four-year grazing trial have shown.
Cell grazing consistently outperformed set stocking plots at Rothamsted Research, North Wyke, Devon, with higher metabolisable energy (ME) content in forage and higher pasture growth, pasture utilisation and stocking rates, resulting in significantly higher output/ha.
See also: Experiment shows potential for cell-grazing herbal leys
This is according to principal investigator for the trial, Dr Jordana Rivero, whose interim technical report also concludes that farmers adopting cell-grazing methods can potentially enhance their soil quality and contribute to climate change mitigation.
In the trial, cell-grazed plots achieved a lower nutrient leaching potential for each unit of liveweight gain and similar soil compaction levels, despite running at higher stocking densities.
Cell grazing also led to an increase in soil organic matter and carbon content.
Top tips for starting cell grazing
For those currently set stocking, trial a controlled approach in one field:
- Subdivide the field into smaller areas
- Assess pasture covers using sward sticks, plate meters, or visually
- Achieve low residuals (about 1,500kg dry matter/ha) by adjusting size of area or stocking density
- Allow each area to rest for at least 21 days.
If there are no obvious results in the first season, give it a second chance; learn as you go.
For those already using rotational stocking, adopt a staged approach rather than a wholesale switch with all the infrastructure and technology:
- Further subdivide existing areas gradually
- Fine-tune the estimation of animals’ demand and the available biomass
- Move animals every one to two days
- Consider additional measures such as allocating a few cells for standing hay for deferred grazing to maintain quality and quantity of the remaining area and create resilience in case of a potential shortfall in herbage.
Source: Dr Jordana Rivero, Rothamsted Research
“We already knew that well-managed rotational stocking methods are more productive than set stocking – which has the least control possible,” says Jordana.
“We also knew that cell grazing could operate at greater stocking rates.
“The surprising results were the increase in soil organic carbon in cell grazing, and the slight decrease in set stocking, since the literature shows inconsistent results.
“Probably, there are several factors influencing this, but it strongly suggests the systems are reacting and adapting to the new management practices, and highlights the relevance of assessing them in the long term.”
The study also shows the importance of careful management when moving to a new grazing system (see “Top tips for cell grazing”, above).
This involves considering suitable animal genetics, sward composition and management tactics prior to implementation.
Key findings
The comparison of grazing methods was carried out on 12ha (30 acres) of grassland on heavy clay.
Dairy beef cattle, bought in April 2018 and April 2020, grazed for two seasons (see “Methodology of grazing trial”, below).
1. System productivity
As pasture grown under set stocking was fairly consistent, stocking rate remained at 1,300-1,400kg liveweight/ha throughout the four years.
Cell grazing produced more pasture growth during the first three years, enabling an increase in stocking rate from 1,800kg liveweight/ha to 3,000kg liveweight/ha – twice the stocking rate of the set stocked areas.
“We knew [cell grazing] could support more kilograms of liveweight and began with a 50% higher stocking rate.
“In the third season, the initial stocking rate was 70% higher in the cell grazing. By year four, we had doubled it,” says Jordana.
Cell grazing achieved a higher liveweight/ha output, averaging 483kg liveweight/ha for the first cohort of cattle and 890kg liveweight/ha for the second, compared with set stocking, which averaged 367kg liveweight/ha and 585kg liveweight/ha, respectively.
Overall, cell grazing achieved a 150% increase in liveweight/ha production compared with set stocking.
On average, the cell-grazing season was three weeks longer than the set-stocked one.
2. Soil structure and health
Soil organic matter increased with both grazing methods by nearly 1%, from 8.4% to 9.4%, having been reseeded in 2013 with perennial ryegrass (PRG) and clover and used for silage and occasional grazing before the trial.
Across the four years, there was a 5t/ha increase in carbon in the cell-grazed paddocks and a decrease of about 2t/ha carbon in the set-stocked areas.
Soil compaction was unaffected by grazing method, suggesting that although cell grazing involves substantially higher stocking densities (up to 120,000kg liveweight/ha), the rest periods between grazing events and grazing seasons allow plants to grow deeper, more active root systems, enabling the soil to recover/retain its original structure.
3. Environment
Nutrient leaching potential was similar between grazing methods despite the cell-grazed areas carrying up to twice the stocking rate.
In terms of botanical composition, PRG content increased from about 40% to 70% of the sward with cell grazing, and by about 17% to 52% with set stocking, from spring 2018 to spring 2021.
“However, in autumn, at the end of the grazing season, the proportion of PRG still increased [in cell grazing], from 40% to 60%-plus, whereas in set stocking, it decreased from 30% to 20%,” Jordana explains.
“In spring, the percentage of clover was low for both; in autumn, where usually clover has a higher presence, cell grazing maintained the cover, while it virtually disappeared with set stocking.”
Enteric methane emissions did not vary between the two methods.
4. Pasture growth and utilisation
Dry matter (DM) production was 39% higher on average for cell grazing across the four years, with a 54% difference (11.5t DM/ha versus 7.5t DM/ha) in year four.
Increased pasture growth was achieved by managing short grazing periods followed by appropriate rest periods of 21-56 days depending on herbage availability, growth and time of year. This resulted in a higher average farm cover.
Cell-grazing plots consistently had a higher ME content, with average values of 11.2MJ ME/kg DM for cell grazing and 11MJ ME/kg DM for set stocking.
Pasture utilisation increased during the four-year period, with cell grazing achieving 76% in years three and four, compared with 63% for set stocking.
5. Animal performance
Transitioning the first cohort of cattle (sourced from multiple farms) from a straw and concentrate ration to the grazing treatments was challenging.
This, together with a very dry summer and extremely wet autumn, resulted in poor daily liveweight gain (DLWG) of 0.34kg/day for cell grazing and 0.47kg/day for set stocking.
The second cohort, from a single farm, arrived before the grazing season and was transitioned onto silage before being turned out to set stocking or cell grazing.
Weather conditions were more normal and growth rates averaged 0.76kg/day for cell-grazed animals and 1kg/day for set-stocked.
Age at slaughter was 25-26 months. Set-stocked animals were heavier going to slaughter but killing-out percentage was very similar between grazing treatments, at about 51%.
Next steps
The seventh grazing season is under way and comparison of the two grazing systems will continue, with additional measures related to biodiversity, soil carbon and animal activity.
Methodology of grazing trial
The 12ha (30-acre) field was divided into three areas, each with set stocking and cell grazing, giving three replicates for both methods.
The set-stocked paddocks were 1.5ha in 2018, increasing to 1.75ha from 2019 onwards.
The cell-grazed paddocks were laid out in 1ha plots divided into two 0.5ha strips, each with 21 fence posts. These posts acted as markers to create up to 42 0.024ha cells.
The field had been ploughed and reseeded in 2013 with perennial ryegrass and white clover and used for silage and occasional grazing.
At the start of the experiment, it still contained the sown species, as well as a range of weed grasses and forbs.
The following applications of inorganic fertiliser were applied each year:
- 50kg/ha a year of P
- 30kg/ha a year of K (except for 2021)
- 100kg/ha a year of N, split into three applications (except for 2018).
Autumn-born dairy beef steer calves were acquired in April 2018 (average weight 255kg) and April 2020 (average weight 219kg) to graze for two consecutive seasons.
They were randomly allocated to the treatment groups, which were balanced for liveweight and breed type.
At the end of their first grazing season (winter 2018 and winter 2020), they were housed together and fed a diet of grass silage, 2kg sugar beet, 0.5kg wheat distillers’ grains and 100g minerals, targeting 0.8kg daily liveweight gain.
The following April they returned to the same grazing method and finished on pasture.
The set-stocking paddocks were grazed continuously from April to October/November. The cell-grazed areas were rotationally grazed on daily moves.
The daily allocated grazing area varied according to pasture growth rate.
Target entry covers for cell-grazed plots were set at 3,500kg dry matter (DM)/ha and grazing residuals at 1,800kg DM/ha.
Set-stocked animals were removed from a paddock when average pasture covers dropped below 2,000kg DM/ha or ground conditions deteriorated.
Cattle were weighed at the start of the grazing season and monthly thereafter.
Pasture covers were recorded weekly using a rising plate meter and fresh pasture samples were collected fortnightly for lab analysis.
Botanical composition of the swards was assessed at the beginning and end of each grazing season.
Soil samples were taken annually, as well as soil compaction assessments and soil groundwater samples.
The grazing trial was partly funded by the European Regional Development Fund through the Agri-Tech Cornwall initiative (2018-2021). Precision Grazing was a project partner. The full report (PDF) is available online.