Grass access worth £26 a cow in 100-day robot trial

A Northern Irish study found margin over feed was £26 a cow greater for cows offered pasture access over a 100-day period.

This was despite a 12% fall in milk yield and cows only supplementing a total mixed ration (TMR) with 5kg dry matter (DM) a cow a day intake of grazed grass.

The study, which took place on the high-yielding herd at the Agri-Food and Biosciences Institute (AFBI), builds on research at the Agriculture and Food Development Authority (Teagasc) at Moorepark, County Cork.

This earlier research found robotic visits and pasture intake can be sufficient in systems combining robotic milking and grazing.

See also: How a dairy farmer has integrated robots and grazing cows

AFBI dairy and grassland researcher Dr Debbie McConnell stressed that, even with low grass intakes, enough was eaten to affect feed cost and show a benefit in margin over feed.

“Even now with the higher fertiliser prices and ammonium nitrate at £600/t or £700/t, high-quality, well-managed grazed grass remains the best feed we have,” Dr McConnell told Farmers Weekly.

Study design

  • 48 cows in two groups
  • 100-day study from June to August
  • High-genetic merit herd of profitable lifetime index (PLI) +£322
  • Cows balanced for calving date, lactation, liveweight, yield, body condition score, PLI and predicted transmitting abilities for milk, fat and protein
  • Cows given three weeks of training for robots and automatic gates
  • All cows were early-lactation, spring-calving cows
  • Pre-experimental yield of 39.8kg

How the study worked

Cows were split into two groups. One group was offered access to pasture through an automatic grazing gate and the other was housed continuously through the summer of 2021.

Pasture was allocated into a morning block and an afternoon block, and each block was split into 24 paddocks.

One paddock in each block was grazed daily. Two electric fences were set up for each paddock each day.

The team measured milk yield and constituents, change in liveweight, concentrate, TMR and grass intake, milking interval and visit behaviour, grazing efficiency and grass utilisation and time at pasture.

A feed cost was calculated that included all costs of forage production and the TMR cost, and a milk price was calculated from yield and milk constituents.

Milk constituents fell in the grazed group, but this was not statistically significant, said Dr McConnell.

Daily routines for housed and grazing cows during 100-day study


Housed group

Grazing group


Full-time housing of 24 cows on a TMR diet (maintenance plus 24 litres) and fed to yield through milking robot at 0.45kg/litre

Grazing by day and housed at night. Fed half the TMR allocation of housed group, targeting 10kg DM a cow a day of grazed grass in two visits to pasture and fed to yield (0.45kg/litre)



Grazing gate opens automatically


Any cows not milked for 10 hours are moved to waiting pen for robot

Cows waiting in shed moved to waiting pen for robot



Grazing gate opens for next paddock automatically

1pm TMR feed allocation to bunker


Any cows not milked for 10 hours are moved to waiting pen for robot

Cows still grazing in paddocks are brought in from the field and placed in the waiting pen for the robot

Take-away messages

  • Cows preferred morning grazing Residuals in morning paddocks were 5% lower than afternoon paddocks. Cows spent five hours nine minutes at pasture in the morning and three hours 39 minutes in the afternoon
  • Distance is important The furthest paddock was only 350m away from the shed gate and allowed more robot visits in the housed cows. Grass utilisation was almost 78% in the morning and 73% in the afternoon. Research from around the world shows visits to robots suffer if cows walk 850m or more to graze
  • Weight gain lower at grass Cows that were housed were 27kg heavier at the end of the trial. But this had no notable impact on subsequent fertility
  • Dry matter intake lower at grass Lower yields from the grazing cows was likely because of a lower dry matter diet (16% versus 38.5%) and increased energy expenditure from walking
  • Fencing is labour intensive Two fences were put up for each paddock every day, which took time and would need factoring in on a commercial farm

Cow performance and margins




Milk yield (kg a cow a day)



Milk fat (%)



Milk protein (%)



Liveweight gain (kg)



TMR intake



Concentrate intake (kg/DM a cow a day)



Silage intake (kg/DM a cow a day)



Feed costs (£ a cow a day)



Milk income (£ a cow a day)



Margin over feed (£ a cow a day)



Robots and grazing

The trial was prompted by a steady growth in robotic milking in Northern Ireland over the past 10 years. About 10% of dairy cows in Northern Ireland are now milked robotically.

Addressing a recent GrassCheck GB webinar (3 February), Dr McConnell revealed an AFBI survey that showed a tendency for robotic conversion to preclude grazing.

Of the farms surveyed, 62.5% grazed the primary milking cow group through the summer prior to installation. After installation, 67.7% provided forage through silage and TMRs and did not graze the main group of cows.  

But Dr McConnell said that a growing body of data could convince farmers to persist with grazing if making the switch.

“Milk from grass is good for the industry’s image,” said Dr McConnell. “Consumers like to see cows grazing in summer.

“We can show more margin and an economic reason for grazing, and we also know that milk from grazed grass has higher levels of vitamin D and polyunsaturated fatty acids, so it produces a better product. There is an economic and environmental win here.”