Archive: HGM cows at grass

Published 29 August 14

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Grass offers significant potential as a cost effective ration component in the high genetic merit (HGM) dairy cow diet, says DairyCo extension officer Tom Goatman, in his project for a Diploma in Ruminant Nutrition at Harper Adams University. But achieving this requires excellent grassland management to ensure the nutritional requirements are successfully fulfilled. 

“On an international scale there is a relationship between an increasing proportion of grazed grass in the diet and a decrease in the total cost of production,” says Tom. “Data from Great Britain also demonstrates that as the total cost of production increases, net margin decreases and that increasing the proportion of milk obtained from forage can reduce total feed costs, positively influencing overall net margin.

“Therefore, there are considerable benefits to increasing the proportion of grazed grass in dairy cow diets, but there can also be significant implications in terms of the nutritional management of cows, particularly those of high genetic merit.”

As part of his diploma, Tom has looked at research, both completed and on-going, into the nutritional management techniques required to achieve the genetic potential of the modern HGM dairy cow at grass, and based on current understanding, there are a number of key points to consider: 

  • High quality pasture and management is the foundation of successful nutritional management of HGM dairy cows at grass
  • The two key factors in fulfilling the genetic potential are optimising DM intake to fulfil energy requirements, while minimising excessive body condition score loss. Managing the HGM dairy cow at grass will require the system to maximise daily herbage intake per cow but also maintain the quantity and quality over the grazing season. Studies have shown that a pasture allowance of 90% of the animals’ voluntary pasture intake level can be a good guideline to reach equilibrium between per cow and per ha milk production. This can be managed through effective management of pre and post grazing sward height
  • HGM dairy cows yielding 30+ litres will require a level of supplementation at grass in order to achieve genetic potential. Milk production greater than 30kg/d without excess mobilisation of body reserves is unlikely without supplementation of additional energy. Research also suggests that compared to lower yielding animals, concentrate supplementation of high yielding dairy cows at pasture will result in a lower substitution rate. This ranged from a 0.4kg to 0.6kg reduction in grass DM intake per kg increase in concentrate DM intake. It will also result in a higher milk yield response of >1.0kg milk/kg of concentrate DM, this is because the animal is unable to fulfil the energy requirement from grazed pasture alone. Adding concentrate supplementation increases energy intake. This will increase rumen fermentation and microbial protein synthesis, which in turn optimises dry matter intake
  • An increase in energy requirement must be factored in, depending on herbage availability, digestibility, distances walked, weather and topography. Studies have demonstrated that energy expenditure under grazing conditions can increase by up to 21% and subsequently the maintenance requirements should also be increased to account for this. This has important implications in HGM animals, as this puts further pressure on the balance between fulfilling requirements within the constraint of intake. It also highlights the importance of good farm infrastructure and layout to allow ease of access to pasture if grazing is to be utilised successfully in the HGM dairy cow diet
  • Studies have indicated no benefit of offering supplement in a partial mixed ration over traditional in-parlour allocation and separate forage
  • There is evidence that the traditional pH thresholds of rumen dysfunction may differ in grass based diets
  • There is evidence that restricting access time to pasture to 3 to 4 hours per session can improve intake. There is also evidence that cows can react to a time constraint at pasture by spending 90% to 95% of the time grazing. This increases pasture intake rate by 30% to 40% compared to full time grazing. 

“HGM dairy cow at grass offers both an important route in controlling the total cost of production but also improve overall feed efficiency and mitigation of environmental effects,” concludes Tom.

For a full copy of Tom’s report Nutritional Management of the High Genetic Merit Dairy Cow at Grass click here.