Palatability of a grass sward is a major factor in the overall performance of ruminants – an often-overlooked characteristic.

What the animal chooses to eat determines intakes, rather than the total forage available. The palatability of the sward is heavily determined by the growth stage of the grass, but also by the species and varieties that are present.

Unfortunately, the palatability of grasses can only be measured by observing grazing performance using live animals – an expensive and challenging operation. However, Watson Seeds embarked on this method of testing grass varieties at their site at Upper Nisbet, Roxburghshire.

Through this work some 30 different varieties of perennial ryegrass over three grazing seasons were observed and measured such that the company now has a good understanding of how they perform under animal intervention as opposed to the National List Trials system and their mechanical cut and measurement protocols.

Work in Ireland at Moorepark, has identified that the system of frequent mechanical defoliation (8-10 harvests/year) to simulate rotational grazing in dairy systems has its weaknesses.

Differences in dry matter consumed has highlighted that certain varieties possess specialised traits that make them better suited to grazing systems. This leads to the argument that the lack of animal intervention in National List Trialling and Recommended Lists is a major disadvantage especially as grass sward performance is realised through animal production.

On-farm trial with cattle

The on-farm trial using animals provides more realistic grazing information with the pre and post grazing heights of varieties measured with an AS Platemeter G1000. Residual grazing height has been determined using the differences between actual and predicted post grazing heights which has been used as a measure of grazing efficiency.

The field was subdivided into 1ha paddocks with six individual single stand varieties of PRG grouped by their heading date (REE) being rotationally grazed. By having the varieties tightly grouped and with diploids growing alongside tetraploids, there was a great opportunity to observe their characteristics under different grazing challenges and weather conditions. Swards with high grazing efficiency are those that are consistently grazed to low post grazing heights, and it is these swards that supported the highest level of utilisation.

Their findings are helping to inform what varieties are included in the Castle Mixtures with the goal of increasing utilisation and driving profitability in grazing systems.

Poorly managed swards with high post-grazing heights are more difficult to manage in subsequent rotations as lower digestible stem builds up in the sward. Traditionally these swards need to be corrected and reset by mechanical intervention (topping) with the associated labour, fuel, and machinery costs as well as the loss of the topped grass.

Pasture Profit Index

The growing importance of palatability is clear with The Irish Department of Agriculture, Food and the Marine now adding a Star system (1-5) for grazing utilisation to its variety evaluation – Pasture Profit Index.

Good grazing management keeps swards in the utilisable stage and minimises waste. The eight paddocks at Upper Nisbet were rotationally grazed on average every 28 days (range 18-40 days). A target entry when the average herbage mass across the six plots was estimated at about 1400kg DM/ha above finished grazing level (3.5cm).

With 45 bulling heifers on the paddocks, with an average daily requirement of 10kg DM head/day had a three or 3.5 day shift per paddock. The post grazing sward heights showed significant differences between varieties across the three seasons, both by observation and measurement.

If the lowest residual is 3.5cm, the trial regularly saw varieties at 4.5-4.7 cm. Tetraploids were consistently lower than diploids in post grazing heights over the three seasons.

The late heading varieties tended to have lower PGSH than the intermediates and which was particularly evident with the diploids, but less obvious in the tetraploids. Within the trials, the tetraploids stood out, but there were also diploid varieties that compared favourably to their tetraploid neighbours.

Care was taken in observing and measuring results as a legacy of higher post grazing sward heights might influence subsequent grazing performance with un-grazed grass being carried into the next rotation.

The overall message from the farm trial site at Upper Nisbet was that there are significant differences in the PRG varieties and their level of grazing efficiency.

Tetraploids have outperformed diploids on yield and better grazing utilisation over three seasons, but the next couple of years will be key to the persistency of the sward over the duration of the farm’s rotation and reseeding policy.

Pat Lambert, Watson Seeds