Challenges of the 2010 grazing season

The extremes in weather since the start of the year have provided very different challenges for grassland farmers. Although many farms have reported below average yields for first cut silage as a result of the low spring temperatures, the predominantly dry conditions since April have provided ideal grazing conditions. The GrassCheck project, jointly funded by AgriSearch and DARD, has been monitoring grass growth throughout this period. However, a number of farmers have highlighted that the trends in grass growth rates measured within GrassCheck during the last few weeks have not always reflected growth trends observed on their own farms. In addition, those who follow the grass predictions closely may have noticed that some of the predictions presented during the last few weeks have not been as accurate as those presented in previous years. This article aims to explore some of these issues.

At each of the six GrassCheck sites there are a total of nine grass plots, and these plots have been monitored since 1 March. Grass growth is measured by cutting and weighing the grass on three of these grass plots each week. This means that the grass growth reported by GrassCheck each week is actually the average daily growth over the previous three weeks. It must also be remembered that these plots are regularly cut to a constant height. This may result in higher growth rates being measured compared to a sward being grazed because the potential negative effects of poaching, lax grazing and irregular grazing intervals are removed.

The total annual fertiliser input on the grass plots is 270 KG N per ha, with fertiliser being applied following every harvest of grass. The week beginning 14 June was the start of the sixth rotation of cuts, and 211 KG N per ha have been applied to date (considerably more than on most farms). However, it is worth noting that the GrassCheck grass plots do not benefit from the recycling of nutrients through the grazing animal (dung and urine), and within an intensive dairy farm this could be equivalent to approximately 80 or 100 KG N per ha per year. Although 270 KG N per ha represents a higher annual fertiliser application rate than most farmers are currently using, dairy farmers who have a high grass demand can operate at these levels, while still meeting the requirements of the Nitrates Directive. Thus the GrassCheck project provides information on the ‘grass growth potential’ of well managed grassland. Farmers applying lower levels of fertiliser N will obviously experience lower growth rates.

Grass growth predictions have been a feature of GrassCheck since 2004, and undoubtedly 2010 has been the most challenging year for this element of the project. Although the predictions were accurate early in the season, they have been highly variable through late May and early June. Growth predictions are generated from a computer model, based on time of year, historical growth rates, fertiliser N application rates and the weather forecast for the next two weeks. The accuracy of the weather forecasts and the way the model interprets these inputs have both played a role in the erratic nature of the predictions in recent weeks.

The main reason that the model has struggled in recent weeks is related to the increasing soil moisture deficit in many of our soils at present. It is recognised that the model does not cope particularly well during periods of soil moisture deficit, and as soil moisture deficits are reasonably uncommon in Northern Ireland, there is not a lot of data available to strengthen the model. The model has identified that moisture has been a limiting factor for growth since 17 May, and we are currently in a severe situation, where growth potential is being restricted by almost 50percent. To remove the limitation of moisture on growth the model suggests that over 50 mm (2 inches) of rainfall is required. The coming weeks are expected to provide a wide range in growth across Northern Ireland, as the effect of soil type and the uneven spread of rainfall across the country begins to have a greater influence.

There is an additional complication currently affecting the predicted grass growth rates, which relates to how grass responds immediately after rain during a period of soil moisture deficit. Research evidence suggests that after a heavy shower of rain grass growth potential may increase to almost maximum for a short period, despite overall soil moisture remaining in deficit. This has been observed by farmers in the past few weeks, with short bursts of rapid growth seen in the grazing fields immediately after quite heavy rain. This sudden burst of growth is something which the model struggles to identify, as growth predicted by the model is mainly driven by the fact that the soil moisture is still in deficit. To compensate for this limitation, additional sward measurements will be carried out on all the sites to try and determine how growth reacts within the current period of moisture deficit, and particularly how it reacts once soil moisture improves.   

This year the GrassCheck project has been expanded to include additional grassland management information from six local dairy herds. The ‘Grazing Management Focus’ section was launched this year to increase awareness and understanding of key grazing management technologies such as the grass wedge. The herds involved represent very different cow genotypes, land types and milk production systems. The differences in stocking rates, grass supply, grass intakes and cow performances between the farms are useful to demonstrate how the grass wedge reacts to these changing situations. It is important however to stress that these farms are not demonstration farms, and have not been selected to demonstrate best practise grazing management. They do however provide real data from ordinary farms around Northern Ireland, and the willingness of these farmers to allow their herd and grassland performance to be published in the farming press is very much appreciated.

Although the GrassCheck project has experienced difficulties in predicting grass growth in recent weeks, it is hoped that with the assistance of additional sward measurements, this weakness within the model can be strengthened. GrassCheck will continue to provide weekly information, which is designed to highlight the key grassland issues to allow grassland managers to respond to whatever further challenges this grazing season has in store.