Reports indicate that there has been a significant increase in the area of forage crops sown this season. This is stemming from excellent weather conditions providing tillage farmers with more opportunities to get crops harvested and forage crops planted earlier. The earlier planting dates and higher soil temperatures have also supported increased growth rates leading to higher volumes of feed being available.

The sharp rise in the cost of concentrates and challenges faced in building autumn grass supplies is generating more interest from sheep farmers in grazing these crops. This is leading to more questions about potential levels of animal performance and establishing the volume of feed available.

Typical performance

A research trial assessing the potential of finishing store lambs on grass and forage crops commenced in Teagasc, Mellows Campus Athenry. The trial, led by researcher Frank Campion and overseen day-to-day by PhD student Mark Dolan, has two years of preliminary results captured.

The daily liveweight gain of hill and crossbred lambs on the trial for 2020 and 2021 are detailed in Tables 1 and 2.

As can be seen, the daily liveweight gain of lambs has averaged in the region of 120g to 140g daily. There are differences in performance across the three brassica forage crops with lambs grazing on forage rape and hybrid brassica ahead of those grazing Kale. As seen in Table 2, crossbred lambs have the potential to perform significantly better than Blackface lambs.

The performance of lambs in the Athenry-based trial is strongly influenced by rainfall levels, which have a direct effect on the level of utilisation attainable. It is not uncommon for lambs accustomed to the crop to be gaining in the region of 150g/day to 250g/day on brassica crops during periods of ideal weather conditions when utilisation is upwards of 60% to 70%.

In contrast, utilisation has fallen to 40% or even lower during periods of sustained heavy rainfall and this is where daily liveweight suffers most, with lambs also hit by lower dry matter levels in crops.

There is more potential to sustain the higher levels of performance in the east of the country, where rainfall levels are lower on average and utilisation is higher.

Frank says it is also important to note that performance in the first couple of weeks after going on to the crop will look poor, ranging from 40g to 70g on average. This is driven by lambs transitioning from grass to brassica crops and an adjustment in gut fill.

Lessons learned

The set-up of the trial in Athenry is focused on assessing lambs with a significant variation in liveweight. This design has restricted the percentage of lambs finished off the crop to in the region of 50% to 60%. Frank advises that where the aim is to finish lambs on forage crops, then it is important to get a rough estimate of the yield of forage available and tailor the stocking rate and type of lamb to suit. This is explained in more detail in the section regarding calculating the volume of feed present.

Lambs in the Athenry trial are offered a source of roughage in the form of straw in a covered rack-type feeder. Consumption levels are low and have been recorded in the region of 50g to 60g daily. Some farmers get away with not offering any source of roughage, or in cases where access to a grass lie-back area or headlands is seen as sufficient.

The greatest benefit in offering a source of roughage appears to be during inclement weather, when crop dry matter levels are lower.

The level of utilisation is maximised in the Athenry trial by restricting access to the crop and moving strip wires every two days to offer fresh material. Mark explains that during periods of high rainfall, this changes to allocating fresh material on a daily basis.

However, this is demanding on labour and a more realistic option for many farmers moving sheep to temporary grazing is likely to be splitting up the crop in to grazing sections. It is possible that utilisation lost in such a move may be balanced out by higher average levels of utilisation in the east of the country.

Estimating yield

The yield of the crop and probable levels of utilisation will determine the stock carrying capacity and help to establish comparable values. A simple exercise can be carried out to determine the yield as follows.

  • Measure an area of 1m2 using a measuring tape or quadrant. Cut and weigh the volume of feed in this area.
  • Multiply this figure by 10,000 as there are 10,000m2 in a hectare.
  • This will tell you the volume of forage available on a fresh weight basis. To convert this to a dry matter basis, multiply by the dry matter content of the crop.
  • Typical values for forage rape and hybrid brassicas are in the region of 12% to 15%, but remember this can drop to 8% to 10% during sustained wet weather. The dry matter of stubble turnips is lower at 8% to 10%, while fodder beet varieties typically range from 18% to 20% dry matter.
  • The next factor to take into account is utilisation and the figure from above should be multiplied by the estimated utilisation.

  • Example: If we take 2.7kg fresh weight equates to 27,000kg on a fresh weight basis, then taking a dry matter content of 13% gives a dry matter yield of 3,510kg or 3.51t. Assuming high levels of utilisation, say 75%, this equates to 2,632.5kg of feed.
  • The next factor to take into account is intake, so that the stock carrying capacity can be determined.

    Lowland lambs weighing 35kg to 44kg are estimated as having a dry matter intake potential of 1.25kg DM daily with light hill lambs weighing 28kg to 35kg in the region of 1kg dry matter intake. This will increase comparable to lowland lambs as lambs increase in weight.

    If we revert to our 2,632.5kg of feed available and use a figure of lambs consuming 90% of their daily requirement in forage and 10% from a source of roughage or concentrates then there is 2,340 grazing days available or 23 days grazing for 100 lambs. The stock carrying capacity can be prolonged with lambs by introducing concentrates or likewise with ewes by offering hay/silage.

    Balanced grazing

    Irish Farmers Journal tillage editor Andy Doyle advises that the optimum method of grazing brassica forage crops is to move animals to fresh forage before they are forced to graze down the stems. This, he says, will provide an opportunity for a level of regrowth and keep a layer of cover on the field to provide soil protection.

    It will also enhance animal performance as they are not being forced to graze lower-quality stem.