The value of lucerne silage

20 July 2018 - 31 January 2019

LucerneThe current dry spell has made farmers think about the need to have “not just grass”. The amount of lucerne seed sold has been increasing over the last few years, especially since pure stands were included in greening rules of the Basic Payment Scheme.

Lucerne or Medicago sativa, commonly known as alfalfa, is a legume and widely grown throughout the world. It is valued for its yield, protein content, digestible fibre and drought tolerance. Like other legumes it does not grow when the soil temperature is below 8°C, so does well between April/May and September in the UK. Yields can struggle in cold spring and – even with its drought tolerance – in dry summers.

Lucerne grows off a tap root which stores nutrients to help the plant regrow after it has been cut or grazed. Like other legumes, such as clover, peas and beans, there are nodules on its roots which contain nitrogen-fixing bacteria so the crop needs no additional applied nitrogen (N) to grow. There have been some reports of lucerne roots going down as far as 15 metres in search of water. There could be some risk to drains when growing this crop.

The nutritive value of lucerne is around 18-22% crude protein (CP) and around 10 megajoules (MJ) per kg DM of metabolisable energy (ME). It can be grown with certain grasses to improve the feed’s energy content or fed with maize silage to make a balanced ration for energy and protein. However, even when grown with fescue or timothy as partner in a sward, competition from the grass can severely reduce the output and survival of lucerne.

The target fresh weight (FW) production per year is 40t/ha (16t/acre), with a target annual DM yield (at 30% DM for silage) of 12t DM/ha (4.8t DM/acre).

Conserving lucerne

Lucerne can be baled or clamped. Under UK conditions, high humidity and the low sugar content of lucerne can make it difficult to ensile. In addition, lucerne is high in protein and calcium which will buffer any changes in pH in the clamp/bale. Good conservation management is key to achieving high quality forage.

In the first year of the crop, one or two cuts may be harvested. After this however, the crop will be suitable for cutting four to five times a year, typically at five week intervals.

Typically in the UK, lucerne can be harvested from mid-May onwards (depending on location). The greatest yields are achieved in the first two cuts (see Table 1).

Table 1. The proportion of total yield from lucerne cuts

Cut

Proportion of total yield (%)

Late May

35

Early July

35

Mid-August

20

Late October/early November

10

Source: Sheldrick et al. 1995

Deciding on the best time to cut can be a compromise between yield, quality and crop persistence. While the protein content of lucerne is greatest at the pre-bud stage, it is currently believed that routine cutting at this time may shorten the lifespan of the crop.

Herbage harvested at full bloom will have a higher proportion of stem. As the crop flowers the stems become more fibrous and the feeding quality of the forage decreases. As a result, most growers choose to harvest the crop at the point of flower (10–30% flowers open). This is to ensure a good compromise between quality and yield and to ensure there is adequate root development.

To ensure persistency, some growers opt to harvest one flowered crop per year. Work from New Zealand supports this approach. However, French growers do not do this, as it means the subsequent cut is of lower quality. It is likely that the NZ guidance is more important for systems based around grazing the crop rather than cutting.

The regrowth of new shoots can also help ascertain optimum time for cutting. Harvesting when new shoots are visible will ensure good regrowth. However shoots should be short enough to avoid damaging these with the mower (See Figure 1).

 

Lucerne Table

Mowing and wilting

A minimum cutting height of 7cm is advisable to avoid damage to the crown of the plant. This will also allow good airflow under each swath, which will help dry the crop.

Direct cut lucerne is high in moisture and often too low in sugar to allow effective fermentation to occur, so it is important to wilt the crop to a minimum DM content of at least 30% (40–50% if baling) to concentrate the sugar content. As a result, lucerne/grass mixtures are thought to be easier to ensile due to the higher sugar content of grasses.

Up to 70% of the protein and 90% of the minerals and vitamins in the plant reside in the leaf so it is important to minimise leaf losses. As the leaf dries out quicker than the stem of the plant, over-wilting or extensive handling can result in large amounts of leaf shatter.

The use of roller-type mower conditioners, which cut and condition the lucerne by crimping or crushing the stem is advisable when harvesting lucerne. These will speed up the rate of moisture loss from the stem without extensive damage to the leaf. However by encouraging more rapid leaf drying it can increase leaf shatter, so care is needed.

Figure 1. The consequence of different cutting times on lucerne plants (Figure 3 from BRP+)

Raking the crop before the dew has evaporated and reducing tractor speed will help reduce leaf losses.

Using an additive is advisable when ensiling lucerne. Seek advice from the local merchant or supplier.

Feeding lucerne

High DM yield, protein and calcium content make lucerne a suitable forage for ruminants and it is more digestible than other similar feedstuffs. The actual digestibility of the forage will depend on factors such as growth stage, cutting frequency, harvesting conditions and fermentation processes.

The high protein content (18–25%) of lucerne silages make it a good replacement for soyabean meal in diets. It is also a good complement to maize silage because it is high in nutrients that maize is low in. For ruminants, the lower readily available starch content and higher buffering capacity of lucerne compared to maize silage also has a beneficial effect on rumen pH.