You’ve got the quantity with first cut – now plan for quality with second cut

June 10, 2022

Target harvest date for high quality first-cut silage is between mid to late May. Nigel Gould, CAFRE Beef and Sheep Adviser states wet weather this year has delayed harvest date on many farms. The delayed cutting date, combined with possible lower application rates of chemical fertiliser due to rising prices means that the quality of first-cut silage may be reduced. With concentrate costs continuing to increase, it is vital to assess silage stocks on farm in terms of both quantity and quality.

Use Tables 1 and 2 to estimate the quantity of silage available and compare this with your likely winter demand. The volume of silage is calculated by multiplying the length of the pit by the width by the height. For example, the volume of silage in a pit measuring 40m by 10m by 3m is 1200 cubic metres. To convert the volume of silage to tonnes multiply the volume by the correct conversion factor (Table 1). For example, if the silage is 25% dry matter multiply 1200 by 0.68. This equates to 816 tonnes of fresh silage. For silage left over from last year, a silage analysis will provide an accurate figure for dry matter percentage. This won’t be possible for this year’s first cut and an estimate based on experience may be necessary.

Table 1: Conversion factors to convert silage volume to tonnes of silage

Table 2: Estimated monthly feed requirement of stock eating 25% dry matter silage

“Where concentrates are traditionally fed on farm to certain classes of stock, is there an opportunity to reduce or eliminate this cost?” For example, feeding excellent quality silage (D-value > 70) can eliminate the requirement for supplementary concentrates for a typical weanling with a target daily live weight gain of 0.6-0.7 kg/day during the housed winter period. This animal may require 2-3 kg of concentrates if fed alongside poorer quality silage. Poorer quality silage alone can be targeted toward less productive stock such as dry Spring-calving cows however, for more productive animals such as lactating Autumn-calving cows and ewes in the final 6-8 weeks of pregnancy, quality silage in addition to supplementary concentrates will be required. “For this reason, thought should go into fertiliser requirements and cutting date for second cut silage.”

 Refer to Table 3 below as a guide to maximum recommendations for P and K fertiliser applications for second cut silage. Allow for P and K in slurry if applied after 1^st cut silage. Typically, 1000 gallons of slurry supplies 12.5 kg (25 units) of K. The amount of available P in slurry will depend on the soil P index. It will supply 2.75 kg (5.5 units) of P at index 1 or below and 5.5 kg (11 units) at index 2 or above). Table 4 summarises the P and K requirements for 2^nd cut following an application of 2000 gallons of slurry per acre.

Table 3: Phosphate and Potash Recommendations (kg/ha) for 2^nd Cut Grass Silage (kg/ha)

Table 4: Phosphate and Potash Requirements (kg/ha) for 2^nd Cut Grass Silage (kg/ha) to supplement beef cattle slurry applied at 22m³/ha (2000 gal/acre)

Nitrogen rates required for second cuts will vary depending on the anticipated yield, desired number of cuts and historical N spread.  For a two cut system with medium levels of N applied in the past, up to 100kg/ha (80 units per acre) will be required from organic manures and chemical fertiliser.

Aim for a silage dry matter content of 30% and a D-value greater than 70. This can be achieved by targeting a harvest date before much seed emergence occurs in the crop and ensuring a good rapid wilt. Sometimes there is a worry that nitrate may still be present in the grass particularly if fertiliser spreading date is later, which can negatively affect silage fermentation. A rule of thumb is that 2 units of nitrogen per acre is used up by the crop daily. Therefore, if two 50kg bags per acre of CAN (27% N) was spread, this equates to 54 units of Nitrogen so would take just under 4 weeks to be used up. It is important to note that this is just a general guide, the rate will be higher during periods of high growth and lower in periods of poorer grass growth. Any nitrate present in grass at harvesting is less of an issue where a good wilt has been achieved.