Completing a simple winter feed budget for a dairy herd
Silage accounts for at least 25% of the annual feed budget for the average spring-calving dairy herd. This will be greater on farms operating high grazing stocking rates or on heavy soils.
Weather shocks such as drought or poor spring growth can create significant short-term problems. These issues often tend to affect many farms simultaneously, increasing risk of feed shortages and price rises. It is important, therefore, that farms complete a winter feed budget and take steps to ensure an adequate supply of winter feed.
Herd feed demand: How much silage is needed?
To estimate total silage demand, a simple approach is to estimate the typical full days on silage for the milking herd and multiply by an average intake of 12kg dry matter (DM). In-calf heifers and cull cows for finishing will eat 1–2kg DM less per day, but for simplicity they can be included in the cow numbers. Allow 0.5 days of silage feeding for every day where cows are usually housed by night in autumn or spring. Weanling heifers will consume approximately 5kg DM silage per day depending on quality. The figures for each stock class can be summed to give an estimate of total silage required (Table 1).
Table 1: Estimating silage requirement for a 100-cow dairy
|Stock||Days||Intake (kg DM)||No. animals||Tonnes silage DM||Fresh silage (22% DM)|
|Of which dry cow feed||77||350|
It is also important to consider the silage quality required by each class of stock. The target dry matter digestibility (DMD) for dry cows is 68–70%, suitable for moderate body condition gain over the dry period. All remaining silage will be fed to milking or growing stock and therefore should be of good quality (73–78% DMD approximately depending on the system).
Assuming that herd average dry period length is approximately 70 days, dry cow silage requirement will be 0.77 tonnes of DM in total (11 kg DM intake per day), or 3.5 tonnes per cow fresh weight at silage dry matter of 24%. Estimate the total dry cow silage requirement and subtract this from the total to calculate the tonnage of good quality silage needed. In the example outlined in Table 1, a 100-cow herd requires 350 tonnes as fed of dry cow silage, meaning 481 tonnes of good quality silage is needed; that equates to 58% of total winter feed. It is also advised that farms build a feed reserve of approximately 400 kg DM high quality silage per cow to mitigate risk of weather shocks.
Measuring winter feed supplies
Pits should be measured (length x breadth x average height in metres) to calculate volume of silage. The estimated feed in the pit will vary due to dry matter, so it is important to also have silage sampled and analysed in the lab. Drier silage will have less fresh weight per cubic metre but will actually contain more feed due to reduced water content (Table 2). Drier silage is also usually better value when purchasing feed as a result.
Table 2: Estimated fresh and dry matter of forage per cubic metre of settled silage pits
|Silage DM||Fresh tonnes m3||Dry matter tonnes per m3|
Multiplying total pit volume by the correct factor will give a good estimate of feed in the pit. For example a pit measuring 25m long, 14m wide, and 3m height at 25% dry matter will have: 25*14*3 = 1,050m3 * 0.70 = 735 tonnes fresh silage or 1,050 m3 * 0.174 = 183 tonnes DM. For baled silage, recent appraisals of bale weights would indicate that silage bales are 800–900 kg fresh or 200–260 kg DM with an average of 220 kg DM. A 220 kg DM bale is equivalent to one tonne of pit silage at 22% DM.
Budgeting winter feed and mitigation of shortages
Winter budgets can be completed manually or using PastureBase. In general, deficits of <10% at the onset of winter are manageable but the situation should be kept under review. An initial feed deficit of 10–15% may seem small, but it can become a major problem by the end of winter depending on spring grazing conditions and local forage availability. It is thus recommended to take early steps to mitigate deficits of this scale. Teagasc advisors are available to assist with completing budgets and to assess options for winter feed.
This article, by Joe Patton, first appeared in the Irish Dairying Delivering Sustainability open day book.
Also read: Setting the signs on milking quality silage in Co. Mayo