Making high quality silage for dairy farms
Silage quality for the milking herd and the replacement heifer enterprise needs to be 74 DMD+, while 68-70 DMD silage is adequate for the dry cow period.
While the focus for pasture-based dairy farms must be on maximising the proportion of grazed grass in the milking cow diet, there are inevitably times of the year where silage constitutes part or all of the daily forage fed to the herd.
Ensuring that high quality grass silage is produced, and a reserve of same is available, is critical to maintaining high levels of performance when grass availability or utilisation is compromised, particularly during early lactation. Silage quality measured as dry matter digestibility (DMD) nationally averages approx. 68% DMD with a range from 58 to 80% DMD observed across all ruminant enterprises, and so there is considerable scope to improve silage quality on Irish farms.
Characterising high quality silage
There are three main components to consider when determining silage quality; the nutritive value, preservation characteristics and the mineral content of the forage. Some of the key parameters and guidelines within these categories are outlined in Table 1 for both milking cows/young stock and dry cows. These parameters influence the physical dry matter intake (DMI) potential of the silage, and the energy and protein intake of the animal, which together play an important role in ensuring optimal performance.
Table 1: Key parameters for assessing quality of grass silage
| Measure | Dry cows | Milking cows and young stock | Comment |
| DMD % | 68 to 70 | 74+ | Key determinant of feed value |
| UFL (energy) /kg | 0.72 to 0.75 | 0.83 to 0.88 | Higher UFL – more milk/weight gain |
| Crude protein (CP) | 12 | 14+ | Low DMD +/- N applied reduce CP. |
| PDIE g/kg (protein) | 75+ | 80+ | Determined by UFL and CP levels |
| Intake value g/kg LW0.75 | 90 to 95 | >105 | Higher values – better intake potential |
| Potassium (K) content | <2.2% for dry cows | >2.4% no issue for milking cows | High K silage fed from 2wk pre-calving creates milk fever risk |
| pH | 4.0 to 4.2 (4.4 for drier crops) | Too high pH – poor preservation, Too low pH – may affect intake | |
| Ammonia | Less than 8% of N | High ammonia indicates poor preservation and reduces intake | |
| Lactic acid | 8-10% of DM | Higher values indicate a stable, palatable silage | |
| Ash | <8% of DM | High ash indicates soil contamination | |
The demand for high quality silage is driven by the overall capacity of the farm to produce grass and the stocking rate (SR), particularly milking platform stocking rate, as additional forage is required to support forage demands in spring/autumn.
Increasing milking platform SR from 2.5 to 3.2 increases high quality silage requirements by 0.3-0.4 t DM/cow. Seasonal challenges such as prolonged wet periods in spring and drought periods during summer months also increase the demand for high quality silage, and a reserve of 0.4 t DM/cow should be maintained on farm for such events.
Farms should determine the requirements for high quality silage for their farm and put a plan in place to achieve adequate levels. If silage deficits occur, acting promptly to source ‘dry cow’ forage to ensure an adequate reserve of high quality feed is maintained for the early lactation dairy cow diet should be prioritised.
Silage Potassium (K) levels are of major importance for ‘transition’ cows during the 2-3 weeks leading up to calving. High K levels in silage can interfere with Magnesium absorption, which plays a critical role in mobilising Calcium at the start of lactation.
There are a number of factors that influence the nutritive parameters as outlined in Table 1. It is recommended therefore to test animal mineral content of dry cow silage, whether produced on farm or purchased.
Cutting date has an important role to play, particularly as the plant begins to ‘head out’ and the proportion of stem and less digestible material in the sward increases. Grazing swards in late autumn for early cutting, or late Feb/early March grazing will ensure a clean leafy base to the sward. Swards with dead material must be grazed in spring as this can reduce DMD by 6-7 units.
The proportion of ryegrass in the sward will also influence digestibility and it is important to rejuvenate silage swards routinely to maintain perennial ryegrass content. Soil fertility and meeting crop nutrient requirements will also play a role in maintaining sward persistency and achieving a balance between total DM yield and quality.
Ensuring optimal conditions for preservation will minimise losses during fermentation and feeding out by creating a stable environment. Mowing in sunny conditions, preferably in the afternoon and wilting leafy swards will increase the concentration of sugars for conversion to acid. This is particularly important where nitrate levels remain high in the plant. Good pit management through filling, sealing and subsequent feeding out will also reduce losses. Optimal preservation and a stable clamp will promote good intakes of forage which is critical to support milk production in early lactation when silage is buffer fed and to ensure heifers meet over winter growth targets.
Testing silage for nutritional quality and mineral parameters is recommended to ensure diets are correctly balanced for each stage of the production cycles, and to assess silage making strategy on farm.
Conclusion
It is important to account for the demand for high quality silage for your farm and have a reserve in place to deal with adverse conditions. Address the factors on your farm that are reducing silage quality such as poor soil fertility or sward quality and delayed cutting date. Decision making at harvest and management at feed out are also important considerations.
The above was prepared by Dr Aisling Claffey and Dr Joe Patton and first published in the Moorepark 2025 Open Day Book (PDF).