Pasture silage is important as it facilitates the removal of pasture surplus to the herd’s immediate needs, enabling the provision of higher quality pasture in late spring and early summer. It also provides a good quality feed supplement for summer and autumn milk production, and autumn body condition score gain.
Making high-quality pasture silage shouldn’t be difficult, and it should be viewed as an investment in supplementary feed rather than a necessary evil to manage pasture. The objective in making silage is to preserve as many of the original nutrients as possible.
The value of silage as a supplementary feed could be reduced if it isn’t made at the optimal time and with enough attention paid to the silage making process.
When grass is cut and left in a heap, it rots. Silage-making is the process of ‘pickling’ pasture to reduce the pH to a level that stops microbial activity, and stops the feed ‘rotting’.
This is achieved by compacting the pasture and covering it with plastic to exclude air, while micro-organisms ‘burn’ the sugars in the grass to produce lactic and acetic acid. When enough of these acids are produced, no further breakdown of the pasture occurs.
The micro-organisms can be naturally present in the grass or added in the form of inoculants.
A high pH (>4.5) in silage indicates inefficient fermentation. If the silage is exposed to air, eg if the plastic tears, a chain reaction happens that reduces silage quality. Yeasts that can’t grow without air become active and break down the acids in the silage (‘heating’). This causes the pH to rise, allowing the bacteria that were suppressed at low pH to grow again. These bacteria use the energy and protein in the pasture, causing spoilage.
These ‘heated’ silages can also have a high concentration of butyric acid, which reduce palatability and drymatter intake and, if fed in early lactation, increase the risk of ketosis.
Most silage analyses provide indicators of how well the pasture was fermented. The following are key things to take note of:
- Drymatter (DM%): pasture that has a DM% of less than 25% is more difficult to ensile well and will lose nutrients through effluent loss. Pasture with a DM% more than 35% is more difficult to compact, especially if not precision chopped, and generally takes longer for the pH to drop.
- pH: this is an indicator of how well the fermentation process has gone. A high pH (>4.5) generally indicates that air was not excluded properly.
- Ammonia-N (NH3-N): this is an indicator of how much protein has been broken down by bacteria. In well-preserved silage, NH3-N should be less than 10%.
- Lactic acid (% DM or % total acid): is an indicator of how successful the fermentation was, how successful your choice of inoculant was, and how palatable the silage will be. In pasture silage, total acids can be 2-10% DM. Ideally silages will be 5-7% total acid, of which more than 50% will be lactic acid.
- Butyric acid (% DM or % total acid): this is an indicator of secondary fermentation and soil contamination. Either air hasn’t been excluded from the stack or the plastic has been ripped. The pasture ensiled was contaminated with soil (pugged paddock not rolled), providing clostridial bacteria that convert sugars to butyric acid. Butyric acid should be less than 1% DM.
Silage quality is important as it has a direct effect on milk production. DairyNZ data suggests that increasing silage quality by 2.3 megajoules of metabolisable energy (ME)/kg DM increases milksolids production by 13%, 17%, and 41% in spring, summer, and autumn respectively. As is recommended for all feeds, the dollar value of silage as a supplement must be based on its quality, that is, its ME content.
It is impossible to produce high-quality silage from low-quality pasture, no matter how good the fermentation is. Both the quality of the ensiled pasture and the quality of the fermentation must be considered.
To achieve high quality silage, areas identified for silage must be closed early, preferably before balance date, grazing residuals should be 1500kg DM or less, and heavily pugged paddocks should be rolled to avoid soil contamination of the silage
Harvesting, compacting and covering of the stack must be done quickly to reduce spoilage. Inoculants may improve the fermentation process, but will not turn poor-quality grass into good silage.
All inoculants do not work the same. Ask your local merchant or contractor for the research results that show the inoculants improve silage quality and-or animal production.
Kevin Macdonald is a DairyNZ senior scientist.
