We all know the expression that you can't make a silk purse out of a sows ear, and that stands true for making silage too. Poor quality grass can't be turned into good quality silage. But we also know that good quality grass can be turned into poor quality silage if things aren't done correctly.
So how do we know if we have a good quality pasture to start with?
The first line in most feed test results that people jump to, and discuss is the ME value (the megajoules of metabolizable energy per kgDM – sometimes also known as the M/D value). This is a measure of all the energy sources in the feed, and generally anything over 12 MJME/kgDM is a very high quality feed, and below about 9.5MJME/kgDM we are talking a low energy feed. For silage the ME value will also be slightly lower than the standing pasture it is made from, as there is energy loss during the wilting and fermentation process. However, it’s important that this silage is made from the best quality grass to start with, as the ME is predicted from digestibility and has a strong correlation with intake potentials.
We know when talking spring ryegrass pastures that the crude protein content is typically high, at over 20%. Typically an early lactation animal will need about 18% of her diet as crude protein, so unless there is a lot of other stuff going into the mix with low protein values, the pasture will deliver to her needs. However, if cows are doing over 2.4kgMS/cow/day they may be getting to a point that they need protein supplementation.
When considering the carbohydrate content of the grass, we look at the structural carbs – the ADF and NDF fibre, and the non-structural carbs – the sugars and starch. The Neutral Detergent Fibre (NDF) is a measure of total fibre, and is the main component that determines the rate of digestion. Acid Detergent Fibre is the fibre in the cell walls, mainly lignin and cellulose, which cannot be broken down in the rumen. As the season progresses we see an increase in the fibre content of the pasture as the grass goes reproductive, so the goal of management is to limit this as much as possible.
Typically pastures have a relatively low level of starch (compared to grains such as wheat or barley), but do have good sugar contents. Remember however, that the sugar content of the grass varies throughout the day, peaking in the late afternoon typically, so the timing of sampling can have an impact on this result!
When we are looking at a silage sample there's a few other results we want to be looking at too:
The Dry Matter % of the silage will depend on a number of factors, including the maturity of the grass that is cut and the wilting process. Ideal DM levels are between 25 and 35% for pit silage, with “wet” silage (25%) having potentially effluent and leaching issues, whereas “dry” silage can have issues with achieving adequate compaction of the pit.
The pH is a measure of how well preserved the silage is. A good fermentation process drops the pH from about 6 (the level in pasture) to around 4 for excellent quality silage. There is an interaction with the dry matter of the silage, with lower DM silage requiring a lower pH for successful ensiling. These wet silages will also have a lower SSS value than drier silage, as more of these carbohydrates are converted to acid in the fermentation process.
Ammonia Nitrogen is a good test of “how well was this silage made” as if conditions mean that spoilage can occur, the plant protein is degraded into ammonia nitrogen and other non-protein compounds. This lowers the nutritional value of the feed, and high levels can lost during the respiration process between cutting and fermentation, and if there is rain during this process this will also decrease these values.
If you'd like to do some pasture or silage tests, want some help understanding your feed test results, or just want to chat about how to manage through this surplus give me a call on 0274645876.