Last spring, five farmers from the North of England and South West Scotland volunteered to participate in a study of how the nutritive value of grass intended for cutting as silage changed during the spring. Here are the results of the study designed and managed by Trouw Nutrition and Carrs Billington. Article as featured in British Dairying in March.
Samples of grass were taken from the same field each week in the six weeks before cutting and a full nutrient analysis obtained. The final analyses were performed the day before cutting and compared with the first cut analyses when the clamps were opened up. The nutrients analysed for were dry matter, protein, sugar and NDF (fibre).
Graph 1 shows how the national average pre-cut samples analysed by Trouw Nutrition changed over time during the summer of 2017.
DM weather dependent
Dry matter is heavily dependent on the weather—although there is good evidence that using a fertiliser containing sodium can improve forage dry matter content where the soil is deficient, which many are. The fall in dry matter does coincide with the end of a prolonged dry spring and the onset of wetter weather in the North of England and Scotland. Protein content remained fairly steady, while sugar levels slowly declined as spring turned into summer.
It is the NDF content which showed the most significant changes, rapidly increasing from 35% to 42% in the two weeks between mid-May and early June. This change in fibre level would equate to a 1.2 MJ/kg dry matter loss in the metabolisable energy content of silage produced.
Every year is different. Graph 2 shows that the average increase in NDF content occurred two weeks later in 2017 than it did in 2016. The two main points to take from this are:
1: Analyse grass on a regular basis one month before you usually cut—not the day before cutting.
2: Be prepared. If the weather is right and the grass analysis is good be ready to go—do not wait as many did this year particularly waiting far too long to cut 2nd cut. The grass grew much faster after 1st cut than most realised.
Big range in cutting dates
Table 1 shows the pre-cut and final silage analyses for each of the five farms and an average of them all. The first thing of interest is the wide range in cutting dates between the first farm and last farm to cut—four weeks in total. The earliest cutting date was on April 23rd and the last on May 18th, yet all farms produced very high quality silage.
What was common however is that all five farms cut their grass for 1st cut relatively young and close to 38 % NDF—it was not just luck that all the silages made from this young low fibre grass had an ME greater than 11.3 MJ/kg DM. Another interesting factor to point out is the loss of nutrients that occurs between cutting grass and opening it later as silage. This is a normal part of the fermentation process but not always appreciated
On average, for these five farms, there was a 0.5MJ/kg drop in ME and 3.5% loss of crude protein. Clamp management was excellent
on all five farms and had there been excessive application of nitrogen too close to cutting, poor clamp management or cutting corners with respect to sheeting these losses would have been much higher.
Interestingly the two silages with a dry matter content below 30% lost significantly more protein between field and clamp, nearly 6% of protein being lost possibly as effluent both in the field and clamp.
Dynamic Energy averaged 6.5 MJ/kg in the first cuts—which is excellent. This is part of the new Nutri-Opt dairy rationing system, developed by Trouw and adopted by Carrs Billington this winter. It measures the amount of energy in a feed which the cow can use for milk production—a good target figure is 6.0 and above. Many forages, particularly second cuts in the North, had quite good energy levels when measured on an ME basis but failed to generate the expected milk yields.
It was common this year for these to have a Dynamic Energy below 6.0 and so this parameter is proving to be an increasingly useful predictor of cow performance. What is the ideal target NDF value of grass for it to be cut for silage? If one is aiming to achieve 70+ D value silage then aim to cut between 38% to 40% NDF. Cutting higher than 40% NDF will result in lower energy silage but cutting at much less than 38% NDF, especially if wilting has proved to be difficult, could increase the risk of silage slippage and a butyric fermentation if too much fertiliser nitrogen was still in the plant at cutting.
Finally, some comfort for those in a saturated North, where winter temperatures have been close to normal with periods of low and freezing weather.
Trouw Nutrition have started looking to see if there is a relationship between winter temperature and average silage quality the following year—particularly first cut after a mild or cold winter—and there does seem to be. Silage digestibility seems to be better after a cold rather than a mild winter perhaps because the grass has already started growing and should be cut sooner at an earlier date after a mild winter rather than at the same date every year. Analysis shows that pre-cut grass analysis has been available as a service to our customers for years but often we are asked to take one sample very close to the intended cutting date to assess if it is safe to cut from a silage fermentation perspective. For this we look at the dry matter, sugar, protein and Nitrate N content to make the call as to whether to delay cutting or not and to decide on the most appropriate silage additive. We are however overlooking a simple but important guide as to when to cut from an overall digestibility perspective—that being the NDF content of the grass. We are now suggesting grass samples are taken one month ahead of the intended cutting date and then two weeks later to help decide if this is a year, on your farm, to cut sooner or later than originally planned.