Knowing exactly what’s in the clamp and balancing rations with care will allow producers to maximise milk production from this year’s variable grass silage. We spoke to a leading dairy nutritionist to find out more.
TEXT PHIL EADES
Grass silage is a notoriously variable feed, but 2021’s crop is particularly varied, meaning silages will require regular analysis during the winter to allow precise balancing. So says Liz Homer from Trouw Nutrition GB, which runs the UK’s largest forage-analysis laboratory.
“It is inevitable grass silage will vary throughout the clamp due to the considerable number of factors interacting during its production, whether in a single cut or across multiple cuts in a season.
“Variation begins in the field. Most units will be harvesting a variety of swards, and their composition influences heading dates and timing of harvest. The result is variations in feed value, depending on the proportion of different grass mixes,” Dr Homer explains. “Weather conditions during the main growing period will also affect maturity. And weather at harvest will potentially delay cutting date and influence the rate of drying down, as well as the number of times the crop is moved in the field.”
“We know the weather at harvest, and even the time of day, affects the rate at which grass dries out, increasing the prospect of variations in dry matter of ensiled grass. And if some grass is left down longer than other cuts, this will affect energy content.” Dr Homer adds that the weather this year has been the dominant factor, affecting growth rates, nitrogen uptake and impacting cutting dates. “The consequence has been a greater range in first-cut quality, which will significantly influence feed values and, most importantly, how grass silage will perform in the rumen.”
Well fermented
Table 1: Early-first-cut, later-first-cut and overall first-cut average silage results for 2021
Table 1, which shows the analyses of more than 2,000 first-cut silages, highlights the difference between early first cuts, harvested typically at the end of April, and later cuts taken throughout May. Dr Homer says crops are typically well fermented with good pH and lactic acid content, combined with low sugars. But she warns care will be needed when feeding drier silages to reduce the risks of moulds and heating.
“There are some significant differences in how silages will feed and perform in the rumen. Overall protein content is low with fewer high-protein crops, which will have implications for diet formulation.
“Later-cut crops have lower crude protein content compared to earlier cuts and totally fermentable protein levels are lower too. This means later-cut crops will almost certainly need careful protein balancing with both rumen fermentable and bypass proteins,” she adds.
The results show NDF levels are averaging 43.3% DM overall, with slightly higher lignin levels than usual, which impacts rumen function and intakes. Unsurprisingly, later-cut crops are typically higher NDF with higher lignin content compared to earlier cuts, reflecting the harvest of more mature crops. “Acid load is higher than we would like to see with variable fibre index values, which are typically on the low side. This will have consequences for energy sources when balancing rations.
Balancing forages
“Later cuts also have lower ME, but there is a significant variation across all cuts. The combination of increased total fermentable carbohydrates and lower crude protein in later cuts means there is more fermentable carbohydrate than fermentable protein, making feeding more fermentable protein a priority.” Dr Homer stresses the differences between cuts will need to be considered when balancing forages, particularly given volatile feed commodity prices. Supplementation must deliver the best return.
“If faced with a lower protein silage, it’s important to understand if fermentable or bypass protein is required. If additional fermentable carbohydrates are required then it will be important to select sources with the potential effect on acid load and rumen fermentation in mind, if intakes and rumen health are to be maintained.
“The key to precise supplementation will be regularly analysing your own clamp or clamps, to ensure rations are based on the silage actually being fed.” Even a clamp containing a single cut will be made up of a different proportions of various swards, likely cut across least two days. And this will affect feed value through the clamp.
Silage making: variation begins in the field
“The variation is clearly greater in clamps with multiple cuts, yet the average sampling interval identified at our laboratory is close to two months, which a recent US study has shown is far too long.” The work, at Ohio State University, set out to define the optimal analysis frequency for forages, which will itself depend on several factors including herd size and clamp capacity.
“From the data, we concluded herds of 500 cows should be analysing clamps around every 11 days. For 250-cow herds the optimum is every 17 days, while for 120-cow herds it is every 26 days.
“Set against the potential benefits of fine-tuning the diet to maintain or improve performance and reduce feed costs, the cost and time of taking samples more regularly is immaterial,” adds Dr Homer. “A sample sent for traditional NIRS analysis will be back on farm within 24 hours and offers all the information required to fine-tune rations and make the most of grass silage.”
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