Forage specialist Dr. Dave Davies spoke at the first of the 2022, Alltech-hosted ‘Making Silage Sense’ webinar series.
The Wales-based consultant specifically highlighted the factors that impact silage crude protein and metabolisable energy (ME) values.
The maturity of grass at harvest is a critical issue in this regard.
He explained: “The starting point is the period between initial field closure and the date of first cut.
“The target should be cut before the start of the sixth week. The bulk of the ME value is determined by the digestibility of the NDF [neutral detergent fiber] fraction within the grass.
“As lignin content within the grass increases, so do NDF digestibility and ME values.”
Grass silage protein values
Davies also confirmed the link between ME and silage protein values.
“There is a clear linkage between ME and crude protein values as grass goes through its various growth stages,” he continued. But in the real world, other factors come into play. This is apparent in the 420 silages analysed by Alltech this year.
“Basically, we can see that there are a significant number of factors impacting on crude protein levels, apart from the digestibility of the grass and the maturity of the crop.
“It is clearly evident that numerous forages with the same ME values have different protein values.”
In explaining these differences, Davies pointed to the fact that nutrient management programmes implemented while growing grass crops will have a fundamental influence on subsequent silage quality.
“This has been a challenging year in terms of fertiliser prices. This challenge is going to increase as the season progresses. So it is an issue that we will have to keep a key focus on.
“Research work has consistently confirmed the linkage between added fertiliser nitrogen (N) and silage protein values.
“This is a factor that farmers must take full account of as they look forward to 2023. Another key grass nutrient is sulphur. It is critically important in allowing grass to synthesise specific amino acids and proteins,” he added.
The specialist explained that if sulphur is not available in sufficient quantities, growth rates will be reduced. This is another important issue that farmers will have to take account of as they consider their fertiliser options for the year ahead.
Other factors for forage
The consultant explained that other factors that impact on silage energy values and protein contents are the harvesting-related dry matter and organic matter losses.
“These values are impacted by wilting times and the management of the forage from ensiling through to feed out,” he said.
Davies went on to question the long-accepted wisdom of cutting in the afternoon, when grass sugar levels are at their highest values. A 24-hour wilt would then follow.
“Yes, sugar levels are at their highest in the afternoon period,” he added. “But this is not really the issue. Making good silage is all about conserving the nutrients in the grass.
“Research has shown that grass left to wilt overnight will lose a high proportion of its nutrient value. Wilting to get grass down to a 30% dry matter value as quickly as possible is important.
“While leaf stomata are open, grass can lose up to 100L of water/t of grass per hour. However, when the stomata close, this figure drops to 20L.
“Stomata only stay open for a couple of hours after mowing. So we need to spread the crop immediately after cutting, irrespective of the weather conditions,” he continued.
This means that the bulk of wilting takes place in the period directly after mowing.
A study carried out in Wales has confirmed that rapid wilting will minimise ME losses, while a further study in Devon confirms that rapid wilting will act to reduce protein losses.
Nutrient losses in forage
According to the silage specialist, farmers must control the nutrient losses from grass. Crucial to this is the need to wilt as rapidly as possible.
“So cutting at 10:00a.m., and then pushing for as rapid a wilt thereafter, will help to maintain the protein values and other key nutrient parameters in grass that is made into silage,” Davies explained.
Clamp management is also key in maximising the nutritive value of grass silage. Here Davies recommends the use of homo-fermentative-only additives in order to control the breakdown of nutrients.
“Hetero-fermentative bacteria will deliver a more inefficient fermentation, taking longer to get the pH of the silage down to the required value,” he continued.
“When we think about silage, it is the acid in the liquid phase that controls the fermentation. Lower dry matter silages contains more water. So, greater quantities of acid will be required to drop the pH.
“Lactic is almost eight times stronger acid than is the case with acetic acid, which is produced during a hetero-fermentative ensiling process.
“The quicker a pH value of 4.0 is reached within a clamp, the greater the level of true protein and sugars that are maintained within the forage.”
Clamp management
The specialist highlighted the importance of the forage clamp becoming anaerobic as quickly as possible.
“This entails good clamp management, the use of sheeting, the use of cling film, an oxygen barrier, top sheet and sufficient top weight,” he stated.
“If we do all that, we allow a good fermentation to take place. We will maximise the retention of protein and sugars, which will optimise the nutritive value of the silage at feed out.
“The sealing of the clamp is very important. It keeps oxygen out. But, more important than this, it also keeps carbon dioxide [CO2] in.
“Carbon dioxide is the additive that is available for free as it acts to reduce degradative processes within the clamp,” he concluded.
