Grass silage is the single most important source of winter feed on Irish farms with almost 25m tonnes ensiled annually, writes Matt O'Keeffe
Grazed grass will continue to be the cheapest fodder available on Irish livestock farms costing €50/t, dry matter (DM) utilised. Surplus grass is regularly grown in this country. Some of it is wasted, but most ends up in pits or bales to be used as winter forage. The yield of grass harvested has a huge impact on the cost of feeding silage. Light-yielding crops make expensive silage, whereas heavy-yielding crops spread the costs over a greater tonnage. On the other hand, light-yielding crops of grass silage tend to be of higher quality and can result in higher weight gain and greater milk output from the livestock consuming it. Most farmers try to hit a happy medium ensuring they have adequate quantities for the expected winter period, while also aiming to have a quality adequate to their requirements. This last point is critical: the needs of a dry suckler cow differ greatly to those of a milking dairy cow.
Cost And Quality
First-cut grass silage can be produced at reasonable costs, estimated at approximately €130/t, DM utilised. Grass silage costs vary considerably depending on yields. Second- and third- cut silages are more expensive forms of fodder with costs per tonne of DM running upwards of €145/t, where machinery has to be hired.
Moreover, the variability in yield and quality of second and third-cut silage has forced many farmers to consider alternatives such as maize, whole crop wheat and fodder beet. The cost per tonne of DM utilised for maize, for instance, is €110 and whole crop wheat is €118. Fodder beet is thought to cost €137/t DM utilised. Trial work on kale has recorded high utilisable yields reducing its cost per tonne of DM to €117. The convenience of growing, storing and feeding, as well as animal performance, are important considerations when deciding which fodder crop to grow.
Protecting The End Product
Grass silage will continue to be the predominant winter forage in Ireland. While getting the quality right at harvesting is critical, the end product may or may not reflect the quality of the grass that went into the pit. Back in the 1970s and 1980s, huge sums were spent on silage additives such as acids, molasses and biological agents. Then the pendulum swung so that very few additives of any kind were applied into the first decade of the 21st century. Since then, there has been a growing interest in cost-effective additives that can be scientifically proven to either improve, or at least maintain, quality in the pit.
Some silage additives will improve preservation, enhance forage quality and minimise storage losses. Legitimate claims that individual silage additives can improve milk yields and live weight gain. One of the more spectacular effects of using a silage additive is reported in a 2012 study of 100 herds and 25,000 cows. The results of this study showed that farmers who treated their silage with Genus ABS Powerstart achieved, on average, a 10-day reduction in calving to conception interval – worth £50 per cow. Dr Dave Davies, a former Senior Scientist at the Institute of Grassland and Environmental Research reviewed the results, which seem to go far beyond merely stabilising or enhancing silage quality.
Raising The Reproductive Response
Now working with Silage Solutions, Dr Davies explains exactly why a silage inoculant can improve how quickly cows get back in calf: "When the trial, involving over 100 herds and 25,000 cows, showed the link between silage inoculant and fertility, the immediate question was why? How can silage production have such an effect on reproduction? The answer lies in the way the silage is fermented and how the forage is utilised in the rumen." Dr Davies explains that Powerstart contains just one strain of bacteria, a unique strain of the homofermentative bacteria, Lactobacillus plantarum, which was selected after extensive independent evaluation of lactic acid bacteria carried out at Aberystwyth by IGER (Institute of Grassland and Environmental Research) in the mid1990s.
"Unlike most strains of Lactobacillus, L plantarum Aber F1 is a fructan degrading strain, which means it is able to make better use of all the sugars available in grass, which results in a more rapid fermentation with two significant consequences for the nutritional value of the silage. The first is that, by making use of all the sugars in grass, less of the available sugar is used during the fermentation. This means that there is more sugar in the resultant silage and, therefore, more available to the cow. The second is that, by accelerating the rate of fermentation, the protein in the grass is preserved more effectively with a higher proportion of true protein and fewer free amino acids, something that will not show up on a silage analysis."
Dr Davies says the impact of faster fermentation and better nutrient retention is very important when silage is digested in the rumen. According to the silage expert, when Lactobacillus plantarum-treated silage is fed, the high proportion of true protein combined with the higher sugar levels means the rumen digests the forage more efficiently. In a typical silage, with more free amino acids and ammonia combined with less available sugar, there can be an imbalance in the rumen fermentation with too much nitrogen released and a shortage of rumen-available energy. He continues: "The consequence is a large proportion of protein is lost as rumen ammonia, which is excreted as urea into the blood and milk. In silage fermented with L plantarum, the improved balance of sugars and true protein results in a more effective rumen fermentation, less surplus ammonia and, consequently, lower urea levels. It is the lower urea levels that are the key to why fertility improves with treated silage."If the rumen is producing a lot of ammonia, it has to be removed, which uses up energy. Converting ammonia into urea wastes energy, which reduces the energy available for milk production. If less energy is available from the diet, the consequence in early lactation is that cows will be in extended negative energy balance and will lose more condition over a longer period of time, which is well understood to have a negative effect on fertility."
High Urea Hits Fertility
Dr Davies claims that high blood urea reduces fertility in three ways. "Firstly, cows with high urea levels have longer intervals between heat periods, which means there are fewer opportunities to breed them. Secondly, high urea and ammonia levels have been shown to hamper the development of the eggs in the ovary, leading to poorer quality eggs, which reduces the chance of fertilisation. Finally, high blood urea levels alter the environment in the uterus and reduce progesterone levels, which mean cows are less able to maintain a pregnancy.
"By closely providing the rumen with what it needs, in the correct balance to allow an effective fermentation, silage fermented using Lactobacillus plantarum has a direct benefit in reducing the reproductive problems associated with high blood urea. It also drastically reduces the energy required to deal with waste urea and, so, leaves more energy for the cow, for production and to reduce negative energy balance."
It is not certain that these benefits extend into the following grazing season, though there may be a residual effect. Most Irish dairy cows are bred at grass in the spring when high urea levels remain a challenge to early, survivable pregnancy.