Grassland guide: Grass nutritional value

Capturing the full power of grass

Understanding the full potential of grass is fundamental to ensuring maximum utilisation.

Grass can be used as grazing or silage as the foundation for most forage-based feeding systems. Quite simply, it is the cheapest source of ruminant feed and, arguably, the most important factor in modern farming profitability. However, Kiwi livestock farmers need to take full advantage of grass by maintaining swards' quality and productivity.

Farmers can manage intakes through grazing and winter-feeding plans. Additionally, it is important to understand the nutritional qualities of grass. Basically, if you do not know the full feed value of grass, then there is a chance it wcan be under-utilised or inaccurately supplemented.

Within this grassland guide, Germinal will outline the key definitions and highlight the importance of different components, while offering advice on how to achieve the best results. Among farmers, grass is also often grown with white clover and red clover, which is why we will also discuss these legumes.

Dry matter

The dry matter (DM) of forage is the proportion of total components, including fibres, proteins, water-soluble carbohydrates, lipids and ash, which remain after water has been removed. Essentially, the lower the dry matter content, the higher the freshweight of the forage that will be needed to achieve a target nutrient intake, whether as grazed grass or conserved forage.

Dry matter can also be used as a term to measure yield. Recorded as kgDM/ha, yield is used as a measure of sward carrying capacity (stocking rate) and is a vital element of effective grazing management. Also, it can be used to measure silage crop yields.

Regarding dry matter content, field and weather conditions cause significant variation, and there are inherent differences between tetraploid and diploid varieties. All other factors equal, diploids have higher DM content (typically 18-26%) than tetraploids (15-20%), as diploids have smaller cells and a lower cell-wall to cell contents ratio. When ruminants are fed entirely on a tetraploid sward, they will need to consume as much as one-third more fresh grass per day to reach the same nutritional intake as a 100% diploid sward.

When analysing dry matter yield, modern ryegrasses are bred for maximum production.

In grazing terms, the goal is to present forage that offers the ideal balance of fresh nutritious growth and the appropriate fibre content for efficient rumen passage. Ultimately, this balance can be achieved by using a grazing rotation of 18-25 days during peak growth season.

Poor sward management, however, will increase the proportion of dying plant material, which results in a significant decline in forage quality and intake potential. While making silage, the aim is to cut at 16-20% DM and ensile at 30-35% (for clamp silage) and 35-40% (baled). This will enable good fermentation and optimum intakes while minimising the risk of aerobic instability.

D-value and ME of grass

The D-value is the measure of digestibility or the level of the forage that can be digested by a ruminant. This digestible part of forage is made up of crude protein, carbohydrates and lipids.

ME is the amount of energy an animal can draw from the feed. It is measured in megajoules of energy per kilogram of forage dry matter (MJ/ kg DM). The ME is directly correlated with D-value as any feed has to be digestible in order for the energy to be available.

Essentially, one percentage point of D-value equates to 0.16 MJ/kg DM of ME. A proportion of ME is available as an energy source for rumen microbes. This is referred to as fermentable ME (FME) and is largely comprised of plant cell walls. D-value will be highest in grass when the sward has fresh leafy growth, though will decline as the plants become more mature.

The decline in D-value will be highest after ear emergence (heading). Typically, grass that's cut for silage will lose 2 percentage points in D-value between cutting and feeding. The higher the D-value and ME in forage, the better ruminant performance will be.

In the UK, NIAB estimates a single point increase in D-value (or 0.16 MJ/kg ME) equates to 0.26 litres of milk per dairy cow per day, 40g/day extra beef liveweight gain and 20g/day of extra lamb liveweight gain. Similar work at Teagasc in Ireland and DARDNI estimates the increase in milk yield is higher at 0.33 and 0.4 litres per cow per day, respectively.

Water-soluble carbohydrate

WSCs are the soluble sugars that are quickly released from grass in the rumen. WSCs provide a readily available source of energy for the rumen microbes that are responsible for digesting forage. Also, these sugars can provide the fuel for silage fermentation.

The higher the sugar, the better the silage will be preserved and the higher the feed value will be for the animal. A higher WSC is the differentiating factor in modern ryegrasses bred at IBERS Aberystwyth University over 30 years. Grass seed varieties that are higher in WSC than conventional varieties are now available in New Zealand as Aber High Sugar Grass.

Relative differences in WSC are maintained between ryegrass varieties even though the content can rise and fall over a season due to varying weather conditions.

On a warm summer day, for example, WSC content can be as high as 35% DM. Meanwhile, on a cool cloudy autumn day, it can be as low as 10% DM. At either end of the spectrum, differences between varieties are maintained.

Generally, a high WSC will mean forage composition is closer to the 2:1 WSC-to-crude protein ratio that animal models suggest is the target for optimum nitrogen use efficiency in the rumen.

Ultimately, this means more of the feed is converted into milk and meat, with less going to waste in urine (and methane). Under ideal growing conditions, Germinal's Aber HSG ryegrasses will achieve the optimum ratio of 2:1 for WSC-to-protein. Wetter silage ultimately uses up more sugar to achieve a stable fermentation and will leave less for the animal.

Pointers on WSC

  • Pick and sow 100% Aber HSG and silage mixtures to maximise WSC
  • Cut silage late in the afternoon to maximise WSC content
  • Avoid making overly wet silage (below 28% DM) as this can result in sugar losses in the effluent and increases the effluent; wet silage also has a greater need for sugars to create a good fermentation and stable silage
  • WSC peaks 3-5 weeks after grazing or cutting
  • Manage grass swards to avoid diseases that will reduce WSC (e.g. crown rust or leaf spot)

The clover effect

  • White clover is generally lower in WSC and higher in protein, so it important to maintain the target of 30% white clover sward content over a grazing season for optimum performance
  • Red clover is generally lower in WSC than ryegrass, so growing it in combination with Aber HSG varieties is beneficial for the silage fermentation process
  • Aim for a minimum dry matter of 30% when ensiling red clover and ryegrass to increase the concentration of WSC in the forage

Protein

Protein in grass is generally reported as total crude protein (CP), which is 6.25 times the nitrogen content. Approximately 80% of the CP in fresh grass is true protein. The remaining fraction is often referred to as non-protein nitrogen.

Each type of nitrogen can be used by an animal but the true protein is used more efficiently for meat and milk production. A larger part of the non-protein nitrogen is used inefficiently and then excreted.

CP can be split into effective rumen degradable protein (ERDP) and digestible undegradable protein (DUP). ERDP is the biggest part of fresh forage protein and can be broken down by rumen microbes and converted into microbial protein for digestion later. DUP will pass through the rumen intact and will later be broken down and digested in the small intestine.

CP content varies in single varieties and between varieties and is influenced by management factors like crop maturity and nitrogen fertiliser.

The CP proportion available as true protein is lowest in the period after nitrogen fertiliser is applied, though rises as grass grows and converts non-protein nitrogen into true protein.

The proportion of crude protein in silage that is available as true protein is impacted the fermentation. A better fermentation results in more of the CP remaining as true protein.

Fibre is measured as NDF (neutral detergent fibre). This is the insoluble fibre fraction (cellulose, hemicellulose, pectin and lignin) that will remain after boiling in a neutral detergent solution. Carbohydrates within NDF aren't as readily accessible as those in the WSC component of ryegrasses. However, NDF is important for predicting ruminant voluntary intake.

The proportion of NDF that can potentially be digested by ruminants is referred to as dNDF, which is a secondary source of slowly released carbohydrate that offers a useful source of fermentable energy for ruminants in the hindgut and rumen.

Grass fibre concentration varies widly during the growing season. It's at its highest when the grass sward is producing reproductive seed heads as opposed to vegetative leaves. Conversely, in early spring, when fresh growth is peaking, fibre content is often at its lowest.

The principle target with fibre is to maximise animal voluntary intake while ensuring sufficient rumen digestion time. For grazing, the optimum NDF content of grass should be in the 30-40% range total DM, with dNDF 20-30% of DM, or approximately 60-75% of the total fibre content in a digestible form.

If grass fibre content falls below these optimum levels, the supplementary feeding of fibre might be necessary to prevent grass passing through the rumen too quickly. When making silage, it's important to cut before the grass becomes too mature (pre-heading) to avoid a significant drop in digestibility.

Pointers on NDF and dNDF

  • Rotational grazing (18-25 days) with the Three Leaf System to determine when to graze will optimise NDF and dNDF levels
  • Topping removes stemmy growth and stimulate fresh growth, and avoid NDF being too high, but good grazing management should avoid the need for topping
  • Take silage cuts before the grass goes to head to avoid NDF rising too high
  • Manage grazing and cutting of grass swards to avoid diseases that will increase NDF at the expense of overall quality

The clover effect

  • The fibre concentration of red clover and white clover is lower than that of ryegrass and has the potential to increase voluntary intake
  • The physical form of fibre in clover typically breaks down in the rumen more quickly than fibre in ryegrass

Lipids

Lipids found in forage grasses contain a high proportion of polyunsaturated fatty acids (PUFA). And these are the good fatty acids, also known as Omega 3 and Omega 9, which have positive human health effects. From an animal production standpoint, increased PUFA supply has been shown to improve animal fertility and result in positive effects on meat quality with a longer shelf life and more desirable colour.

Furthermore, there is evidence of reduced methane emissions from ruminants consuming high PUFA diets, which is an effect that's positive for the environment. Early data suggests the total fatty acid content of grass varies from about 2.5-5% of forage DM, with the PUFA component making up 65-78% of the total lipid content.

Lipids can have approximately twice the energy content of carbohydrates and are an important source of energy for livestock. Germinal's breeding programmes at IBERS Aberystwyth University have identified lipid concentration and fatty acid profile as important objectives for the future.

The clover effect

  • White clover lipid content is usually reported to be slightly lower than that of ryegrass (a range of 2-4.4% of forage DM)
  • Red clover is typically higher in polyunsaturated fats than ryegrass

Make sure to check out our Knowledge Hub if you want to read more on the effectiveness of sowing forage seeds.

 

 

 

 

 

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