The Nutritional Value of Grass

Capturing the full power of grass

Understanding the full value of grass is key to maximum utilisation.

As grazing or silage, grass is the foundation for most forage-based systems. It is the cheapest source of ruminant feed and the most important factor in modern dairy, beef and sheep farming profitability. Livestock farmers however need to take full advantage of grass by maintaining sward quality and productivity. They need to manage intakes through effective grazing and winter-feeding plans. It is also important to have a good understanding of the nutritional qualities of grass. If you don’t know the full feed value of grass, there is a chance it will either be under-utilised or inaccurately supplemented – its potential will then be lost.

In this guide, we provide simple definitions and highlight the importance of different components and offer pointers on how to attain the best results. Grass is also commonly grown with white and red clover, so we provide references where applicable to the contribution of these legumes.

Dry Matter

Why is dry matter important in grass?

The dry matter (DM) content of forage (measured as a percentage) is the proportion of total components (fibres, proteins, ash, water soluble carbohydrates, lipids, etc) remaining after water has been removed. Knowing the dry matter percentage of forage is important. The lower the dry matter content, the higher the freshweight of forage required to achieve a target nutrient intake, whether this is grazed grass or conserved forage. Dry matter is also used as a term to measure yield. Recorded as kgDM/ha, this is used as a measure of sward carrying capacity (stocking rate) and is an essential element of effective grazing management. It is also used to measure silage crop yields.

In terms of dry matter content, field and weather conditions will cause significant variation, and there are also inherent differences between diploid and tetraploid varieties. All other factors being equal, diploids have higher dry matter content (typically 18-26% DM) than tetraploids (15 – 20% DM), due to diploids having smaller cells and a lower cell wall to cell contents ratio. This means ruminants fed entirely on a tetraploid sward will need to consume as much as one-third more fresh grass per day to achieve the same nutritional intake as from a purely diploid sward.

Looking at dry matter yield, modern ryegrasses have been bred for maximum production.

In grazing terms, the aim should be to present grazing that offers the ideal balance of fresh nutritious growth with the appropriate fibre content for optimal rumen passage. This balance is best achieved by using a grazing rotation of 18-25 days in peak growth season. Poor sward management will increase the proportion of dead and dying plant material, resulting in a significant decline in forage quality and intake potential. When making silage, the aim should be to cut at 16-20% dry matter and ensile at 30 – 35% dry matter (for clamp silage) and 35 – 40% (baled). This will ensure a good fermentation and optimum intakes, and minimum risk of aerobic instability.

 

D-Value and ME

Why are the D-value and ME of grass important?

D-value is the measure of digestibility, or the proportion of the forage that can be digested by a ruminant. This digestible part of the forage is made up of crude protein, carbohydrates (including digestible fibres and sugars) and lipids (oils). ME is the amount of energy that an animal can derive from the feed. It is measured in megajoules of energy per kilogram of forage dry matter (MJ/ kg DM). ME is directly correlated with D-value because any feed has to be digestible in order for the energy to be available. 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 wallsD-value is highest in grass when the sward has fresh leafy growth and declines as the plants become more mature (stemmy).

The decline in D-value is highest after ear emergence (heading). Grass cut for silage will typically 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 that 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 put the increase in milk yield higher at 0.33 and 0.4 litres per cow per day respectively.

 

Water Soluble Carbohydrate

Why is water soluble carbohydrate (WSC) important in grass?

Water soluble carbohydrates are the soluble sugars that are quickly released from grass within the rumen. These sugars provide a readily available source of energy for the rumen microbes that are responsible for digesting forage. These sugars also provide the fuel for silage fermentation. The higher the sugar, the better the silage is preserved and the higher the feed value for the animal. Higher WSC is a major differentiating factor in modern ryegrasses bred at IBERS Aberystwyth University over 30 years. Varieties higher in WSC than conventional varieties are now available as Aber High Sugar Grass.

Relative differences in WSC are maintained between ryegrass varieties even though the content typically rises and falls over a season, with varying weather conditions and even over the period of a day. On a warm sunny summer day, WSC content can be as high as 35% of dry matter, whilst on a cool cloudy autumn day it can be as low as 10%, but at either end of the spectrum differences between varieties are maintained.

A high WSC will generally 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. This means that more of the feed is converted into milk and meat, with less going to waste in urine (and methane). Under ideal growing conditions, modern 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, leaving less for the animal.

Pointers on WSC

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

Why is protein important in grass?

Protein in grass is generally reported as total crude protein (CP), which is 6.25 times the nitrogen content. Typically around 80% of the crude protein in fresh grass is true protein. The remaining fraction is often referred to as non-protein nitrogen. Both types of nitrogen can be used by the 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 is excreted by the animal.

Crude protein can be split into effective rumen degradable protein (ERDP) and digestible undegradable protein (DUP). ERDP, which is by far the biggest part of fresh forage protein, can be broken down by rumen microbes and converted into microbial protein that is digested later. DUP passes through the rumen intact and can be broken down and digested in the small intestine.

Crude protein content can vary within single varieties and between varieties and is influenced by management factors such as nitrogen fertiliser applications and crop maturity.

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

In silage, the proportion of crude protein that is available as true protein is affected by the fermentation. A better fermentation results in more of the crude protein remaining as true protein.

Fibre is measured as NDF (neutral detergent fibre), this being the insoluble fibre fraction (cellulose, hemicellulose, pectin and lignin) that remains after boiling in a neutral detergent solution. Carbohydrates within NDF are not as readily accessible as those in the WSC component of ryegrasses. However, NDF content is important for predicting ruminant voluntary intake.

The proportion of NDF that can be digested by ruminants is referred to as dNDF. This is a secondary source of slowly released carbohydrates that provides a useful source of fermentable energy for ruminants within the rumen and hind gut. Grass fibre concentration can vary greatly during the growing season. It is at its highest (and the grass least digestible) when the sward is producing reproductive seed heads rather than vegetative leaves. Conversely, during the early spring when fresh growth is at its peak, fibre content is typically at its lowest (grass is most digestible).

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

When grass fibre content falls below these optimum levels (e.g. early spring flush) supplementary feeding of fibre may be necessary to prevent grass passing through the rumen too rapidly. When making silage, it is important to cut before grass becomes too mature (pre-heading) to avoid a significant reduction in digestibility.

 

Pointers on NDF and dNDF

  • Rotational grazing (18 – 25 day) using the Three Leaf System to determine when to graze will optimise both NDF and dNDF levels in grass
  • Topping will remove stemmy growth and stimulate fresh growth (and avoid NDF being too high) but good grazing management should ideally avoid the need for topping
  • Take silage cuts before grass goes to head to avoid NDF rising too high
  • Manage grazing and cutting swards to avoid diseases (e.g. crown rust, leaf spot) that will increase NDF at the expense of overall quality

 

The clover effect

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

 

Lipids

Why are lipids important in grass?

Lipids in forage grasses contain a high proportion of polyunsaturated fatty acids (PUFA). These are the ‘good’ fatty acids, better known as Omega 3 and Omega 9, which have positive human health effects. From an animal production perspective, increased PUFA supply has been shown to improve animal fertility and result in positive effects on meat quality (longer shelf life and a more desirable colour). There is also evidence of reduced methane emissions from ruminants consuming high PUFA diets, an effect that is positive for the environment. Early data suggests total fatty acid content of grass varies from about 2.5 to 5% of forage dry matter, with the PUFA component making up 65 – 78% of the total lipid content. Grass fed livestock will naturally consume more polyunsaturated fatty acids (Omega 3 and Omega 9) which is believed to improve the colour of aged meat and potentially extend shelf life.

Lipids have approximately twice the energy content of carbohydrates (WSC and fibre) and are an important source of energy for livestock. Current and future grass breeding programmes at IBERS Aberystwyth University have identified lipid concentration and fatty acid profile as important objectives.

The clover effect

  • White clover lipid content is generally reported to be slightly lower than that of ryegrass, with a range of 2 – 4.4% of forage dry matter
  • Red clover is generally reported to be higher in polyunsaturated fats than ryegrass

 

 

 

 

 

 

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