Nutritional value of grass guide

The ability to grow grass so successfully throughout most of the year gives Ireland’s ruminant production systems a very strong competitive advantage over both European and international competitors. For many years now, the value of grazed grass has been highlighted. This nutritional value of grass guide will cover the key aspects.

Grass is by far the cheapest feed available and the better you can manage it, the cheaper you can grow it. Grass is also adding value to Irish agriculture by helping portray the clean green image associated with Irish animal-based food production.

To achieve the full benefit of grass, livestock farmers must manage it across the full year to optimise the balance between grass supply and demand. Livestock farmers must also ensure sufficient quantities are conserved as silage to provide adequate feed when animals are housed.

In this guide, we aim to help livestock farmers gain a better understanding of the nutritional composition of grass. We provide simple definitions and highlight the importance of different components for maximum livestock performance. We also provide pointers on how to attain the best performance from your sward.

While effective as a stand-alone crop, grass is also commonly grown with white clover as a companion species. Throughout, we will also highlight the nutritional contribution of white clover where applicable.

Contents

1. Dry matter
2. Grass quality
3. Water-soluble carbohydrate
4. Protein
5. Grass fibre
6. Lipids (oil)
7. Minerals and vitamins in grass

Dry matter

Why is dry matter in grass important?

The dry matter (DM) content of grass and forage is measured as a percentage and represents 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 grass and forage is important. The lower the dry matter content of grass, the higher the fresh weight of forage required to achieve a target nutrient intake, whether this is grazed grass or conserved forage.

DM is also used as a term to measure yield. Recorded as kg DM/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.

Fresh grass requirements at different DM contents

The range of dry matter in grass

In terms of dry matter content, field and weather conditions will cause significant variation, and there are also inherent differences between diploid and tetraploid grass varieties. All other factors being equal, diploids have higher dry matter content (typically 2% higher) than tetraploids, 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 more fresh grass per day to achieve the same nutritional intake as from a purely diploid grass sward.

Looking at dry matter yield, modern ryegrasses have been bred for maximum production. The best-rated perennial ryegrass varieties on the Recommended List are now capable of grazing or conservation yields in excess of 14 t DM/ha while weed grasses (e.g. creeping bent or annual meadow grass) can yield as little as 2 t DM/ha.

Production from ryegrasses over a season follows the classic growth curve, peaking at around 120 kg DM/ha per day in May and typically dropping to approximately one-third of peak levels by early autumn.

Making the most of dry matter

In grazing terms, the aim should be to present a sward 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 18-21 days in peak 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% DM (for clamp silage) and 35-40% (baled). This will ensure good fermentation and optimum intakes, and minimum risk of aerobic instability.

Seasonality of grass production

Increasing grass production at the shoulders of the season (in the spring and again in the autumn) has the potential to improve farm profitability due to the reduced requirement to buy in more expensive feed such as concentrates and reduce the reliance on silage.

Grass growth is dependent on soil temperatures reaching above 6°C, while clover requires temperatures above 8°C. In Ireland, due to the climate, we typically have very low grass growth rates in the spring and again in the autumn. Maximum growth rates are achieved in May.

Although there is little we can do about the weather in spring and autumn, there are measures we can take to improve grass production at the shoulders of the season.

Dry matter pointers

• Use seasonal and total DM yield values when selecting the best varieties from the Irish Recommended List
• Remember animals on wet pasture will have to consume more fresh weight of forage per day
• Delaying cutting for silage increases yield but decreases quality

The clover effect: DM

Overall DM yield from a grass and clover sward with optimum white clover content is broadly compatible with a straight ryegrass sward, assuming average Irish nitrogen fertiliser applications.

Grass quality

What is digestibility (D-value)?

The quality of grazed grass is described by its digestibility value (D-value). Digestibility is the proportion of the forage that can be potentially digested by a ruminant. The digestible part of the forage comprises a combination of crude protein (CP), carbohydrates (including digestible fibres and sugars) and lipids (oils).

The digestibility of grazed grass

Digestibility of grass is highest when a sward comprises fresh leafy growth and declines as the plants become more mature (stemmy). During May to June when grass is starting to turn reproductive (seed heads emerge) the digestibility can reduce to as low as 67%. The top-ranking varieties on the Recommended List will be higher at 72-75%.

Swards managed within an 18-25-day rotational grazing period will have a higher D-value, typically in the region of 74-77%, with the top-ranking varieties correspondingly at 77-80%. Weed grasses will be substantially lower in D-value than modern ryegrasses.

What is metabolisable energy (ME)?

Metabolisable energy (ME) is the energy an animal can derive from feed. ME is measured in megajoules of energy per kilogram of forage dry matter (MJ/kg DM). It is directly correlated with digestibility because any feed must be digestible for the energy to be available.

One percentage point of D-value equates to 0.16 MJ/kg DM of ME.

Making the most of digestibility and metabolisable energy

The higher the D-value that can be achieved, the better the ruminant performance will be in terms of milk production or growth rates. In Ireland, Teagasc estimates a one-unit increase in digestibility value equates to 0.25 litres of milk per dairy cow per day.

Grass species nutritional content

Source: J. Frame, 1991

Farm feed nutritional content

D-value and ME pointers

• Select the highest-ranking varieties on the Irish Recommended List
• Use a rotational (18-25-day) paddock grazing system with entry determined by the ‘Three-Leaf System’
• Consider soil nutrient availability and when applying fertiliser always apply in accordance with best practice
• Cut for silage prior to stem thickening, or approximately one week before heading
• When necessary, top grazing swards to prevent heading

The clover effect: D-value & ME

White and red clovers typically have D-values comparable to the highest-ranking ryegrass varieties, with a greater proportion of the digestible material being in the form of crude protein (e.g. less carbohydrate).

The optimum white clover content in a grazing sward is an average of 30% over a grazing season.

If including red clover in a silage ley, it is important to use compatible ryegrass varieties to achieve the best overall D-value at cutting. As with ryegrass, red clover’s D-value declines rapidly with increased crop maturity with the target being to cut when no more than half the plants are in bud.

Water soluble carbohydrate

Why are water soluble carbohydrates (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.

The range of WSC in grass

Higher WSC is a major differentiating factor in modern ryegrasses bred at IBERS Aberystwyth University over the last 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, while 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.

Making the most of WSC

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.

Average WSC over five years

Effect of DM at ensiling on WSC content of silage as a result of fermentation

WSC pointers

• Select and sow grass and silage mixtures that have high digestibility and high WSC values
• Avoid over-use of fertiliser by following best practice guidelines
• 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 amount of effluent (wet silage also has an increased need for sugars to create a good fermentation and stable silage)
• WSC generally peaks three to five weeks after grazing or cutting

The clover effect: WSC

White clover is generally lower in WSC and higher in protein than perennial ryegrass. So, it’s 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 perennial ryegrass, so growing it in combination with high WSC grass varieties is beneficial for silage fermentation.

Protein

Why is protein in grass important?

Protein is a large and expensive component of livestock rations, and reliance on imported sources (e.g. soya) leaves businesses vulnerable to price volatility and supply. Greater use of homegrown protein is therefore desirable.

Grass protein 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 nonprotein 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.

Protein range in grass

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 fermentation. A better fermentation results in more of the crude protein remaining as true protein.

Making the most of protein in grass

Animal research has shown that typically only about 20% of protein consumed by ruminants is used to maintain the animal and produce meat or milk. The rest is lost in waste products and excreted from the body.

A better balance of protein and energy supply to the rumen will improve the proportion of protein that is used. Feeding forage (as grazed grass or silage) with a higher sugar (WSC) content has been shown to improve protein utilisation in ruminants.

Given the optimum balance of protein and energy sources, dietary crude protein concentrations can routinely be as low as 12-14% DM without any detriment to livestock productivity (14% for milk production).

Grass protein pointers

• Apply fertiliser in line with best practice guidelines as soon as possible after grazing or cutting
• Avoid making overly wet silage (below 28% DM) as this may result in soluble protein losses in the effluent
• Optimum protein concentrations occur 3-5 days after cutting or grazing

The clover effect: Grass protein

White clover is generally higher in protein than ryegrass. It is important to maintain an optimum balance in grazing swards of an average 30% of dry matter over the season.
Red clover is a high protein forage (typically 22% crude protein). It contains an enzyme (PPO) that in silage helps to maintain the proportion of true protein.

Grass fibre

Why grass fibre is important

Fibre is essential in the diet of ruminants to provide the ‘scratch factor’ essential to stimulate rumen function. There is an important balance to be achieved in all diets for optimum performance.

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 hindgut.

The range of fibre in grass

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).

Making the most of fibre in grass

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 around20-30% 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.

Grass fibre and dry matter intake

Grass fibre pointers

• 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: Fibre

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 (oil)

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.

Making the most of lipids in grass

Lipids have approximately twice the energy content of carbohydrates (WSC and fibre) and are an important source of energy for livestock. Ruminant diets are frequently supplemented with high-lipid feeds as a means of increasing the energy content of the diet.

The range of lipids in grass

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.

Current and future grass breeding programmes at Germinal Horizon have identified lipid concentration and fatty acid profile as important objectives.

Lipid pointers

• Fresh grass provides a better PUFA profile than many dry feeds
• Forage-based systems have the potential to produce better-quality human food due to the favourable PUFA profile in grass
• When silage-making, rapid wilting will increase the level of lipids retained in the forage

The clover effect: Lipids

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.

Minerals and vitamins in grass

Why are minerals and vitamins important in grass?

Minerals include various elements like calcium, selenium and iron. These basic elements, like the more complex vitamins, have important roles in the health and performance of livestock. Understanding the mineral and vitamin content of grass is important in the context of any additional supplementation that may or may not be required.

Making the most of mineral and vitamins in grass

Accurately managing a sward for mineral and vitamin content will require soil analysis for each paddock. Where any mineral is found to be deficient, provision of supplemental licks or mineral boluses can overcome most deficiencies.

The range of minerals and vitamins in grass

The mineral content of a sward will depend largely upon the mineral availability in the soil and the pH. Mineral and vitamin content will not usually change in silage, though in very wet crops some losses may occur in effluent.

While many vitamins are synthesised by rumen microbes, some lipid-soluble vitamins must be obtained from feed (vitamins A, D and E) and all vitamins provided by feeds can be a useful addition to the ruminant’s diet.

When turning stock into lush pastures of rapidly growing grass, particularly in the spring, it is advisable to monitor them to further reduce the risk of tetany. Supplement rations with minerals in line with silage analysis.

Grass nutrient availability chart

If you have any queries about the nutrional value of grass, contact one of our agricultural grass seed experts.

Originally published on Germinal.co.uk as the 'Nutritional value of grass'.