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Planning your Nutrition
Published 26 January 10
A detailed knowledge of rumen physiology and ruminant metabolism is not essential for improving herd performance through better feeding. However, a broad understanding of rumen fermentation, nutrient requirements, feed intake and utilisation, energy balance, milk production and fertility needs, metabolic disorders and feed resources is important for anyone wishing to make the most of the many opportunities available for nutritional improvement in the vast majority of herds.
This chapter will cover:
- Ruminant Physiology
- Nutrient Requirements
- Feed Intake & Utilisation
- Nutrition & Milk Production
- Nutrition & Fertility
- Metabolic Disorders
- Feeding & Breeding
- Feed Resources
Ruminants are distinguished from other animals by having a four-compartment stomach, comprising rumen, reticulum, omasum and abomasums.
The Rumen
Located on the left side of the body, the rumen makes up over 65% of an adult cow's total stomach volume. It is, in effect, a huge fermentation vat containing a soup of around 130 litres of chewed-up feed with large amounts of saliva and micro-organisms - primarily bacteria and protozoa. Floating on top of this soup is a fibrous mat of coarser solid material which acts as a filter. Feed particles are regurgitated and re-chewed until they are small enough to fall through the fibre mat into the rumen liquor below.
The rumen liquor commonly contains between 109 and 1011 bacterial per ml, together with 105 -106 protozoa. These break down degradable feed materials to produce Volatile Fatty Acids (VFAs), ammonia and a variety of long chain fatty acids.
Ammonia is used as a nitrogen source for microbial growth and VFAs absorbed from the rumen are a key energy source for the cow. Increasing the rumen-available energy content of the diet in the form of sugar and starch stimulates papillae growth, improving VFA absorption. While rumen fermentation allows good use to be made of fibrous feeds that could not otherwise be digested, it does mean only around 70-85% of the energy in the feed is available to the animal - 6-15% commonly being lost as gases (mainly methane) and 6-7% as heat.
The Reticulum
Small in comparison to it, the reticulum is a continuation of the rumen with a honeycomb structure. Microbial fermentation continues as the feed moves through the reticulum and into the omasum - a globe-shaped structure containing page-like folds of tissue from which water and some nutrients are absorbed.
The Omasum
Moving through the omasum, the mixture of feed and rumen micro-organisms becomes progressively drier. Excessive intake of minerals or low quality fibre (such as sunflower hulls) can cause compaction of the omasum.
The Abomasum
Finally the abomasum or 'true stomach' secretes hydrochloric acid and digestive enzymes to begin breaking down feeds that have escaped microbial digestion together with microbes
excreted from the rumen. From the stomach the digesta moves into the small intestine where most of the digestive enzymes are secreted to break down both feed and microbial nutrients into simpler nutritional building blocks. These are absorbed across the intestinal lining and into the bloodstream through small finger-like projections (villi) which increase its surface area
Bacterial fermentation of some undigested feed occurs in the final section of the digestive tract - the large intestine - which also absorbs both VFAs and water.
Rumen Dynamics
The contents of the rumen are continually mixed by the rhythmic contraction of its walls, a healthy rumen contracting around twice a minute. As well as bringing feed and bacteria into close contact with each other, the contractions move smaller, denser feed particles out of the rumen while bringing larger, lighter particles up to the fibre mat at the top surface for rumination.
Cows commonly spend 8-10 hours/day ruminating, the extent of rumination depending on the roughage content of the diet. The cycle of rumination involves four distinct elements:
- Regurgitation - coarse material (the scratch effect) at the upper end of the rumen stimulates a bolus of feed (cud) to be returned to the mouth.
- Chewing - each cud of regurgitated food is chewed many times to grind it down into particles small enough to pass out of the rumen.
- Salivation - chewing stimulates the secretion of buffer-containing saliva (as much as 75 litres/day) which is mixed with the cud to stabilise rumen pH.
- Swallowing - once the coarse material has been ground down sufficiently by chewing it is swallowed and sinks to the bottom of the rumen to pass into the reticulum.
When cows are resting (not eating or being milked) over 60% should be ruminating. If the diet contains adequate long fibre cows should chew at least 30 times (ideally 60) before re-swallowing.
VFA Production
Volatile Fatty Acids (VFAs) produced from microbial fermentation of feed carbohydrates in the rumen are the primary source of energy for ruminants. Three distinct volatile fatty acids are produced by rumen fermentation: acetate, propionate and butyrate.
Under optimal conditions the acetate:propionate ratio should be greater than 2.2 to 1. High levels of acetate indicate a high fibre/low starch ration, producing a generally slower, more stable fermentation. High levels of propionate indicate a high starch/low fibre ration producing a faster rate of fermentation which can lead to reduced rumen pH, depressed fibre digestion and even acidosis.
Rumen Efficiency
With fibre-digesting bacteria thriving best at pH 6.0-6.8 and starch-digesting bacteria at 5.5-6.0, the best balance of fibre and starch digestion occurs at a rumen pH of around 6.0.
Factors affecting rumen pH and fermentation efficiency include:
- Forage to concentrate ratio. High forage diets stimulate higher rates of saliva production, better rumen buffering and greater acetate production which supports butterfat levels. Excessive amounts of concentrates, on the other hand, increase propionate production, decrease rumen pH, reduce feed intake and microbial production and depress butterfats.
- Concentrate type. Starch-based concentrates may depress rumen pH to a greater extent than those based on digestible fibre.
- Physical form of feeds. Grinding, pelleting, chopping or over-mixing can reduce the particle size of feeds to a level at which the rumen mat cannot be maintained, depressing rumination, saliva production and pH. Finely ground concentrates increase microbial fermentation (favouring lactic acid-producing bacteria, in particular), reducing rumen pH and increasing the risk of acidosis.
- Higher feed intakes mean more material available for bacterial fermentation and higher levels of VFA production. The amount of saliva produced per unit of feed dry matter may also decline, leading rumen pH to drop.
- Wet feeds can reduce rumen pH because less saliva is needed to lubricate the feed for swallowing. Rumen pH can also be adversely affected with very dry diets because of low intake levels.
- Degradable fat levels. Fatty acids in vegetable and fish oils can coat fibre particles, reducing their digestibility. They can also be toxic to fibre-digesting bacteria. Grinding or extruding oilseeds tends to make these effects worse by rupturing cell walls and releasing oil into the rumen, while feeding whole oilseeds can reduce this risk.
- Feeding method. Total Mixed Rations (TMR) can be beneficial in stabilising rumen pH, providing a balanced supply of nutrients for the most efficient bacterial fermentation, increasing dry matter intake and minimising feed selection. When high levels of concentrates are included, rumen pH may still be below optimum and fibre digestion compromised. If concentrates are fed separately, it is important to limit the amount to 3 kg/meal and avoid high starch levels and finely processed grains to prevent large fluctuations in rumen pH.
It is vital to appreciate the dynamic state of the rumen environment and the extent to which changing feeds or feeding systems can alter rumen conditions - for better or worse.
Cows require four main groups of nutrients to live, grow, produce and reproduce - water, energy, protein, and minerals and vitamins.
Water Requirements
Comprising 50-80% of a cow's body, depending on age, and essential for all cellular functions as well as milk production, the transport of nutrients and excretion of waste products, water is the single most important dairy nutrient. It is also vital to the regulation of body temperature.
Cows require at least 60 litres of water/head/day and may need 100 litres or more depending upon yield. Cows also have a good sense of smell and taste it is important to ensure water supplies are of sufficient quality; poorly-sited wells or bore holes can become contaminated by slurry stores, septic tank outflows, carcase burial pits and even landfills. Salmonella and other coliform bacteria can survive for long periods in leach into otherwise clean water supplies from some distance.
Water Quality Testing
All non-mains water should be tested annually for pH, total dissolved solids, total coliform bacteria, faecal coliform bacteria, total plate count and key minerals using clean, sterile sample containers from a testing laboratory. Samples for bacteriological testing must be refrigerated, insulated, and delivered to the laboratory within six hours. Other samples can be delivered or mailed using a standard overnight service.
Energy Requirements
50-80% of the energy cows require to power all their bodily functions comes from Volatile Fatty Acids (VFAs) produced by the fermentation of feed carbohydrates in the rumen, with the remainder derived from carbohydrates, fats and proteins that escape rumen degradation.
Ruminant energy requirements and feed energy supplies are generally expressed in terms of Metabolisable Energy (ME) - the energy available to the cow after accounting for losses in digestion, gases and urine.
Fermentable Metabolisable Energy (FME) is the proportion of the ME potentially available in the rumen.
Bacteria cannot use either fermentation acids or fats/oils as an energy source, so the right balance of dietary sugar, starch and fibre is essential for efficient rumen function.
Imbalances of the main energy sources can cause the following problems:
- Sugar and starch: Too high risk of acidosis; fat cows. Too low risk of low milk protein; thin cows.
- Fibre: Too high intakes drop. Too low risk of acidosis; displaced abomasums.
- Fat: Too high poor fibre digestion and low intakes. Too low risk of low milk yield and fat.
Protein Requirements
Essential to every aspect of body maintenance, reproduction and milk production, so called Metabolisable Protein (MP) is supplied to the cow as a combination of microbial protein from the rumen and dietary protein that passes through it undegraded.
Ruminant protein requirements and feed protein supplies are generally expressed in terms of Crude Protein (CP) which includes non-protein nitrogen as well as true protein.
Rumen Degradable Protein (RDP) describes the protein supply available to the rumen microbes, while Digestible Undegraded Protein (DUP) is the protein available from the feed which escapes rumen degradation.
Although not considered to be a reliable guide for fertility purposes, Milk Urea Nitrogen (MUN) concentrations can provide useful indications of the efficiency with which protein is being utilised in the rumen.
Mineral & Vitamin Requirements
Minerals are inorganic compounds needed for a whole host of regulatory and structural functions in the cow. They are provided in different quantities are supplied in a range of feed supplements
Macro Minerals
Required in relatively large amounts - grams/cow/day - and expressed as a percent of ration dry matter, include calcium, phosphorus, magnesium, potassium, sodium and sulphur. Sodium, potassium and sulphur salts are ionic and affect the acid-base balance in cows, critical to the maintenance of many bodily functions.
When dry cows are fed rations high in potassium producing positively-charged ions, for instance, the availability and absorption of magnesium can be reduced, leading to milk fever type symptoms. There is good evidence that feeding anionic (negatively-charged) chlorine or sulphur salts using a Dietary Cation-Anion Balance (DCAB) approach helps prevent milk fever in these circumstances. If dry cow rations contain potassium at over 2% in the forage DM, however, it is often better to change the forage rather than adding anionic salts, since their poor palatability can depress appetite.
Key Trace Elements
Are only required in relatively small amounts and measured in milligrams/day include cobalt, copper, iodine, iron, manganese, selenium and zinc. They can be supplemented in either inorganic (eg, zinc oxide) or organic (eg, zinc methionine) forms.
Inorganic minerals are most commonly used because they are less expensive and more concentrated than organic minerals. The many interactions between minerals and the fact that some can be toxic at relatively low levels makes providing them in excess of requirements as harmful as failing to correct deficiencies.
Vitamins
Organic compounds needed in small amounts for a variety of chemical reactions in the body. Fresh forages are good sources of fat soluble vitamins but dried, stored and ensiled forages have little vitamin content remaining so diets based upon them must generally be supplemented.
Digested feed clearly has a major effect on nutrient supply, the most important factor governing the extent to which cows can meet their energy, protein and other nutrient needs is the amount of feed they consume. This is especially important in the early stages of lactation when the energy demand for production is higher than intake can support, creating an increasingly negative energy balance which cows have to meet from body reserves, milking off their backs.
Feed Intake
Feed consumption is generally expressed in terms of Dry Matter Intake (DMI) - the weight of feed material consumed excluding the moisture it contains. A large number of different animal, food and management factors affect DM intake.
Key animal factors affecting DMI include:
- Cow size - Big cows eat more than small cows.
- Cow breed - Some breeds of cow eat more for their size than others.
- Cow age - Heifers eat less than mature cows.
- Milk yield - Higher yielding cows eat more than lower yielders.
- Cow condition - Fat cows eat less than thin cows.
- Stage of lactation - Early lactation cows eat less than those in mid and late-lactation.
Key food factors affecting DMI include:
- Fibre content - Cows eat less when fibre levels are too high.
- Protein content - Cows eat more when protein levels rise.
- Processing - Cows eat more when the feed particle size is smaller.
- Moisture content - Cows eat most at a ration DM of 45-55%.
- Diet composition - Cows eat less when rations are poorly balanced.
- Digestibility - Cows eat more when rations are more digestible.
Key management factors affecting DMI include:
- Feed access - Cows eat more when feed is available ad-lib.
- Water access - Cows eat more when palatable water is readily available.
- Feeding frequency - Cows eat more when fresh feeds are provided more frequently.
- Ration palatability - Cows eat more the tastier they find the feed.
- Feed spoilage - Cows eat less when feeds are spoiled by decay or contamination.
- Ration changes - Cows eat less when rations are changed abruptly.
Substitution Rates Prioritising Energy Balance
The clear limit to the amount of Non-Digestive Fibre (NDF) cows can consume in a day means intakes of low fibre feeds like wheat (12% NDF) are potentially four times those of higher fibre feeds like good quality silage (48% NDF). As well as having important implications for overall intakes, this means cows will eat less forage when supplements are available, the extent of this substitution depending on the type of supplement.
Concentrate feeds generally displace relatively small amounts of forage from the diet, so supplementation will generally allow daily DM intakes and performance to be increased. A kilogram of wheat (12% NDF) will, for instance, displace only 0.25 kg of 48% NDF silage from the daily intake (12% ÷ 48% = 0.25 substitution).
In contrast, higher fibre feeds have higher substitution rates - a kilogram of sugar beet pulp (32% NDF) displacing 0.67 kg (32% ÷ 48%) of the same silage.
When buffer feeding grazed grass, higher fibre feeds can lead to substitution rates of greater than 1.0, reducing daily intakes and compromising performance.
Energy Balance
Nutrients absorbed from the gut are continually being partitioned within the cow to maintain its body functions and support the production of milk and body reserves. Over and above the nutrients required for maintenance, milk production receives the clear priority in early lactation, with shortages of nutrients from the diet made-up by the mobilisation of body reserves. Thereafter, there is a progressive re-ordering of priorities, with milk production declining and a greater proportion of nutrients being directed to rebuilding body reserves - primarily fat.
The challenge of feeding high yielding cows is underlined by the fact that in a single lactation they are likely to produce more dry matter in the form of milk than their body contains. This challenge is particularly intense in early lactation as milk yields build rapidly to a peak about 4-8 weeks after calving whereas maximum voluntary feed intake is only reached after around 10-12 weeks. Ensuring high DM intakes as soon as possible after calving is a key priority if high levels of both production and fertility are to be achieved. The inevitable rise in energy demand for milk production ahead of energy intake in the first few weeks of lactation has not been found to cause problems as long as intake catches-up with production by around the sixth or eighth week of lactation, at which time the negative energy balance ceases and cows stop losing weight.
The fact that early lactation intakes have not risen in line with milk yields in recent years has meant increasingly deeper and longer periods of negative energy balance - in excess of 20 weeks in studies with very high yielding herds.
At peak milk yields of up to 40 litres/day most cows have relatively little difficulty consuming sufficient feed to support their production. Once daily DM intakes of 24 kg or more are required by yields much in excess of 40 litres/day, however, increasing problems arise, even with particularly high energy density diets and relatively high daily weight losses. Considerable research into nutrition and fertility performance has pinpointed more pronounced and protracted early lactation energy deficits as a major factor in the lower fertility experienced by many high yielding cows. Research and experience indicates a daily DM intake of 3.5% of body weight should be achieved by five weeks after calving for optimum performance in high yielding herds.
Cow Condition
Body condition scoring is widely accepted as a practical way of assessing body fat reserves, providing a good measure of a cow's energy balance to inform feeding and management.
A semi-subjective assessment to an 11-point scale of half units, condition scoring is best carried out by the same person on each occasion to eliminate operator differences. As the change in condition score is more important than the absolute value, scoring should be undertaken regularly. For example: as calving, prior to first service, in mid-lactation and at drying-off.
Research and experience has established the following ideal Body Condition Score targets for key stages in the lactation cycle:
- At calving: 2.75-3.0
- Prior to first service: 2.25-2.5
- In mid-lactation: 2.5-2.75
As a guide cows should maintain condition during the dry period, lose no more than 0.25 Condition Score to 4 weeks post calving; and, lose no more than 0.25 Condition Score from 4 to 8 weeks post calving.
Most of the major constituents of milk - lactose, fat and protein - are synthesised in the mammary gland from precursors selectively absorbed from the blood and transported either from the digestive system or from body reserves. Since the amount of water secreted by the mammary gland is directly related to lactose levels, lactose synthesis is the principal driver of milk volume. The primary building blocks of milk fat are the VFAs, acetate and butyrate, with glucose supplying the glycerol required. Milk protein - primarily casein - is produced from amino acids.
Influencing Milk Components
The levels and ratios of individual VFAs produced by the digestive system can have a marked influence on milk fat and protein percentages.
There are a number of ways of manipulating milk solids levels through feeding, although the cost-effectiveness of ration adjustments always needs to be assessed against the specific milk contract.
Milk protein percentages can best be increased by:
- Increasing the energy density of the ration.
- Feeding high ME silages with good intake potential.
- Increasing the protein content of the ration.
- Feeding mixed forages.
- Increasing the degradable starch content of the ration with ingredients like rolled wheat.
- Increasing the by-pass starch content of the ration with ingredients like crimped maize.
- Increasing the by-pass protein of the ration with ingredients like protected soybean meal.
- Using both protected starch and protein.
- Feeding protected methionine.
- Avoiding added fat (even protected fat).
- Calving cows in optimum condition.
Milk fat percentages can best be increased by:
- Increasing the forage to concentrate ratio.
- Feeding high fibre forages.
- Sufficient long fibre.
- Feeding high digestible fibre concentrates.
- Feeding concentrates little and often to stabilise rumen pH.
- Avoiding high oil by-products like distillers and brewers grains.
- Avoiding whole oil seeds like full fat soya and whole rape seed.
- Avoiding fish oil products and/or Feeding small amounts of a protected fat.
Protected Fats
Fats are protected from rumen degradation either by conversion into a rumen insoluble soap or naturally by virtue of a high melting point which makes them relatively inert in the rumen. The form of protection must, of course, ensure they are available for breakdown and absorption lower down the digestive tract.
The fatty acids making up protected fats can be a relatively pure source of 16 carbon chain molecules - palmitic acid (known as C16s) - or a mixture of C14, C16 and C18 molecules (usually referred to generically as protected fats). C16 fatty acids can be directly converted into milk fat to boost butterfat percentages.
Nutrition in general and energy nutrition in particular has a major effect on fertility. Cows in too much of a negative energy balance in early lactation tend to be difficult to get back in calf. This results both from a delay in the resumption of normal oestrus cycling and a lower conception rate. Cows that are too fat at calving encounter particular problems since their early lactation appetites tend to be poor, resulting in excessive body fat mobilisation which can result in ketosis or fatty liver syndrome. For optimum fertility, cows should calve down at Body Condition Score 2.75-3.0 and lose no more than half a Condition Score by service.
Feeding for Fertility
Increasing starch and reducing fat supply, for example, has been shown to increase bulling activity and insulin levels. Because higher insulin levels have a detrimental effect on embryo quality, however, cows subsequently need diets higher in saturated fat to stimulate progesterone production and lower in starch to minimise insulin production. This implies that feeding for fertility in this way is likely to require more complex grouping of stock than may be practicable for many.
Excess protein is not, itself, a major cause of poor fertility, however, almost always exacerbates energy deficits as additional energy is required to get rid of it. There is some evidence that high blood ammonia reduces early embryonic growth which could lead to pregnancies being lost within the first 10 days.
If fertility problems persist despite cows being in the correct body condition and rations being correctly balanced for energy and protein, it is advisable to check the mineral and vitamin status of both animals and rations. In view of the many interactions between different minerals it is vital to analyse the mineral status of forages and seek specialist advice before undertaking additional supplementation.
A number of disorders linked to incorrect diet or feeding can have adverse effects on dairy cow health and welfare as well as productivity.
For example:
- Acidosis
- Displaced abomasums (DA)
- Hypocalcaemia (milk fever)
- Hypomagnesaemia (grass staggers)
- Ketosis (acetonaemia)
- Fatty liver
- Laminitis
- Retained foetal membranes (retained cleansings).
All the common metabolic disorders have a knock-on effect on fertility and it can take many months for the reproductive system to recover from metabolic disorders. Metabolic disorders can invariably be prevented by ensuring the best possible dietary balance and particularly careful management of cows at drying off, during the dry period and in early lactation.
There are clear differences between the breeds in their ability to produce milk and milk components which need to be accounted for in their feeding. Equally, differences in bodyweights mean differences in daily dry matter intake capacities. Some breeds are considered to be better at looking after themselves and replacing condition more easily than others, making them better suited to systems involving out-wintering or extended grazing.
Lighter animals may be valuable for causing less poaching and deeper-bodied cows with larger rumen capacities better adapted to high forage grazing systems. Specific selection pressures (historically in New Zealand, for example, and now in certain UK studs) are likely to produce bloodlines with better inherent grazing abilities than those selected under predominantly housed production regimes.
Compared to their average generitc mertic contemportaries, the cattle used in investigations at Langhill, have higher intake capacities and need not necessarily produce lower fat or protein percentages. They are more efficient at converting feed energy into milk energy and generate substantially higher feeding margins. Overall, the animals bred for combined weight of fat and protein produced similar yields from 1 tonne of concentrates as their unselected contemporaries did from 2.4 tonnes, resulting in substantially higher margins.
Crossbreeding studies in New Zealand and North America have further highlighted the potential for improving overall dairy productivity - particularly in terms of fertility, health and survivability - by harnessing the power of hybrid vigour.
While different types of stock may be better suited to different production systems, regardless of breed the key to cost-effective feeding is meeting the animals' particular performance requirements within their specific intake capacities.
A wide range of forages, concentrates, moist feeds and supplements are available to meet dairy cow energy, protein, mineral and vitamin requirements. These need to be selected and utilised on the basis of specific analyses and careful rationing to ensure the best balanced diets.
Further information about nutrition can be found in chapter two of the DairyCo pd+ folder