ASB 221 - Lesson 11

Dry Matter Intake in Ruminants

Objectives

• Define dry matter intake.
• Describe feed, environmental, and animal factors that influence dry matter intake in livestock.

Dry Matter Intake (DMI)

• Dry matter (DM) intake of grazing animals is the amount of feed without water consumed by an animal.
• Ruminants generally consume 2-3.0% of their body weight as DM.
• The intake of concentrates is likely to be controlled by chemostatic mechanisms involving the concentration of Volatile Fatty Acids (VFAs) absorbed from the rumen.
• Acetic and Propionic acid infused into the rumen depress DM intake.

Factors Influencing Dry Matter Intake

• Receptors of Acetic and Propionic acid exist on the luminal side of the reticulo-rumen.
• Control of roughage intake appears to depend on the digestive system and feed characteristics.
• Feed intake is influenced by characteristics of the feed, the animal, and environmental factors.

Feed Characteristics

• Retention of feed in the rumen or the rate of passage affects DMI.
• Digestibility of the feed is positively related to the DM intake.
  • Feed intake is closely related to the rate of digestion.
  • The primary nutrient which affects the rate of digestion is the Neutral Detergent Fiber (NDF).
  • NDF is a measure of the cell wall content of the plant.
  • Feeds rich in NDF have a low rate of digestion.
• Addition of concentrates to roughage of low digestibility improves the intake of the roughage, but where the roughage has high digestibility, it's replaced by the concentrate.
• Feeds with similar digestibility but differing in NDF have different DM intakes e.g., legumes and grasses.
• In legumes, lignification occurs in the vascular bundle, whereas in grasses, it's more widespread on the entire plant, leading to an inhibitory effect on intake.

Variation of DM Intake with Digestibility in Cattle

• Blaxter et al., 1961 showed the relationship between digestibility and DM intake.

Other Feed Characteristics

• Physical form of the feed.
• Processing (mechanical or chemical).
• Part of the plant (e.g., leaves vs. stems).
  • Cell walls in the leaves are more easily broken down than those in the stems.
• Nutrient deficiencies that retard rumen microbial activity may reduce feed intake (e.g., protein or N).
• Other nutrients with a similar effect are Sulfur (S), Phosphorus (P), Sodium (Na), and Cobalt (Co).
• Badly fermented silages and feeds with an off-smell are poorly consumed.
• Moisture content.
• Palatability.

Dry Matter Intake of Goats Fed on Guinea Grass

• Devendra & McLeroy, 1987 showed the DMI (% of BW) of goats fed on Guinea grass at different days of growth:
  • 16-19 days: 2.8%
  • 21-28 days: 2.8%
  • 28-35 days: 2.4%
  • 35-42 days: 2.4%
  • 42-49 days: 2.2%

Animal Factors Influencing DMI

• Physiological state.
• Fat deposits on the animals body.
  • Excessively fat animals have a stable DM intake compared to their metabolic body weight.
  • This may be attributed to abdominal fat which restricts rumen expansion or increased secretion of the enzyme leptin.
  • Emaciated animals, on the other hand, have higher DM intakes per metabolic body weight.
• During pregnancy, the increased need for nutrients for foetal development causes DM intake to increase.
  • In late pregnancy, as the foetus increases in size, the volume of the abdominal cavity is reduced, which reduces the DM intake especially on roughage diets.
• Lactation: the onset of lactation is accompanied by an increase in DM intake.
  • Intakes of Gross Energy (GE) by lactating cows may be up to 50% higher than in non-lactating cows.

Dry Matter Intakes of Cows in Mid and Late Lactation

• The table shows how DMI (kg DM/day) varies with body weight (kg) and milk yield (kg).
  • For example, a cow with 450 kg body weight and producing 15 kg of milk has a DMI of 12.8 kg DM/day.

Other Animal Factors

• Capacity of the rumen.
• Breed.
• Grazing behaviour/Selection.
• Ill health reduces feed intake.
• Parasitic infestations (internal and external) also reduce DM intake.

Environmental Factors Influencing DMI

• DM intake is influenced by the bite size, bite rate, and the grazing time.
• Normal duration of grazing in cattle is 8-10 hours.
• 0.6g DM is the normal bite size with a bite rate of 1 bite/second.
• Suitable distribution of herbage enables the animal to achieve optimum bite size and bite rate.
• Short and dense swards allow maximum bite size and rate for cattle.
• Tall prickly plants restrict bite size.
• Low plant density (<1500 kg DM/ha for small ruminants).
• Contamination with animal excreta.
• Environmental temperature.
  • Feed intake decreases at temperatures above the thermoneutral zone and it increases when temperatures fall below that zone.
  • High temperatures tend to adversely affect high producing animals in particular.
  • e.g., in temperate breeds, DM intake falls by 2% for every unit increase in mean daily temperature above 25°C.

Heat Production

• The rate of heat loss from animals is influenced by environmental temperature.
• The graph shows the relationship between heat production, sensible heat loss, evaporative heat loss, and environmental temperature.
• Key points on the graph include:
  • Lower Critical Temperature (LCT)
  • Upper Critical Temperature (UCT)
  • Thermoneutral Zone (TNZ)

Other Environmental Factors

• Day length affects DM intake in some species and not others (e.g., Deer > Sheep > Cattle).

Factors That Determine Feed Intake in Ruminants

• Feed Factors
  • Particle size of feed or the degree of processing
  • Digestibility and the moisture content of the feed
  • Nutrient imbalance
  • Palatability of feed
• Animal Factors
  • Capacity of the rumen
  • Age of the animal
  • Physiological state
  • Ill health and parasitic infestation
  • Psychological state
• Environmental Factors
  • Day length
  • Environmental temperature
  • Humidity

Further Reading

• https://www.daf.qld.gov.au/business-priorities/agriculture/animals/dairy/nutrition-lactating-cows/affecting-feed-intake