Nutritional Content of grass Flashcards
What is the source-sink dynamic?
The energy the plant gains from the sun is transformed through photosynthesis into carbohydrates.
The plant absorbs energy and it is stored as starch until the plant is ready to use it.
This dynamic can change hourly due to temperature, light intensity, the weather, drought and stage of growth
How is grass used as an energy source?
- it is a source of both non-structural carbohydrates (sugar/starch) and structural carbohydrates (fibre)
- all grasses contain sugar (sucrose can make up 12-15% of dry matter)
- sugar increased when the plant is stressed
- starch is stored in the leaf tissue, especially in high heat situations
- WSC content may be 15-30% of dry matter
How much energy does grass typically contain?
On average it contained 8-10 MJ DE/kgDM. This may increase to 14 MJ in high WSC ryegrasses.
A horse can typically eat about 1 kg of dry matter in an hour, and the fibre that is ingested is fermented in the large intestine, turning into VFA (acetate, propionate, butyrate), which is then used in the production of anaerobic energy.
Where is fibre found in grass?
95% of fibre is found in the cell walls of plants - made up of the primary cell well, the secondary cell wall and the middle lamella.
The middle lamella contains pectins (about 2%). These are degradable.
The primary cell wall contains cellulose (20-30%) and hemiceullose (10-30%). These are degradable.
The secondary cell wall contains lignin (not degradable). Lignin increases as the plant ages, making it less digestible the older it gets
How is protein gained from forage?
Forage contains protein - the amount is dependent on the age of the plant and the soil nutrient content. Protein provides essential amino acids. Protein has to be digested in the small intestine to be useful to the horse
How is the crude protein level in grass affected by the time of year?
The crude protein levels in grass decrease as it ages:
In Spring, when the grass is young - 300g protein/kg DM (30% protein in dry matter)
In Summer - 100-140g protein/kg DM (10-14% protein in dry matter)
In Autumn - 140-200g protein/kg DM (14-20% protein in dry matter)
In Winter, when the plant is old - <100g protein/kg DM (< 10% protein in dry matter
What are the characteristics of fresh grass?
- has high leaf:stem ratio
- has high water soluble carbohydrate content
- may have high CP levels
- may have high water content
- low levels of lignin, cellulose, hemicellulose and pectins
- easily digested and fermented
When is the nutritional content of grass altered?
As it ages the cell wall of the plant thickens and this increases the amount of lignin, lowers levels of WSC and lowers protein levels
What is the vitamin and mineral content in grass?
The total content varies in grass. The mineral uptake is dependent on:
- maturity
- species and stage of growth
- soil type
- pH
- fertiliser used
What is voluntary feed intake?
The amount of food that us consumed by a horse - there is a relationship between live weight and gut capacity. Horses will in theory eat between 1.8-3% of total live weight per day. Ponies can eat up to 5%. The nutrient levels in the bloodstream gained from grass can affect the horses desire to eat more.
What factors specifically affect VFI?
- volume of different parts of the GIT
- rate of passage of digesta (how quickly food moves through the GIT)
- concentration of products of digesta in the blood and in the small and large intestine
- energy demands (affected by amount of work done and natural energy levels)
