Grass is made up of approximately 83% water and 17% dry matter. This dry matter is made of cell walls (fibre portion) and the content of these cells is made up of sugar, protein, fats, minerals and other compounds.
The majority of energy in grass is provided by the fibre and sugar, with a smaller portion provided by oil and protein. The proportion of leaf to stem influences the overall energy content of grass. The higher the leaf portion the more energy the grass will contain. This is because grass leaf contains more sugar and digestible fibre than stem. As grass becomes stemmy the digestibility of fibre decreases, lowering the energy content. If we provide grass with over 80% leaf to grazing dairy cows we maximise animal performance.
If we maintain a high proportion of perennial ryegrasses in swards and manage them by avoiding grazing heavy covers and grazing tightly (4cm), we can maintain leafy swards throughout the grazing season. Quality grass is sufficient to meet a cow’s energy requirement for most of the year, with shortfalls usually occurring in early spring and late autumn.
Fibre in grass – a healthy rumen is fundamental for health and good performance. Ruminants can digest fibre from grass and other forages and it is this fibre that maintains a stable, healthy rumen. When a cow’s fibre requirements is not met, the rumen pH becomes unstable and animal performance declines. You need to get the balance right; too much fibre reduces dry matter intake, and thus energy intake. The knock on consequences is reduced body condition gain and production losses. Dairy cows need a minimum of 30% fibre (NDF) to maintain a healthy rumen, while beef cattle can reach high levels of performance on much lower levels of fibre in the diet.
We describe the fibre levels in grass using a term called NDF% (neutral detergent fibre). Usually leafy grazed grass contains around 35% NDF while stemmy grass could contain upwards of 50%. Generally, there is adequate fibre contained in grazed grass. Even though rumen pH may be slightly lower than the optimum in grazed grass diets, there is no evidence to suggest supplementing a grazed grass diet with extra roughage (fibre) will improve animal performance.
Protein quantity in grass is typically between 16 – 28%, and the factors that influence this are sward type, the growth stage, time of year, plant stress (drought) and how the grass is fertilized. We should always look at both the quantity and quality of protein supply to the animal as not all protein is equal in its nutritional value.
Protein is necessary for appetite, milk production, reproduction and growth. Young, growing cattle and lactating cows need most protein. Young stock need 13-15% Crude Protein (CP) in the diet, lactating cows 14-17%, depending on yield and finishing cattle need 11-12% CP. In many cases the protein supply from grass is over and above the animals requirements. This has a drawback in that excess dietary protein must be excreted in the urine, and can reduce the energy available for production. Therefore, supplementary feeding at grass needs to be carefully considered to avoid over feeding protein.
Grazed grass protein quality is not an issue for young stock, suckler cows or finishing cattle. Freshly calved cows in early spring need some quality protein from supplementary feed for the first 6 weeks of lactation. Autumn grass has adequate protein for late lactation spring calving cows, but freshly calved autumn calving cows need some supplemented quality protein to meet their dietary requirements.