Balance feed quantity and quality

 However, since grazing ruminant animals can survive on a wide range of pasture and forage feeds, the objective becomes optimising the ‘efficiency of conversion’ of feed into animal products (such as milk, meat and wool) by management of the quantity/quality trade-off.

That’s why chemical analysis of pasture and forage feeds to determine ‘feed quality’ is increasingly important in the New Zealand farming scene. Getting a feed test gives you the information you need to make sound decisions and increase productivity. 

Plus, analytical testing can be used to predict how well a particular feed will meet animal requirements. 

Keep in mind the type of livestock and the production objectives will influence the quantity and quality of feed required in the complete diet to achieve top production.  And remember the critical time to optimise dairy cow diet for best economic response is the three months between calving and mating.

This article outlines the types of tests available to help analyse pasture and forage feeds.

Feed Quality

For optimum productivity, the following properties of the feed are important to track and should be tested:

Dry matter intake

Crude protein content

Carbohydrate composition

Digestibility

Energy yield from the digested feed

Mineral and trace element content

Dry Matter

Another measure to test for is the residual dry weight of pasture, forage or silage after removal of moisture.  This is usually expressed as ‘percentage of the fresh weight.’

The dry matter intake of a cow, for example, depends on many variables including: live weight, stage of lactation, level of milk production, environmental conditions, body condition and the quality of the feed.

Crude Protein

The protein content of the pasture or forage is a good test to include in your testing regime.  It is directly related to the nitrogen content, which varies with growing conditions, plant species, and maturity of the plant. 

Once you know your pasture’s protein content, you can monitor animals’ intake, keeping in mind crude protein requirements are dependent on the class of livestock being fed. For example, a maintenance requirement for a dairy cow may be as low as 12% protein, whereas a range of 16–20% protein is needed for growth and lactation.

Plant carbohydrates

Testing plant carbohydrate levels is another useful test. Plant carbohydrates may be conveniently classified as structural carbohydrates and non-structural carbohydrates. Levels of structural carbohydrates increase with increasing plant maturity with a corresponding decrease in plant digestibility.

The key non-structural carbohydrates in forages are the soluble sugars such as sucrose, glucose and fructose. Soluble sugars are important for stimulating microbial activity in ruminant animals.  Plant soluble sugars fluctuate diurnally with highest levels generally found in the early to mid-afternoon period – typical levels for temperate grasses can range from 5 – 15 %. 

Acid detergent fibre (ADF) and neutral detergent fibre (NDF)

Knowing your pasture’s levels of ADF and NDF can provide estimates of the less digestible structural carbohydrates in your forage. ADF consists mainly of cellulose and lignin with small amounts of nitrogen and minerals. The NDF fraction includes the hemicelluloses in addition to the ADF component of plant tissue. Very high fibre levels slow the rate of digestion and limit dry matter intake, but a certain amount of fibre is required to stimulate rumen activity. 

Digestibility

Feed digestibility is defined as the proportion of forage dry matter able to be digested by the animal. It is largely influenced by the maturity of the plant species and declines as the plant matures. Within pastures, the species type also influences digestibility. For example, clovers retain a higher leaf: stem ratio with increasing maturity and so maintain a higher digestibility relative to grasses.

Digestibility is measured in two quite distinct procedures:

In vivo digestibility – determined directly by animal feeding trials by way of a mass balance from what is consumed, digested and  excreted.

In vitro digestibility – determined by wet chemistry using rumen fluid taken from research animals, or using purified cellulase enzymes.

In vivo digestibility provides the most meaningful estimate of animal performance, but the cost of setting up animal trials for measuring in vivo digestibility, is prohibitive. 

Therefore, most laboratories measure in vitro digestibility by incubating samples with enzyme preparations and use these data to predict in vivo digestibility from in vivo standards.

Metabolisable energy (ME)

ME is a useful test to run – an estimate of the energy content of the feed potentially available for maintenance and production in ruminant animals. It is that proportion of feed energy absorbed from the digestive tract and retained for metabolic processes and the value is expressed as a proportion of the dry matter (MJ/kg).