Forage analysis crucial to accurate ruminant rations

The use of NIR analysis aids in combating forage variability by identifying accurate nutrient and mineral parameters in dairy rations.


Forage nutrient content can vary substantially, making analysis a critical part of rationing ruminant livestock. Analysis includes the basic parameters of energy and protein, as well as dry matter (DM) and can be extended to minerals and assessment of hygienic quality.

Without accurate analyses, including forages in rations will be relying on guesswork.   

Forage analysis overview

Analytical parameters for forages vary slightly according to type of forage, region and rationing system being used. However, they are still assessing the same basic components: DM, energy, fiber, protein and fat. Ash content, starch, sugars and individual minerals, as well as more dynamic parameters, such as degradation values, are also often included in a forage analysis. Additionally, estimations (or calculations) of potential intake value, rumen stability values and potential acid loads are included in some regions.

Near-infrared reflectance spectroscopy (NIRS) has become the analytical method of choice over traditional wet chemistry in the analysis of many feed materials, particularly forages. It relies on measuring the amount of energy required to vibrate chemical bonds between atoms within molecules. Different chemical bonds require different amounts of energy and that variation is measured via differences in reflectance at different wavelengths, termed a spectrum, across the NIR region in the electromagnetic spectrum (780 to 2,500 nanometers). These spectra can then be used to determine which chemical bonds are present and, subsequently, the molecular structure of compounds within the sample. This technology is much more rapid and cost effective than wet chemistry and requires little sample preparation in contrast to wet chemistry.

However, NIRS accuracy is reliant on a detailed database of samples and calibration for individual groups of material, e.g. grass silage, corn silage, grass. The results are only as good as the calibration behind them, so it’s crucial to ensure that calibration equations are derived from a sufficient number of samples from relevant forages. For example, calibrations for grass silage must be built on values from grass silages, not from corn or alfalfa silages. Region will also have an effect, e.g. corn silage in the United States will not be representative of a corn silage in the United Kingdom.

Historically, NIRS has been performed by bench-top units, requiring samples to be collected, packaged and sent to an appropriate laboratory for analysis. This injects a delay of at least five working days, if not more, from taking the sample on-farm to being able to give reformulation and/or nutritional advice, meaning that the forage analysis can be out of date by the time the results are returned. To this end, hand-held, field-side devices have been developed. The aim is to give accurate, real-time results to enable nutritional advice to be given there and then, reducing lead time and cost.   

Principles of rationing

The overriding principle behind creating diets for cows is to ensure supply meets demand with regard to nutrients and various physical dietary attributes. Feedstuff variation and feeding accuracy have a substantial effect on farm profitability and nutrient excretion. Forage is obviously a major component of ruminant diets and, as such, forage composition and quality can have a substantial effect on overall diet properties. In fact, forage has the greatest influence on diet nutrient composition due to the huge variability between and within different forage types compared with cereals and protein feeds, such as rapeseed and soybeans.

One of the greatest variations is in DM, particularly with grass silage. Ranges of 12.7 to 65 percent DM have been noted when comparing baled grass silage across different regions in Ireland, for example. Although these values represent effects of differing management systems and geographical regions, it’s well known that silage DM can vary on the same unit, from the same field or bale and even within the same bale or across the clamp face.

Speaking at the Total Dairy seminar in the U.K., Dr. Dave Davies, director of Silage Solutions and a leading global expert in forage production, highlighted the huge variation possible. Grass silage sampled at different points across the same clamp on the same day showed DM varying from 18.5 to 26.4 percent. This variation was mirrored by values for metabolizable energy (ME), which exhibited a range from 9.76 to 10.74 MJ/kg DM.

Davies went on to demonstrate the potential effect this has on the amount of DM the animals actually receive. He explained that, if 25 kg of silage per cow are put into the mixer wagon daily at a DM of 20 percent, this would represent 5 kg DM, yet 6.5 kg DM if the silage was slightly drier at 26 percent. If we assign an ME value of 10 MJ/kg DM then the difference of 1.5 kg in DM intake (DMI) would equate to more than 2.5 liters of milk/cow/day.

Equally, for beef producers, the difference in ME intake would represent ~0.3 kg of daily gain. As these equations used a consistent energy value for the example silages and it’s known that this is often not the case, then one can further see how this situation can be exacerbated.        

Silage protein levels

Of course, it’s not just energy that is affected by differences in DM. Level of protein supplied will not only vary according to silage DM, but also the protein level per se of the silage, and the latter can also vary significantly.

Numerous factors affect protein levels in silage, including climate, fertilizer and region. A study evaluating the effect of year on nutrient quality of corn silage found a difference of nearly 10 grams of crude protein (CP)/kg DM across a period of seven years. While it is less surprising to see variation across years, this level of variation can and does occur within year and within the same bale and/or clamp. Within the same year, silage protein levels can vary substantially and differences of 50 to 100 grams CP/kg DM have been found in some grass silages. Having an accurate idea of exactly how much protein is in the forage part of the ration is essential to be able to correctly balance the diet with other protein sources. Knowledge of the split between rumen degradable and undegradable protein (RDP and RUP, respectively) is also critical to balancing the ration fed to the cow.

Forage vitamins and minerals

Energy and protein are the underpinning parameters used to create diets for cows, but minerals and vitamins also need to be considered when thinking about forage analysis. Mineral content and availability in forages is related to soil conditions, most notably mineral content and pH.

While mineral levels can be a little more stable over time compared with DM and protein levels, it’s still crucial to be aware of the mineral content of forages to tailor mineral supplementation. Mineral content and ratios between key minerals are important when rationing animals, particularly through the dry and transition period in dairy cattle, for example, the ratios of potassium to sodium and calcium to phosphorous. Watching out for excess potassium in close-up dry cows is also key, especially when dealing with grass silage and fresh grazing. Trace minerals can also be deficient in some forages, selenium being a good example. Many geographical areas have soils that are selenium deficient, including Scandinavia and the U.K. and Ireland. Diets including forages from these soils will need to be aware of selenium levels in order to avoid deficiency or sub-optimal performance.

Equally, high levels of minerals, such as molybdenum, iron and aluminium, must be taken into account due to their antagonistic action toward other, required minerals thus reducing the availability of those minerals. For example, molybdenum forms insoluble complexes with copper and sulphur, called thiomolybdates, in the rumen reducing the availability of dietary copper to the animal. In severe cases, copper deficiency symptoms will be seen. Therefore, it is beneficial to know levels of antagonists present in forages to inform the specification of mineral supplementation.

Forage quality, safety

One aspect that is often overlooked is the analysis of the hygienic quality of the forage. Yeast, molds and mycotoxins all reduce the quality of the forage and can have negative impacts on animal health and performance.

One of the biggest impacts is reduction in DMI with subsequent reduction in performance. Mycotoxin contamination, in particular, is known to elicit a drop in DMI.

Analysis of the hygienic quality of forages can help with management decisions regarding feeding of that forage.

Utilize NIRS for accuracy

NIRS forage analysis is a common practice. Assuming the calibrations behind the NIRS come from a sufficient number of representative samples, then results can be very accurate. Preparing diets for ruminants is based on matching nutrient demand with supply, and forages make up a large proportion of the nutrients in a ration. One of the greatest sources of variation is forage DM, which can vary substantially. Knowledge of variations in forage nutrient content makes matching supply to demand much more accurate and less of a guessing game.

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