When it comes to rations and supplementation programs for beef cattle, sampling forages for nutrient analysis is frequently discussed. Yet, it is a topic with many continually asked questions. While producers and industry professionals alike recognize why forage sampling is important and generally agree with the oft-cited mantra of, “You cannot manage what you do not measure,” taking sufficient and accurate samples often falls to the bottom of the priority list.
Agronomists expend considerable effort collecting soil and plant samples to better understand field fertility and production responses to precision application of inputs. In much the same way, sampling is critical to aiding our understanding of how animal nutrient requirements interact with what is supplied from forage and what we may or may not expect to see in terms of performance. This article aims to discuss key principles for forage testing and analysis to keep in mind as you evaluate your resources to feed cattle this fall.
What information are you looking to gain by collecting a forage sample? This is an important first question that needs to be answered because it will dictate the proper analysis to be performed by the laboratory to which it is submitted, as well as the cost. In general, forage samples can be analyzed by either wet chemistry or near-infrared reflectance spectroscopy (NIRS) methods. While wet chemistry analyses come with higher fees and longer turnaround times, it is a very reliable and highly accurate methodology. Wet chemistry is often preferred or required for analysis of a total mixed ration (TMR), mixed forage sample or trace minerals. This is because NIRS uses an infrared light spectrum process to assess the nutrient content of a forage sample relative to an established database specific to an individual laboratory. This method is less reliable when multiple or different feedstuffs are included in a single sample. However, NIRS is typically highly accurate for forage samples of a single source, but some labs may have accurate databases for mixed samples. A conversation with your lab of choice can help determine whether NIRS or wet chemistry should be conducted. Likewise, clearly and accurately labeling samples is very important because it enables lab personnel to conduct a more robust analysis, particularly with NIRS.
Most analysis reports will have moisture/dry matter, protein, macrominerals (i.e., calcium and phosphorus) and fiber measurements, and provide estimates of energy – represented as a percentage of total digestible nutrients (TDN), net energy for maintenance (NEm) or net energy for gain or growth (NEg) – based on such fiber measurements. Crude protein is based on the amount of nitrogen multiplied by 6.25. Samples analyzed by NIRS will be less expensive, require less time for processing, and generally supply the basic information needed to formulate rations for beef cows – moisture/dry matter, crude protein, TDN and/or NEm, acid detergent fiber (ADF) and neutral detergent fiber (NDF). Remember, the energy content of a feedstuff is challenging to measure, and the lab-reported energy values are derived from equations based on fiber measurements. For convenience and flexibility, most laboratories offer both package analyses, in which multiple tests can be conducted on a sample and reported together, or analysis of individual items. While we all recognize the value of having analysis information, there is also no reason to spend money unnecessarily on a sample if the chosen analysis does not provide the information needed to answer your feeding question. The key take-home point is to understand what information we are hoping to gain by sampling and how that information will help us make an informed feeding decision.
Every year, certain areas of the U.S. experience drought. During such conditions, producers may harvest and feed (often at higher than typical levels) forages unfamiliar to them, including failed grain crops, crop residues or hays containing high amounts of weeds. Avoiding formulation error is critical in these situations and further emphasizes the need for accurate sampling of these challenging forages. During drought, there are always numerous questions regarding nitrates and hydrocyanic (prussic) acid. Aside from the fact that these compounds can be analyzed by commercial laboratories and forages containing either can be fed safely to cattle, keep the following key points in mind:
- The risk of toxicity from either nitrates or prussic acid is greater during drought conditions, although numerous stressors can cause levels of both compounds to accumulate in forages.
- Nitrate and prussic acid toxicity are two distinct conditions. Yet, in both conditions, the animal does not receive oxygen at the cellular level, ultimately resulting in asphyxiation.
- Many plant species can accumulate only nitrates, some can accrue only prussic acid and some can accumulate both. Only plants that contain dhurrin can produce prussic acid, which primarily includes sorghums, sudangrass, sorghum-sudan hybrids and other related species.
- Critical differences between these conditions regarding plant physiology dictate our approach to management and sampling.
- Ensiling can reduce both compounds.
Forages are highly variable due to soil type, fertilization, moisture and stage of maturity at harvest, aside from the type of plant itself. This inherent variation is why we recommend using a core sampler or probe on baled hay; it takes a cross-section of the bale representative of the length of the field that produced the bale, and it provides for a more consistent leaf-to-stem ratio in the sample. Additional resources for sampling procedures and equipment selection can be found on the Kansas State University website.
Our goal is to obtain a sample most representative of the forage in question. Remember to identify and test lots separately; a lot is forage produced from the same cutting, field and stage of maturity. There are guidelines to help determine the number of bales necessary to subsample and composite into one larger sample based on forage type and degree of precision, and these can also help avoid unnecessary oversampling. Always properly and clearly label samples and submit them to the lab as soon as possible after collection, or keep them cool until they are submitted. Forage sampling allows for feeding values to be determined and subsequent informed decisions to be made.
To answer the question proposed in the title of this article, no, we cannot overemphasize the importance of forage analysis. Keep these key forage testing principles in mind as you consider nutrition plans for your cow herd this coming fall and winter.
