Calcium Phosphorus Magnesium Potassium
Many areas of Western Canada have come through a dry summer. Weather conditions have reduced perennial and annual forage production up to 75 percent in some areas of Alberta and Saskatchewan. Forage costs have increased dramatically from the previous year and farmers are trying to optimize the use of their feed resources.
Mixing feeds is one way producers can stretch their feed supply. To do this efficiently, feed testing each forage type and grain is required. With tight economic times in the cow-calf sector, many producers have turned to the Near Infrared Red Spectroscopy (NIRS) system for analysis instead of wet chemistry. This change reduces costs by approximately $40 per forage sample submitted. Turn around time is significantly reduced when NIRS is used.
In dry years, plants mature more rapidly than in a normal year. A lack of moisture restricts total plant growth and the amount of starch or sugar deposited. Acid-detergent fibre and neutral-detergent fibre levels increase more rapidly than in a year with normal moisture, reducing overall energy content in the forage.
Mineral content of the plant is also impacted. For minerals to be absorbed by the plant, they must be in water solution. When moisture levels are high in the soil, phosphorus is absorbed into the plant early in the growing season and excess phosphorus is stored in the plant. As the year progresses and the plant grows; phosphorus is transported from older plant material to younger material.
Calcium is absorbed by the plant throughout the growing season. It is not stockpiled in the plant tissue. Absorption in the early part of the year is adequate. If drought occurs, soil water levels decrease and the ability to mineralize (get calcium and other nutrients out of the soil) is also reduced. This limits the amount of calcium transported into the plant with the soil water. If the drought continues, it is not uncommon for calcium levels to be 50 to 75 per cent lower in greenfeed and hay compared to a normal year.
HOW DOES NIRS WORK ?
NIRS measures the nutrient content of forages and grains by the amount of light that is absorbed or reflected off the sample. Light energy absorbed by the hydrogen-containing bonds in the feed is measured by the machine and the scan results are related through statistical correlation and calibration equations, to predict the nutritional content of the feed.
Reference samples are analyzed by wet chemistry and are used to develop calibration curves for the NIRS system. These reference samples are from a wide range of locations, different stages of plant maturity and environmental conditions. If the range of samples used to develop the curves is too narrow, the NIRS machine may incorrectly report the nutritional value of the sample in question or may not recognize the sample and consider it an “outlier” such as samples grown under drought conditions. The accuracy of the prediction is in question if the calibration curves are not developed properly.
HOW ACC URATE
The accuracy of NIRS predictions depends on the calibration curves developed from the wet chemistry analysis. NIRS results for protein, acid detergent fibre (ADF) and neutral detergent fibre (NDF) of forages are used widely in North America for ration-balancing purposes.
Plants grown in a drought year may be unique in their fibre composition as well as the starch/sugar contents. This may influence the ability of NIRS to accurately predict feeding values of these forages.
Measuring mineral composition (calcium, phosphorus, magnesium, potassium, and sodium) by NIRS is more difficult. These nutrients do not absorb light in the near-infared spectrum unless they are bound in a molecule which contains a hydrogen bond. Unlike CP and ADF, NIRS is not recognized as an official method for the determination of mineral content in forages by the Association of Official Analytical Chemists (AOAC).
To illustrate how weather conditions and calibration curves can impact test results, portions from the same sample of a barley greenfeed grown under drought conditions was analyzed by both NIRS and wet chemistry. Results are in the table below. Reported concentrations for calcium, phosphorus and magnesium were considerably different between wet chemistry and NIRS while the potassium and sodium results are identical.
MINERAL TEST RESULTS
The NIRS phosphorus test result for this barley greenfeed sample was 0.38 per cent. The 10-year Alberta average value for phosphorus in greenfeed samples is 0.22 per cent. The NIRS predicted value is 73 per cent higher than the wet chemistry average.
Test results for calcium by NIRS analysis is 0.45 per cent is very similar to the 10-year average Alberta average of 0.46 per cent. The reported value by NIRS of 0.45 per cent must be compared to the 0.16 per cent value reported by wet chemistry. As discussed above, calcium absorption can be reduced during a drought, and the AOAC does not recognize NIRS as an accurate method to determine mineral content in forages. Thus these results must be considered with caution.
Magnesium results of 0.23 per cent is very similar to 10 year Alberta average values for six row barley greenfeed but is much higher than the average 0.16 per cent value for two-row barley greenfeed.
Test results for calcium, phosphorus and magnesium must be considered carefully before values are used for ration balancing purposes.
Calcium and phosphorus are the two most important macro minerals in a beef ration. If the amounts of calcium and phosphorus are not in the proper ratio with calcium being deficient; weight gains can be reduced in growing animals. For mature cows fed a calcium deficient ration you may experience the following conditions; milk fever or downer cows, reduced milk production, winter tetany, stillborn calves, and retained placentas. Calcium can be mobilized from the cows’ bones which can cause osteoporosis.
If phosphorus is deficient in the ration, feed intake can be reduced resulting in lower milk yield in cows and growth rates in calves. Additional cow concerns are silent heats, longer times to start cycling and low conception rates in cows. A phosphorus deficiency related production problem will be noticed quicker than a calcium deficiency.
If a cow-calf producer was to provide the barley greenfeed as the sole feed after calving and use the NIRS results, a lactating cow would require 113 grams (four ounces) of limestone per head per day to maintain a 2:1 calcium to phosphorus ratio. Phosphorus and magnesium levels appear to be sufficient when using Cowbytes to balance the ration.
When the wet chemistry results are used, feeding the same ration, on a per-head per-day basis, 50 grams (1.75 ounces) of a 2:1 mineral is needed to increase the phosphorus content in the ration, along with an additional 95 grams (3.3 ounces) of limestone to have a 2:1 calcium-to-phosphorus ratio. Magnesium is deficient in the ration before supplementation. To prevent tetany problems, 23 grams (0.8 ounces) of magnesium oxide is also required per head per day.
There can be large discrepancies in feed test results between the analytical systems. Depending on which results you use, the supplementation program required can be considerably different and have impact on the long term performance of the cow herd. Consult with a nutritionist, or extension agent, to obtain a second opinion of what is required to provide a balanced supplementation program.
Barry Yaremcio, is a beef and forage specialist with Alberta Agricutlure’s Ag-Info Centre, in Stettler, Alberta.