Making Sense of Feed Labels
Part I: Balancing trace mineral deficiencies in your goat’s diet
You are at the feed store looking for a mineral supplement for your goats because you have heard that grass and hay in your area are deficient in minerals. You read the label of a recommended supplement with a “guaranteed analysis” expressed in terms of % (percent), ppm (parts per million), IU (International Units) and USP (U.S. Pharmacopeia) and have absolutely NO idea how all these numbers relate to your goat’s diet. Sound familiar! Here’s an example taken from the bag label of a ready-mixed local goat supplement (recommended daily ration 0.5-1.5 ounces).
Table 1. Guaranteed Analysis Mineral/Vitamin Supplement
|Calcium (Ca) (Min.)||15.5%|
|Calcium (Ca) (Max.)||18.5%|
|Phosphorous (P) (Min.)||8.0%|
|Salt (NaCl) (Min.)||18.5%|
|Salt (NaCl) (Max.)||22.0%|
|Magnesium (Mg) (Min.)||1.5%|
|Potassium (K) (Min.)||1.0%|
|Selenium (Se) (Min.)*||26 ppm|
|Copper (Cu) (Min.)||1450 ppm|
|Copper (Cu) (Max.)||1850 ppm|
|Zinc (Zn) (Min.)||7500 ppm|
|Iodine (I)||125 ppm|
|Iron (Fe) (Min.)||3600 ppm|
|Vitamin A (Min.)||300,000 USP/lb|
|Vitamin D (Min.)||45,000 IU/lb|
|Vitamin E (Min.)||400 IU/lb|
* Trace mineral concentrations are highlighted in blue. Because the mixing process can affect feed homogeneity, minimums (Min) and/or maximums (Max) from average concentrations are used by feed companies to meet regulatory standards. If trace mineral concentrations in your mineral supplement are expressed in percent (%) rather than ppm you can convert % to ppm by multiplying by 10,000.
Let’s start from the beginning. Before you decide on any supplement you must first know the mineral concentrations in your hay and pasture. The only way to know this is to test or analyze your goat’s forage. This is not expensive and provides a wealth of information. For example, a complete nutrient profile for hay (major and trace minerals plus crude protein, digestible energy (calories) and carbohydrates) is only about $30.00 from forage labs such as Dairy One. Table 2 compares the mineral content of two hays fed to dairy goats on our farm.
If you are not familiar with the metric system, some of the weight conversions between metric and English units are shown below.
1 gram (g) = 1/28 ounce (oz) = 0.0357 oz, or 28 g = 1 oz
1/1000 g = 0.001 g = 1 milligram (mg) = 0.0000357 oz
1/1,000,00 g = 0.000001 g = 1 microgram (mcg)
1000 g = 1 kilogram (kg) = 2.2 pound (lb)
1 part per million (ppm) = 1 mg/kg = 1 mg/2.2 lb
Table 2. Comparison of mineral content of alfalfa-rich (70%)/orchard grass hay (Hay 1) and mixed grass hay (Hay 2) (“as sampled”) from Central, Indiana.
Note: % and ppm “as sampled” or “as fed”; ppm = parts per million where 1 ppm = 1 mg/kg, or 1 mg/2.2 lb. For example, 6.6 lb (3 kg) Hay 2 contains 8 mg/2.2 lb copper ´ 6.6 lb = 24 mg copper. If you are working in the metric system, multiply ppm by 3 to find weight (mg). Analyses carried out by Equi-Analytical Laboratories (Dairy One), Ithaca, New York. Interpretation of chemical symbols: Ca (Calcium), P (Phosphorous), Mg (Magnesium), K (Potassium), Na (Sodium), S (Sulfur), Fe (Iron), Cu (Copper), Zn (Zinc), Mn (Manganese), Mo (Molybdenum), Se (Selenium); n/a (not analyzed).
Because of the interest in copper (Cu) deficiencies in recent Farmyard blogs, let’s use this trace mineral as our first example. Assuming 3 kg (6.6 lb) hay per day per goat, the daily Cu intakes from our hays are 27 (9 mg/kg x 3 kg) and 24 (8 mg/kg x 3 kg) mg Cu for alfalfa-rich and grass hay, respectively. This is generally considered adequate at maintenance level (Himba 2013). Slightly higher Cu amounts (50 mg per day) are usually recommended for breeding, pregnant and lactating goats. However, Cu requirements are extremely variable in goats because dietary Cu is not fully utilized if significant concentrations of interfering (antagonistic) substances (primarily iron, sulfur, and molybdenum) are present in feed and water (Spears 2004).
In contrast to other ruminants, such as sheep and cattle, minimum nutritional requirements for goats are poorly known, and are based primarily on goats in confinement, or estimated by comparison with cattle.
Let’s see how well our mineral supplement satisfies the Cu requirement based on the recommended range in dairy cattle (10-15 ppm Cu = 10-15 mg/kg dry matter basis). The term dry matter means that the sample (feed) has been dehydrated, or completely dried, before analysis. If water (moisture) is not excluded, it is difficult to compare nutrients in different feeds because the percentage of water can vary widely. For example, hay normally contains < 10% water by weight whereas grass and other pasture plants are ~ 75%-90% water by weight. Without correcting for the weight of water, the Cu concentration in our pasture grass is 3 ppm (80% water) versus 15 ppm dry matter. The hays in Table 2 have essentially the same water content and are compared ‘as sampled’ or ‘as fed’ (not dried).
Balancing Cu deficiency in goats based on minimum nutritional requirements of dairy cattle (adjusted for differences in body weight)
Step 1: Express cattle range (10-15 mg/kg dry matter) in terms of daily ration. Assume minimum concentration 10 mg/kg and large breed cattle body weight 1500 lb (peak dry matter intake ~ 4% x body weight = 60 lb)
10 mg/kg x 60 lb = 10 mg/2.2 lb x 60 lb = 273 mg Cu per day
Step 2: Adjust Cu amount based on approximate body weight difference (150 versus 1500 lb = 1/10)
273 mg Cu per day /10 = 27.3 mg Cu
Step 3: Add daily goat ration of Cu from forage (3 kg = 6.6 lb) and mineral supplement (1 oz at minimum (Min) Cu concentration)
Step 4: Contribution from alfalfa-rich hay
9 ppm x 3 kg = 9 mg/kg x 3 kg = 27 mg Cu per day
Step 5: Contribution from mineral supplement
1450 ppm x 1 oz = 1450 mg/kg x 1 oz = 1450 mg/2.2 lb x 1oz/16 oz =
1450 mg/2.2 lb x 0.0625 lb = 41 mg Cu per day
Step 6: Total intake = 27 + 41 mg = 68 mg Cu per day
Total Cu intake per day with supplementation exceeds minimum requirement. Hay without supplementation satisfies minimum requirement assuming 3 kg hay per day per goat.
If you are feeding a high protein dairy concentrate, Cu concentrations usually range from 20-25 ppm. In this case, 2 lb feed per 1 gallon milk would add about 20 mg/2.2 lb x 2 lb = 18 mg Cu to daily ration for a total of 45 mg Cu per day (27 + 18).
This is a general calculation and might not apply in all cases depending on the Cu concentration in your forage and the effect of interfering substances, which limit Cu absorption, or reduce its activity in biological compounds (e.g., enzymes) (Gould and Kendall 2011). For a good example, refer to Rose Bartiss’s Farmyard article on Cu deficiency in the Northeast U.S.
Zinc (Zn) and selenium (Se) are two other trace minerals commonly deficient in hay and pasture. Applying the above calculation for 3 kg mixed grass hay (Table 2) and 1 oz mineral supplement, Zn and Se amounts are 273 mg per day and 1 mg per day, respectively. The calculated daily minimums for dairy cattle (1500 lb body weight, intake 4% x body weight) are 627 mg Zn (23 mg/kg dry matter) and 2.7 mg Se (0.1 mg/kg dry matter), respectively.
If we estimate Zn and Se requirements based on extrapolation of cattle amounts adjusted to approximate differences in body weight, daily minimums for goats are 63 mg Zn and 0.27 mg Se, which is provided by our mixed grass hay (3 kg) without supplementation.
We feed hay and a mineral supplement free choice to all adult goats plus a dairy concentrate to lactating goats. Our goats are not confined and regularly browse areas consisting of woody plants, weeds and grass. In our case, it’s difficult to determine trace mineral intake from “pasture” because of the diversity of plants available for browsing. However, daily mineral intake from all sources is significantly above minimum requirements, as defined above.
Evaluating Cu bioavailability: Example from our farm
Interfering substances that could affect Cu bioavailability in goats include:
- Iron (Fe) in ready-mixed commercial supplements and feed (Table 1), forage (Table 2) and soil (ferric iron (Fe3+) in iron oxide/hydroxide)
- Ferrous iron (Fe2+) and sulfate (SO4)2- in drinking water
- Molybdenum (Mo) and sulfur (S) in forage (Table 2)
- S in mineral supplement (Table 1).
The major source of Fe for our diary goats is alfalfa-rich hay (Table 1) (200 mg/kg x 3 kg = 600 mg Fe). The mineral supplement (1 oz) and mixed grass hay (3 kg) contribute 102 mg (3600 mg/2.2 lb x 0.0625 lb) and 228 mg (76 mg/kg x 3 kg), respectively. As in most animal diets, Fe is in excess. In the presence of S, Fe decreases Cu bioavailability via some type of Cu-Fe-S interaction (Gould and Kendall 2011). If possible, purchase low-Fe hay (< 100 ppm) and choose a mineral supplement with Fe concentration listed in the guaranteed analysis. There are no specific requirements for Fe in goats but it is generally added as a coloring agent and to provide minimal protection against Fe-deficient anemia caused by internal parasites, such as the barber pole worm (Haemonchus contortus).
Mo and S can form poorly absorbed insoluble compounds with Cu in the rumen, or be absorbed into the body and bind to Cu-containing substances, such as enzymes, reducing their activity. Typical Mo and S concentrations for hay in this area are shown in Table 2 for Hay 2 (1.2 mg/kg and 0.19% = 1.9 g/kg for Mo and S, respectively). In sheep diets, when rumen S concentration is low (1.0 g/kg), Mo does not affect Cu bioavailability (Spears 2004). However, substantially higher concentrations of Mo (4.5 mg/kg) and S (4 g/kg) reduce bioavailability by 40-70%. The S concentration in the supplement is 1.22%, or 12.2 g/kg, which is equivalent to 0.35 g S per 1-oz supplement. This is about 10 times less than that contributed by Hay 2 per day (1.9 g/kg x 3 kg = 5.7 g).
Note that all of the trace minerals with the exception of Mo become less bioavailable to plants when the soil pH approaches neutrality (pH = 7). For soil pH ~ 7 or above, Mo becomes more bioavailable to plants than Cu.
Concentrations of Fe2+ and (SO4)2- (converted to sulfide (S2-) in rumen) in our well water are 0.1 mg/L (milligrams per liter ~ milligrams per kilogram) and 13 mg/L, respectively. The (SO4)2- concentration is due primarily to the presence of gypsum (CaSO4·2H2O) in the limestone (CaCO3) aquifer. Based on these very low (trace) concentrations, our drinking water is not a significant factor in reducing Cu bioavailability.
As you can see, the extent to which chemical interactions in the rumen limit Cu bioavailability is difficult to quantify. This is why it is a good ‘insurance policy’ to supplement at levels beyond minimum requirements. Keep in mind that diet must be evaluated regularly in terms of your goat’s body condition and overall health to ensure that your nutritional program is effective.
Next time we will focus on major minerals and how we can express their concentration in terms of weight rather than percent (%), a more meaningful measurement for balancing deficiencies and excesses in your goat’s diet.
George Lager is Professor Emeritus of Geosciences, University of Louisville, Louisville, Kentucky. He has raised dairy goats for about 10 years to satisfy his craving for unpasteurized milk and raw goat cheese! He and his wife Marjorie live on a small farm near Corydon, Indiana where, in addition to goats, they raise wild mustang horses, donkeys, one pet hog, chickens and some crazy geese (not to mention two German Shepherds and 10 cats)! Visit www.mitchellplainfarm.com for more articles about their animals and farm projects.
You must be logged in to post a comment.