When considering the type of supplement to use for beef cattle on a high forage diet, understanding how the rumen works will simplify the decision.

Rumen microbes shift their population to either forage digesting species or starch digesting species, depending on the diet. When a grain supplement is fed to a ruminant on a forage diet, the rumen population shifts away from full-time forage digestion to digest the starch. Consequently, feeding grain may decrease forage digestibility and intake, depending on the quantity of grain fed and the quality of the forage. This depressed digestion may be due in part to a lack of adequate protein or nitrogen in the rumen, which reduces the number of rumen microbes, as well as a reduction in rate of passage through the digestive tract. When the rate of passage slows down, the animal reduces its intake.

Research at Oklahoma State University and the University of Nebraska indicates that the ratio of total digestible nutrients (TDN) to crude protein (CP) dictates the type of supplement needed, given the forage condition. A forage analysis will yield the needed information. Using dry matter values, divide TDN% by CP%. A ratio of 5 to 7.5 is an optimum balance of energy and protein for high roughage diets, which includes forages and certain types of supplements.

Forages with a TDN/CP ratio of 5 or below that meet the TDN and CP requirement of the grazing animal, indicate that mineral supplementation will be adequate. If the forage does not meet the requirements, or if it has a limited availability and needs to be stretched, grain/protein supplementation will be required. Starch or grain supplementation of forages with a TDN/CP ratio below 5 does not change total tract digestibility or fiber digestibility but does depress forage intake when fed above 0.25% of body weight (Jones et al., 1988, University of Arkansas).

This example demonstrates the mathematics involved in choosing a supplement to match a particular forage.

A TDN/CP ratio of less than 7.5 indicates that a small amount of grain, less than 0.25% of body weight, will not depress forage digestibility or intake. However, grain fed at rates higher than 0.25% body weight will decrease forage digestibility and intake (Krysl et al., 1989, New Mexico State University and University of Nevada-Reno). This is useful under conditions where forage is limited (drought conditions). Beyond 0.25% bodyweight, grain substitutes for forage. Feeding 5 lb. of grain to an 1100 lb. cow will reduce the amount of forage consumed by 3-5 lb. Frequently used high-energy supplements are corn, oats, barley, corn or sorghum silages and molasses-based liquids. TDN/CP ratios of ingredients such as these are above 7.5

Research from Oklahoma State University (Chase and Hibberd, 1987) and the University of Nebraska (Sanson and Clanton, 1989) indicates that when the TDN/CP ratio of the forage is greater than 7.5, feeding as little as 0.25% body weight in grain depresses forage intake and digestibility. Consequently, overall energy intake is not improved. Mineral supplementation is appropriate for these situations provided the forage meets the animal’s requirements.

If the forage provides at least 45% TDN and the protein is below 9%, protein supplementation will increase forage intake and digestibility (McCollum and Galyean, 1985, New Mexico State University; Barton and Hibberd, 1984, Oklahoma State University).. Consequently, total energy intake is increased. When forage TDN is below 45%, voluntary intake is reduced, leading to weight loss under most environmental conditions. Examples of protein ingredients are cottonseed meal, sunflower meal and soybean meal. TDN/CP ratio of these ingredients is below 5.

However, if the TDN/CP ratio of the forage exceeds 7.5 and TDN is below 42%, protein supplementation will not increase TDN intake enough to maintain body condition or increase body weight. Research from Oklahoma State University (Martin and Hibberd, 1990) and the University of Missouri (Grigsby et al., 1991) indicates that structural carbohydrates may be the best supplement under these circumstances. Structural carbohydrates will not affect forage intake nor digestibility, resulting in a higher energy intake. Additionally, structural carbohydrates supply energy and a good deal of protein. Examples of structural carbohydrates are soybean hulls, wheat midds, corn gluten feed and whole cottonseed. Their TDN/CP ratios are below 7.5.

Quality of the forage will affect voluntary consumption. Cows on poor-quality forages will voluntarily eat 0.8—1.5% of their body weight. Cows on medium-quality forages will eat 1.5—2% of their body weight in forage. Consumption of good-quality forages may range from 2—2.7% of body weight. Use these figures to estimate dry matter intake in situations where it cannot be measured directly.

If supplemental feed is required, use a lab analysis of the available forage, and availability of forage to determine the type of supplement (protein, structural carbohydrate or grain) needed for the feeding situation. If cows need to gain extra weight before calving, calculate TDN intake based on a forage analysis and estimated intake. Protein supplements will increase forage intake and digestibility, resulting in increased TDN intake. Supplements containing large amounts of structural carbohydrates or digestible fiber do not reduce forage intake or digestibility. However, use of high-grain supplements may. If the amount of TDN the cows are eating is less than desired, proper supplementation will be necessary to increase energy intake and body condition.

This information can be used in practical feeding situations to determine the right type of supplementation program for a given situation. The tables below summarize this information:

BIBLIOGRAPHY

Barton, K.J. and C.A. Hibberd. 1984. Effects of protein supplementation on ruminal fermentation of low quality native grass hay in steers. Oklahoma State Animal Science Research Report MP-116, pp 165-167.

Bulgin, M.S. and M.M. Nussbaum. 1991. Winterizing: Advance planning reduces losses when Mother Nature sends an artic blast. Large Animal Veterinarian, September/October.

Chase, C.C. and C.A. Hibberd. 1987. Utilization of low-quality native grass hay by beef cows fed increasing quantities of corn grain. J. Animal Science 65:557-566.

Grigsby, K.N., M.S. Kerley and J.A. Paterson. 1991. Effects of soybean hull substitution on nutrient utilization of a low quality bromegrass hay-based diet. J. Animal Science 69(1):268.

Jones, A.L., A.L. Goetsch, S.R. Stokes and M. Colberg. 1988. Intake and digestion in cattle fed warm- or cool-season grass hay with or without supplemental grain. J. Animal Science 66:194-203.

Kartchner, R.J. 1981. Effects of protein and energy supplementation of cows grazing native winter range forage on intake and digestibility. J. Animal Science 51:432-438.

Krysl, L.J., M.E. Branine, A.U. Cheema, M.A. Funk and M.L. Galyean. 1989. Influence of soybean meal and sorghum grain supplementation on intake, digesta kinetics, ruminal fermentation, site and extent of digestion and microbial protein synthesis in beef steers grazing blue grama rangeland. J. Animal Science 67:3040-3051.

Martin, S.K. and C.A. Hibberd. 1990. Intake and digestibility of low-quality native grass hay by beef cows supplemented with graded levels of soybean hulls. J. Animal Science 68:4319-4325.

McCollum, F.T. and M.L. Galyean. 1985. Influence of cottonseed meal supplementation on voluntary intake, rumen fermentation and rate of passage of prairie hay in beef steers. J. Animal Science 60:570-577.

Sanson, D.W. and D.C. Clanton. 1989. Intake and digestibility of low-quality meadow hay by cattle receiving various levels of whole shelled corn. J. Animal Science 67:2854-2862.