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By-Product Feed Utilization by Grazing Cattle
Vet Clin Food Anim 23 (2007) 41–52
By-Product Feed Utilization by Grazing Cattle Dan Loy, PhD Department of Animal Science, Iowa State University, 301 Kildee Hall, Ames, IA 50010, USA
Supplementation of grazing cattle is a routine management practice that serves several purposes. Forage growth is seasonal and vulnerable to environmental factors. Temperature, moisture stress, and nutrient availability impact forage quality and quantity. Forage quality declines with advancing forage maturity. Conversely, cattle requirements for dry matter (DM), energy, and other nutrients may increase as the grazing season progresses. Supplementation can be used to fill the gaps created by seasonal deficiencies in forage growth and quality. Supplementation can also extend pasture availability during drought, increase the carrying capacity of the pasture, and provide nutrients that are inadequate or missing in the forage. Supplementation can also be used effectively to dilute anti-quality factors present in certain forages. Locally available by-products of the grain and food-processing industries can provide a cost-effective source of nutrients to balance the nutritional needs of grazing cattle. It is expected that the availability and cost effectiveness of these feeds will only grow as cattle compete with the biofuels industry for grain in the future.
Requirements of grazing cattle Cattle requirements are dictated by several factors, including class, physiologic status, level of production, level of activity, biologic type, and environmental conditions. Cattle operations that incorporate grazing as a source of nutrition are typically either cow–calf or stocker enterprises. In wellmanaged operations, cattle nutrient requirements typically increase in relative amounts per day as the animals grow and produce weight gain in the form of stocker gain, or calf growth in a cow–calf enterprise. The extent
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of this growth depends on the genetic potential of the animals, the environmental conditions, and the nutrient supply provided through the forage base and supplemental feeds. The National Research Council (NRC) [1] has published models that are intended to predict specific nutrient requirements, based on a description of these factors. Although the complexity of this model does not allow the requirements of grazing cattle under all conditions to be presented in this article, the following discussion describes a few examples that may be useful in developing supplementation programs for grazing beef cattle. Shown in Table 1 are the nutrient density requirements for beef cows, based on NRC requirements [1]. This table assumes a 1400-pound mature-weight beef cow producing 10 pounds of milk at peak lactation. Calf requirements are not accounted for in this table. This important consideration must be addressed when estimating the DM intake demands for the cow–calf pair. Beef cow nutrient requirements reach their apex with peak lactation during the second month after calving: 59.1% total digestible nutrients (TDN), 10.3% crude protein (CP), 0.3% calcium (Ca) and 0.2% phosphorous (P). TDN are an index of the energy availability from feeds and the energy requirements of animals; it is related directly to digestible energy. The NRC model actually uses net energy and metabolizable protein to calculate requirements; however, the corresponding CP and TDN values are used here for simplicity. The lowest requirements for the beef cow production cycle occur just after weaning: 45% TDN, 6% CP, 0.16% Ca, and 0.12% P. By examining only cow nutrient requirements, it would seem that the nutrient demands of spring calving cows decrease as the grazing season progresses; however, growth and forage consumption by the calf must be considered in any reckoning of forage demand. Boggs and coworkers [2] conducted a study measuring forage and milk intake by calves over the grazing season in Kansas. Forage intake increased from 1 pound per calf during the second month of life to almost 8 pounds per calf at 6 months of age. The cows in this study were Herefords, with Table 1 Dietary nutrient densities for beef cows (1400 lb mature weight, 20 lb peak milk production) Month following parturition Nutrient
1
2
3
4
5
6
7
8
9
10
11
12
TDN (% DM) 58.00 59.10 56.80 55.50 54.10 53.00 45.00 45.80 47.30 49.50 52.60 56.60 DMI (lb/d) 29.50 30.50 31.30 30.30 29.40 28.60 27.20 27.00 26.90 26.80 27.00 27.60 CP (% DM) 9.76 10.31 9.56 8.94 8.29 7.73 6.00 6.20 6.53 7.04 7.80 8.88 Ca (% DM) .28 .30 .28 .26 .24 .22 .16 .16 .16 .16 .27 .26 P (% DM) .19 .20 .19 .18 .17 .16 .12 .12 .12 .12 .17 .17 Abbreviations: Ca, calcium; CP, crude protein; DMI, dry matter intake; P, phosphorous; TDN, total digestible nutrients. Adapted from National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996.
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
43
a peak milk production of 15 pounds. By combining the observations of Boggs and coworkers [2] with those of the NRC [1], it is possible to estimate the net forage demand by month of the grazing season for both the cow and the calf (Fig. 1). The net amount of forage needed to meet both cow and calf intake requirements trends upward over the grazing season. Example nutrient requirements for stocker cattle for two different energy levels are shown in Table 2. The energy levels represent low (50% TDN, 0.45 Mcal/lb NEg) or high (60% TDN, 0.61 Mcal/lb NEg) growth performance. CP requirements range from 6.5% of diet DM for the heavy, slow-growing cattle, to more than 10% of diet DM for 660-pound cattle gaining 2 pounds per day. Similarly, Ca requirements range from 0.19% to 0.36% of diet DM, and P requirements range form 0.12% to 0.19% of diet DM, depending on body weight and rate of growth. Under these nutrient intake scenarios, a requirement for forage DM intake is also calculated in Table 1. These figures for the high gain and low gain stocker calves are also plotted in Fig. 1, along with the DM intake requirements of the cow and suckling calf. DM, energy, and protein demands are key factors in developing a supplementation program, particularly one using by-products; however, other minerals and vitamins are also required by the animal. These additional nutrients may be deficient in the local forages. A good reference for regional trace mineral status across several forages and regions in the United States is the National Animal Health Monitoring Service Beef Cow–Calf Health and Productivity Audit [3]. Trace minerals identified as being widely deficient in forages include copper, manganese, zinc, cobalt, selenium, and iron. Producers may also need to supplement magnesium, sodium (ie, salt), Vitamin A, Vitamin D, and Vitamin E. 35
Dry Matter Intake, Lb.
30 25 20 15 10 5 0
Apr
May
June
Jul
Aug
Sept
Month Cow-calf pair High gain stocker
Low gain stocker
Fig. 1. Example dry matter needs of grazing cattle.
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Table 2 Diet nutrient densities for growing cattle (1200 lb and 28% body fat at finishing or maturity) Body weight (lb)
TDN (% DM)
NEm NEg DMI ADG (Mcal/lb) (Mcal/lb) (lb) (lb)
CP (% DM)
Ca (% DM)
P (% DM)
660
50 60 50 60 50 60 50 60 50 60 50 60
.45 .61 .45 .61 .45 .61 .45 .61 .45 .61 .45 .61
7.3 10.2 7.1 9.7 6.9 9.2 6.8 8.3 6.6 8.4 6.5 8.1
.22 .36 .21 .34 .20 .32 .20 .30 .19 .28 .19 .27
.13 .19 .13 .18 .13 .17 .13 .16 .12 .16 .12 .15
720 780 840 900 960
.20 .35 .20 .35 .20 .35 .20 .35 .20 .35 .20 .35
17.5 18.4 18.6 19.7 19.8 20.9 20.9 22.1 22.0 23.3 23.1 24.4
.72 2.00 .72 2.00 .72 2.00 .72 2.00 .72 2.00 .72 2.00
Adapted from National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996; with permission.
The forage supply: quantity and quality The nutrient requirements of cattle depend on physiologic factors, but forage growth and nutrient availability depend on seasonal fluctuations in temperature and moisture. Fig. 2 shows typical growth curves of several forages [4]. Generally, cool- and warm-season grass growth peaks in the late spring and subsequently declines through late summer. Cool-season grasses undergo a second growth surge during the fall, as temperatures moderate and precipitation increases. The seasonal trend in forage growth and availability is one of surplus during the early part of the growing season and shortage during the latter part. This trend runs counter to the needs of a productive cattle enterprise. Historically, this conundrum has been managed by stockpiling surplus forage, harvesting surplus forage, reducing stocking rates when forage supplies diminish, or supplementing cattle when forage supplies diminish. The monthly forage quality of Midwestern pastures is shown in Table 3 [5]. These data were collected on research and demonstration pastures in Iowa over a period of years. Generally, forage quality and nutrient density declined with advancing maturity. Similar trends occur with warm-season native grasses. The protein content of native grasses can range from 18% in May to 4% in September [6].
By-product feeds for grazing cattle Feeds used in the past to supplement grazing cattle have included grain as a primary energy source, mineral and vitamin supplements, and by-products of the grain, food, and fiber-processing industries. Depending on the analysis,
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
45
Fig. 2. Seasonal dry matter yield patterns of common forages. (Adapted from Barnhart SK. Guide to year round forage supply. Ames (Iowa); Iowa State University Extension publication Pm 1771; 1998; with permission.)
by-product feeds may be a primary energy source or a protein source, or provide several needed nutrients. This article emphasizes the most widely available grain processing by-product feeds. These include soybean hulls, corn gluten feed, distillers grains, and wheat middlings. The nutrient content of
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Table 3 Crude protein and total digestible nutrient concentration by month for cultivated pasture forages April
May
June
July
August
September
21.7 11.8 14.2 22.7 15.5 15.5
13.8 11.6 10.8 13.1 12.5 13.2
15.4 14.3 10.9 14.0 13.1 14.0
14.7 13.6 11.4 14.7 12.7 14.4
13.8 13.0 11.9 16.6 d d
Forage type
CP (%)
Alfalfa–brome Brome–orchard–trefoil Southern Iowa pastures Smooth brome Smooth brome–red clover Tall fescue–red clover
d 15.8 17.1 d d d
Forage type
TDN (%, calculated from ADF)
Alfalfa–brome Brome–orchard–trefoil Southern Iowa pastures Smooth brome Smooth brome–red clover Tall fescue–red clover
d 38.5 55.8 d d d
64.1 57.2 57.1 67.1 52.6 57.4
53.8 58.0 52.0 57.7 57.0 58.4
54.3 58.7 48.7 57.0 58.2 59.3
55.0 57.8 47.1 58.6 56.1 57.4
53.5 56.6 46.3 59.4 d d
Abbreviation: ADF, acid detergent fiber. Data from Strohbehn D, Loy D, Morrical D, et al. A summary of monthly nutrient values for research pastures in growing months. Ames (IA); Iowa State University Animal Industry Report. 2004:ASL R187.
several by-products of the grain and food-processing industries are shown in Table 4. This table includes not only the grain processing by-products but also the locally important by-products of other food-processing industries. Beet pulp Beet pulp is the fibrous material remaining after sugar beets have been processed. Beet sugar production occurs in five regions of the United States: Great Lakes (primarily Michigan), Red River Valley (Minnesota and North Dakota), Great Plains (Northeast Colorado through Southeast Montana), Northwest (Idaho, Washington, and Oregon) and California [7]. Most sugar beet processing plants are located in these production areas. In 2006, the United States produced 33.6 million tons of sugar beets, according to the US Department of Agriculture National Agricultural Statistics Service. This quantity of beets should produce 1.18 million tons of dried beet pulp and 1.5 million tons of molasses annually. Beet pulp has a modest protein content (9.8%) and is rich digestible fiber. At 74% TDN, beet pulp is an excellent energy source for grazing cattle, but it is virtually devoid of starch. Citrus pulp In 2005–2006, more than 8 million tons of citrus fruits were processed in the United States [8]. Dried citrus pulp represents approximately 10% of the original weight of the processed fruit [9]. Citrus pulp is primarily a
47
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
Table 4 Nutrient content of selected by-product feedstuffs Nutrient content (DM basis) Feed
DM (%)
TDN (%)
NEm (Mcal/lb)
NEg (Mcal/lb)
CP (%)
Ca (%)
P (%)
Beet pulp Brewer’s grains Citrus pulp Dried distillers grain þ solubles Corn gluten feed Molasses, beet Molasses, cane Soyhulls Wheat middlings
91 90 91 90 90 78 74 990 89
74 66 82 90 80 75 72 77 69
.80 .68 .91 .99 .88 .81 .77 .84 .73
.52 .41 .61 .68 .59 .53 .50 .55 .45
9.8 29.2 6.7 30.4 23.8 8.5 5.8 12.2 18.7
.68 .29 1.88 .07 .07 .15 1.00 .53 .17
.10 .70 .13 .95 .61 .03 0.10 .18 1.01
Adapted from National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996.
by-product of orange and grapefruit processing. Similar to beet pulp, the protein content of citrus pulp is modest (6.7%). Although it contains little or no starch, it is an excellent high-fiber energy source (82% TDN). Citrus pulp is also rich in Ca. Brewer’s grain More than 195 million barrels of beer were produced in the United States in 2005 [10]. Brewer’s grains may be dried and used as a supplement for grazing cattle; however, most of the product is used in its wet form by dairies and feedlots located within a few miles of the various sites of production. Brewer’s grains, considered an intermediate protein source (29.2%), are relatively low in energy (66% TDN) among the grain and food-processing by-products. Soybean hulls Soybean hulls are the seed coat of the soybean kernel. Soybean hulls represent approximately 5% of the original weight of the soybean kernel, and yield about 3 pounds per bushel of whole, raw soybeans [11]. In 2005, 61.7 billion bushels of soybeans were processed in the United States. [12]. Soybean meal may or may not be dehulled in the oil extraction process. Generally, soybean meal that has not been dehulled is marketed as 44% CP soybean meal; so-called ‘‘high-protein soybean meal’’ (48% CP) has been dehulled. Soybean hulls are an excellent source of highly digestible fiber for forage-based beef cattle diets. On average, soybean hulls contain 77% TDN and 12.1% CP. Wheat middlings During the wheat milling process, 70% to 75% of the grain, by weight, is converted to wheat flour. The remainder of the kernel is used as animal feed
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[13]. This animal feed by-product can be referred to by several names; the most common is ‘‘wheat middlings.’’ In 2005 and 2006, 914 million bushels of wheat were processed for food, yielding approximately 8.5 million tons of wheat middlings [14]. The nutritional character of wheat middlings may vary according to wheat variety and processing method but, in general, they are moderate in energy (69% TDN) and moderate in protein (18.7%). Molasses Molasses is produced as a by-product of the beet and cane sugar-processing industries. Sugarcane production in 2005 and 2006 totaled nearly 25 million tons, according to the US Department of Agriculture. Sugarcane production, coupled with the 33.6 million tons of sugar beets processed that year, produced as much as 2.3 million tons of molasses. Molasses contains very little protein (O5% CP) but is rich in sugar-based energy (72%–75% TDN). In the feed industry, most molasses is used as a basis for dietary supplements that contain natural protein, urea, minerals, vitamins, ionophores, and antibiotics (ie, micro ingredients). Often, molasses is a major ingredient in free-choice, intake-limited supplements specifically developed for grazing cattle. Corn gluten feed Corn gluten feed is a by-product of the wet corn–milling industry. Wet corn milling is designed to facilitate the extraction of starch from the corn kernel and its conversion to dextrose. Subsequently, the dextrose is converted to high fructose corn syrup, or fermented to produce ethanol. Livestock feeds produced from the wet-milling process include corn gluten meal, corn germ meal, condensed steepwater solubles, and corn gluten feed. Corn gluten feed is a combination of corn bran and the condensed steepwater solubles. The wet-milling process yields roughly 13.5 lb (DM) of corn gluten feed, 2.5 lb of corn gluten meal, 1.6 lb of corn oil, and 33 gallons of corn sweetener per bushel of corn [15]. Alternatively, 2.3 gallons of ethanol may be produced in lieu of the corn sweetener. In 1999, 9.45 million tons of corn gluten feed (DM) were produced in the United States. Corn gluten feed is moderate in protein (16%–20%) and a good energy source for cattle (80% TDN). Dried distillers grains with solubles The ethanol industry is undergoing a rapid expansion in the United States. According to the Renewable Fuels Association, 3.9 billion gallons of ethanol were produced in the United States in 2005. The National Corn Growers Association estimated that production could swell to 15.9 billion gallons by 2015. Distillers grains and distillers solubles are the primary by-products of ethanol production by way of dry corn milling. In this process, the grain
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
49
(primarily corn or sorghum) is ground, and ethanol is fermented from starch in the grain. Because corn is approximately two thirds starch, the remaining nutrients (eg, protein, fiber, trace minerals) are concentrated in the thin stillage, the material remaining after extraction of alcohol. The solid components of thin stillage become distillers grains, and the liquid becomes distillers solubles. Solubles are condensed through heating and evaporation and may be sold for livestock feed as condensed distillers solubles. Most solubles, however, are added to the distillers grains to create distillers grains with solubles. Distillers grains with solubles may be available locally in the wet form (approximately 70% moisture), as modified or partially dried distillers grains (DDG) with solubles (approximately 50% moisture), or as dried distillers grains with solubles (DDGS; 10% moisture). DDGS are an excellent protein source (30% CP) for grazing cattle. Because the fiber and oil in the original grain are concentrated in the DDGS, it is also an excellent source of fiber-based energy (90% TDN). Rapid expansion in the ethanol industry is changing world perspectives on corn. The grain is no longer chiefly a source of feed energy for livestock. It has become a raw material for both food and fuel. Competition with the renewable fuels industry for feed grains represents a new era for the livestock industry. Fortunately, by-products of corn processing are excellent protein and energy supplements for grazing cattle. Prospects for an increased supply of these feeds, along with dwindling availability of feed grains, suggest that economic use of grain processing by-products will increase in the future.
Response to supplementation The response of cattle to various supplementation programs can be assessed in different ways. The response of stocker cattle to supplementation may be measured in terms of improved daily gain or increased carrying capacity of the forage base. Similarly, the response of cows to supplementation may influence pasture carrying capacity, cow body condition, cow reproductive performance, and calf growth. By-products of the grain processing industry typically contain little or no starch. Rather, the energy that they contain is a highly digestible form of fiber or cellulose. Cellulose is also the major carbohydrate in forages. Several studies have been conducted that demonstrate a depression in digestion of plant cellulose when feeds that are rich in starch (ie, grains) are fed with forage-based diets [16]. Fiber-rich by-product feeds do not interfere with ruminal digestion of plant cellulose. As a result, numerous studies of forage-fed cattle report that by-product supplements support production that is equal to, or greater than, grain supplements, even though by-products appear to have lesser energy yields than grains [17]. The response of forage-fed animals to supplementation with by-product feeds and grains is influenced by forage
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quality [18]. By-product feeds and grains tend to promote similar levels of animal response when added to high-quality forage diets. Conversely, byproduct feeds tend to promote a much more vigorous response, in terms of intake and digestion, than do grains, when added to low-quality forage diets. Kunkle and coworkers [19] reviewed a number of studies in which the performance of forage-fed cattle supplemented with high-fiber by-products was compared to that of grain-fed cattle. It was noted that gain responses were similar to grain when cattle were fed soy hulls, corn gluten feed, beet pulp, or citrus pulp. In a 3-year summary of studies, a combination of wheat middlings and soybean hulls were compared with grain-based supplementation fed to stocker cattle grazing wheat pasture (ie, a high-quality forage) [20]. Daily gains were 2.03, 2.32, and 2.38 lb per day for unsupplemented, grainsupplemented, and by-product-supplemented cattle, respectively. The supplemented cattle were fed at a rate of 0.75% of body weight per day; stocking rates on wheat pasture were increased 22% to 44% by supplementation. Supplementation of high-quality forages generally reduces forage intake, allowing the supply of forage to be extended, or stocking rates to be increased. Conversely, supplementation of low-quality forages with high-protein supplements (ie, O30% CP) generally increases forage intake. This change in forage consumption associated with supplemental feeding has been termed the ‘‘substitution coefficient,’’ and has been reviewed [21]. The substitution coefficient is defined as the change in basal forage intake divided by supplement intake. The extent of the change in forage intake with supplementation is related to the quantity of supplemental energy intake, the energy-to-protein ratio of the forage, and the level of voluntary forage intake [22]. Responses of beef cows to supplementation may include improved cow condition and reproductive performance [16]. Corn gluten feed has been used effectively to maintain cow condition for cows grazing stockpiled winter forage [23]. Distillers grains are also effective in this role. In a Nebraska study, DDG were supplemented at rates of 0, 1.0, 2.1, 3.1, or 4.2 lb per day to heifers grazing bromegrass (ie, a high quality forage) [24]. For each pound of DDG that was fed, 1.72 fewer pounds of forage were consumed (DM basis). Heifer weight gains increased 0.06 lb per day for each pound of DDG consumed. The economic relationship between DDG and forage value was calculated as DDG value ð$=TÞ ¼ 5:07 forage value ð$=animal unit month ½AUMÞ þ 66:08 Standardized Performance Analysis summaries reported an average economic value of pasture grazing in the Midwest for cow–calf producers of $14.26 per AUM [25]. The value for corn stalk grazing was $3.43 per AUM. Using these values and the Nebraska equation, DDG have a value of $138.38 and $83.47 in terms of pasture and cornstalk grazing, respectively.
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
51
Knowledge of pasture productivity and quality is the first step in developing a sound supplementation program. Once the patterns of pasture growth and nutrient yield have been established, animal requirements and production goals can be determined. An assessment of the expected response to supplementation, taking into account the anticipated value of additional gain, is essential in determining the potential returns on investment in a supplementation program. Pasture systems are complex, and each operation has a different set of costs and constraints. Knowledge of the basic animal requirements and forage-supplement tradeoffs lead to better, more informed decisions.
References [1] National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996. [2] Boggs DL, Smith EF, Schalles RR, et al. Effects of milk and forage intake on calf performance. J Anim Sci 1980;51:550–3. [3] Corah LR, Dargatz D. Forage analyses from cow/calf herds in 18 states. Beef cow/calf health and productivity audit. Fort Collins (CO): National Animal Health Monitoring System. USDA; 1996. [4] Barnhart SK. Guide to year round forage supply. Iowa State University Extension Publication Pm 1771; 1998. [5] Strohbehn D, Loy D, Morrical D, et al. A summary of monthly nutrient values for research pastures in growing months. Iowa State University Animal Industry Report. ASL R187; 2004. [6] Smith E. Growing cattle on grass. Kansas State University Agricultural Experiment Station Bulletin 638; 1981. [7] Ali B. Characteristics and production costs of U.S. sugarbeet farms. United States Department of Agriculture Statistical Bulletin SB974-8; 2004. [8] USDA. Citrus-fruits (2006 summary). Agricultural Statistical Bulletin Fr Nt 3-1(06); 2006. [9] Goodrich RM, Braddoah RJ. Major by-products of the Florida citrus-processing industry. University of Florida IFAS Extension FSHN05-22; revised 2006. [10] Robinson M. Brewers almanac. Washington, DC: Beer Institute; 2006. [11] Blasi D, Drouillard J, Titgemeyer EC, et al. Soyhulls: composition and feeding value for beef and dairy cattle. Kansas State University Agricultural Experiment Station Mt-2438; 2000. [12] USDA. Agricultural Outlook - Statistical Indicators. Economic Research Service, 2006. Available at: http://www.ers.usda.gov/publications/aotables/2006/12Ded/Ao1206.pdf. [13] Blasi DA, Reed CL, Dionisia M, et al. Wheat middlings: composition, feeding value and storage guidelines. Kansas State University Agricultural Experiment Station MF-2353; 1998. [14] USDA. Wheat Outlook. ERS WHS-06; 2006. [15] Loy D, Miller W. Ethanol coproducts for cattle: the process and products. Iowa State University Extension Publication IBC-18; 2002. [16] Winger H, ZoBell DR, Olson KC. Supplementation of energy and protein for beef cattle, a literature review. Utah State University Extension Publication. AG/Beef/2006-01; 2006. [17] Merchen NR. Digestion, absorption and excretion in ruminants. In: Clark DC, editor. The ruminant animal. Englewood Cliffs (NJ): Prentice Hall; 1988. p. 172–201. [18] Summer P, Trenkle A. Effects of supplementing high or low quality forages with corn or corn coproducts upon digestibility of dry matter and energy by steers. Iowa State University Beef Research Report. ASL-R1540; 1998.
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[19] Kunkle WE, Johns JT, Poore MH, et al. Designing supplement programs for beef cattle fed forage-based diets. Proceedings American Society of Animal Science 1999. Available at: http://www.asas.org/jas/symposia/proceedings/0912.pdf. [20] Horn GW, Beck PA, Andrae JG, et al. Designing supplementation for stocker cattle grazing wheat pasture. J Anim Sci 2005;83(E Suppl):E69–78. [21] Caton JS, Dhuyvetter DV. Influence of energy supplementation on grazing ruminants: requirements and responses. J Anim Sci 1997;75:533–42. [22] Moore JE, Brant MH, Kunkle WE, et al. Effects of supplementation on voluntary forage intake, diet digestibility and animal performance. J Anim Sci 1999;77(Suppl 2):122–35. [23] Russell J, Driskoll R, Moricall D, et al. Effects of stocking rate and corn gluten feed supplementation on performance of two-year old cows grazing stockpiled forage during winter. Iowa State University Animal Industry Report. ASL-R2065; 2006. [24] MacDonald JC, Klopfenstein T. Dried distillers grains as a grazed forage supplement. Nebraska Beef Report; 2004. p. 25–7. [25] Miller A, Knipe R. SPA. Summary beef cow business record final report. Available at: http: www.iowabeefcenter.org/content/SPASummary.pdf. 2004.
By-Product Feed Utilization by Grazing Cattle Dan Loy, PhD Department of Animal Science, Iowa State University, 301 Kildee Hall, Ames, IA 50010, USA
Supplementation of grazing cattle is a routine management practice that serves several purposes. Forage growth is seasonal and vulnerable to environmental factors. Temperature, moisture stress, and nutrient availability impact forage quality and quantity. Forage quality declines with advancing forage maturity. Conversely, cattle requirements for dry matter (DM), energy, and other nutrients may increase as the grazing season progresses. Supplementation can be used to fill the gaps created by seasonal deficiencies in forage growth and quality. Supplementation can also extend pasture availability during drought, increase the carrying capacity of the pasture, and provide nutrients that are inadequate or missing in the forage. Supplementation can also be used effectively to dilute anti-quality factors present in certain forages. Locally available by-products of the grain and food-processing industries can provide a cost-effective source of nutrients to balance the nutritional needs of grazing cattle. It is expected that the availability and cost effectiveness of these feeds will only grow as cattle compete with the biofuels industry for grain in the future.
Requirements of grazing cattle Cattle requirements are dictated by several factors, including class, physiologic status, level of production, level of activity, biologic type, and environmental conditions. Cattle operations that incorporate grazing as a source of nutrition are typically either cow–calf or stocker enterprises. In wellmanaged operations, cattle nutrient requirements typically increase in relative amounts per day as the animals grow and produce weight gain in the form of stocker gain, or calf growth in a cow–calf enterprise. The extent
E-mail address: [email protected] 0749-0720/07/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.cvfa.2007.01.001 vetfood.theclinics.com
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of this growth depends on the genetic potential of the animals, the environmental conditions, and the nutrient supply provided through the forage base and supplemental feeds. The National Research Council (NRC) [1] has published models that are intended to predict specific nutrient requirements, based on a description of these factors. Although the complexity of this model does not allow the requirements of grazing cattle under all conditions to be presented in this article, the following discussion describes a few examples that may be useful in developing supplementation programs for grazing beef cattle. Shown in Table 1 are the nutrient density requirements for beef cows, based on NRC requirements [1]. This table assumes a 1400-pound mature-weight beef cow producing 10 pounds of milk at peak lactation. Calf requirements are not accounted for in this table. This important consideration must be addressed when estimating the DM intake demands for the cow–calf pair. Beef cow nutrient requirements reach their apex with peak lactation during the second month after calving: 59.1% total digestible nutrients (TDN), 10.3% crude protein (CP), 0.3% calcium (Ca) and 0.2% phosphorous (P). TDN are an index of the energy availability from feeds and the energy requirements of animals; it is related directly to digestible energy. The NRC model actually uses net energy and metabolizable protein to calculate requirements; however, the corresponding CP and TDN values are used here for simplicity. The lowest requirements for the beef cow production cycle occur just after weaning: 45% TDN, 6% CP, 0.16% Ca, and 0.12% P. By examining only cow nutrient requirements, it would seem that the nutrient demands of spring calving cows decrease as the grazing season progresses; however, growth and forage consumption by the calf must be considered in any reckoning of forage demand. Boggs and coworkers [2] conducted a study measuring forage and milk intake by calves over the grazing season in Kansas. Forage intake increased from 1 pound per calf during the second month of life to almost 8 pounds per calf at 6 months of age. The cows in this study were Herefords, with Table 1 Dietary nutrient densities for beef cows (1400 lb mature weight, 20 lb peak milk production) Month following parturition Nutrient
1
2
3
4
5
6
7
8
9
10
11
12
TDN (% DM) 58.00 59.10 56.80 55.50 54.10 53.00 45.00 45.80 47.30 49.50 52.60 56.60 DMI (lb/d) 29.50 30.50 31.30 30.30 29.40 28.60 27.20 27.00 26.90 26.80 27.00 27.60 CP (% DM) 9.76 10.31 9.56 8.94 8.29 7.73 6.00 6.20 6.53 7.04 7.80 8.88 Ca (% DM) .28 .30 .28 .26 .24 .22 .16 .16 .16 .16 .27 .26 P (% DM) .19 .20 .19 .18 .17 .16 .12 .12 .12 .12 .17 .17 Abbreviations: Ca, calcium; CP, crude protein; DMI, dry matter intake; P, phosphorous; TDN, total digestible nutrients. Adapted from National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996.
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
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a peak milk production of 15 pounds. By combining the observations of Boggs and coworkers [2] with those of the NRC [1], it is possible to estimate the net forage demand by month of the grazing season for both the cow and the calf (Fig. 1). The net amount of forage needed to meet both cow and calf intake requirements trends upward over the grazing season. Example nutrient requirements for stocker cattle for two different energy levels are shown in Table 2. The energy levels represent low (50% TDN, 0.45 Mcal/lb NEg) or high (60% TDN, 0.61 Mcal/lb NEg) growth performance. CP requirements range from 6.5% of diet DM for the heavy, slow-growing cattle, to more than 10% of diet DM for 660-pound cattle gaining 2 pounds per day. Similarly, Ca requirements range from 0.19% to 0.36% of diet DM, and P requirements range form 0.12% to 0.19% of diet DM, depending on body weight and rate of growth. Under these nutrient intake scenarios, a requirement for forage DM intake is also calculated in Table 1. These figures for the high gain and low gain stocker calves are also plotted in Fig. 1, along with the DM intake requirements of the cow and suckling calf. DM, energy, and protein demands are key factors in developing a supplementation program, particularly one using by-products; however, other minerals and vitamins are also required by the animal. These additional nutrients may be deficient in the local forages. A good reference for regional trace mineral status across several forages and regions in the United States is the National Animal Health Monitoring Service Beef Cow–Calf Health and Productivity Audit [3]. Trace minerals identified as being widely deficient in forages include copper, manganese, zinc, cobalt, selenium, and iron. Producers may also need to supplement magnesium, sodium (ie, salt), Vitamin A, Vitamin D, and Vitamin E. 35
Dry Matter Intake, Lb.
30 25 20 15 10 5 0
Apr
May
June
Jul
Aug
Sept
Month Cow-calf pair High gain stocker
Low gain stocker
Fig. 1. Example dry matter needs of grazing cattle.
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Table 2 Diet nutrient densities for growing cattle (1200 lb and 28% body fat at finishing or maturity) Body weight (lb)
TDN (% DM)
NEm NEg DMI ADG (Mcal/lb) (Mcal/lb) (lb) (lb)
CP (% DM)
Ca (% DM)
P (% DM)
660
50 60 50 60 50 60 50 60 50 60 50 60
.45 .61 .45 .61 .45 .61 .45 .61 .45 .61 .45 .61
7.3 10.2 7.1 9.7 6.9 9.2 6.8 8.3 6.6 8.4 6.5 8.1
.22 .36 .21 .34 .20 .32 .20 .30 .19 .28 .19 .27
.13 .19 .13 .18 .13 .17 .13 .16 .12 .16 .12 .15
720 780 840 900 960
.20 .35 .20 .35 .20 .35 .20 .35 .20 .35 .20 .35
17.5 18.4 18.6 19.7 19.8 20.9 20.9 22.1 22.0 23.3 23.1 24.4
.72 2.00 .72 2.00 .72 2.00 .72 2.00 .72 2.00 .72 2.00
Adapted from National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996; with permission.
The forage supply: quantity and quality The nutrient requirements of cattle depend on physiologic factors, but forage growth and nutrient availability depend on seasonal fluctuations in temperature and moisture. Fig. 2 shows typical growth curves of several forages [4]. Generally, cool- and warm-season grass growth peaks in the late spring and subsequently declines through late summer. Cool-season grasses undergo a second growth surge during the fall, as temperatures moderate and precipitation increases. The seasonal trend in forage growth and availability is one of surplus during the early part of the growing season and shortage during the latter part. This trend runs counter to the needs of a productive cattle enterprise. Historically, this conundrum has been managed by stockpiling surplus forage, harvesting surplus forage, reducing stocking rates when forage supplies diminish, or supplementing cattle when forage supplies diminish. The monthly forage quality of Midwestern pastures is shown in Table 3 [5]. These data were collected on research and demonstration pastures in Iowa over a period of years. Generally, forage quality and nutrient density declined with advancing maturity. Similar trends occur with warm-season native grasses. The protein content of native grasses can range from 18% in May to 4% in September [6].
By-product feeds for grazing cattle Feeds used in the past to supplement grazing cattle have included grain as a primary energy source, mineral and vitamin supplements, and by-products of the grain, food, and fiber-processing industries. Depending on the analysis,
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
45
Fig. 2. Seasonal dry matter yield patterns of common forages. (Adapted from Barnhart SK. Guide to year round forage supply. Ames (Iowa); Iowa State University Extension publication Pm 1771; 1998; with permission.)
by-product feeds may be a primary energy source or a protein source, or provide several needed nutrients. This article emphasizes the most widely available grain processing by-product feeds. These include soybean hulls, corn gluten feed, distillers grains, and wheat middlings. The nutrient content of
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Table 3 Crude protein and total digestible nutrient concentration by month for cultivated pasture forages April
May
June
July
August
September
21.7 11.8 14.2 22.7 15.5 15.5
13.8 11.6 10.8 13.1 12.5 13.2
15.4 14.3 10.9 14.0 13.1 14.0
14.7 13.6 11.4 14.7 12.7 14.4
13.8 13.0 11.9 16.6 d d
Forage type
CP (%)
Alfalfa–brome Brome–orchard–trefoil Southern Iowa pastures Smooth brome Smooth brome–red clover Tall fescue–red clover
d 15.8 17.1 d d d
Forage type
TDN (%, calculated from ADF)
Alfalfa–brome Brome–orchard–trefoil Southern Iowa pastures Smooth brome Smooth brome–red clover Tall fescue–red clover
d 38.5 55.8 d d d
64.1 57.2 57.1 67.1 52.6 57.4
53.8 58.0 52.0 57.7 57.0 58.4
54.3 58.7 48.7 57.0 58.2 59.3
55.0 57.8 47.1 58.6 56.1 57.4
53.5 56.6 46.3 59.4 d d
Abbreviation: ADF, acid detergent fiber. Data from Strohbehn D, Loy D, Morrical D, et al. A summary of monthly nutrient values for research pastures in growing months. Ames (IA); Iowa State University Animal Industry Report. 2004:ASL R187.
several by-products of the grain and food-processing industries are shown in Table 4. This table includes not only the grain processing by-products but also the locally important by-products of other food-processing industries. Beet pulp Beet pulp is the fibrous material remaining after sugar beets have been processed. Beet sugar production occurs in five regions of the United States: Great Lakes (primarily Michigan), Red River Valley (Minnesota and North Dakota), Great Plains (Northeast Colorado through Southeast Montana), Northwest (Idaho, Washington, and Oregon) and California [7]. Most sugar beet processing plants are located in these production areas. In 2006, the United States produced 33.6 million tons of sugar beets, according to the US Department of Agriculture National Agricultural Statistics Service. This quantity of beets should produce 1.18 million tons of dried beet pulp and 1.5 million tons of molasses annually. Beet pulp has a modest protein content (9.8%) and is rich digestible fiber. At 74% TDN, beet pulp is an excellent energy source for grazing cattle, but it is virtually devoid of starch. Citrus pulp In 2005–2006, more than 8 million tons of citrus fruits were processed in the United States [8]. Dried citrus pulp represents approximately 10% of the original weight of the processed fruit [9]. Citrus pulp is primarily a
47
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
Table 4 Nutrient content of selected by-product feedstuffs Nutrient content (DM basis) Feed
DM (%)
TDN (%)
NEm (Mcal/lb)
NEg (Mcal/lb)
CP (%)
Ca (%)
P (%)
Beet pulp Brewer’s grains Citrus pulp Dried distillers grain þ solubles Corn gluten feed Molasses, beet Molasses, cane Soyhulls Wheat middlings
91 90 91 90 90 78 74 990 89
74 66 82 90 80 75 72 77 69
.80 .68 .91 .99 .88 .81 .77 .84 .73
.52 .41 .61 .68 .59 .53 .50 .55 .45
9.8 29.2 6.7 30.4 23.8 8.5 5.8 12.2 18.7
.68 .29 1.88 .07 .07 .15 1.00 .53 .17
.10 .70 .13 .95 .61 .03 0.10 .18 1.01
Adapted from National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996.
by-product of orange and grapefruit processing. Similar to beet pulp, the protein content of citrus pulp is modest (6.7%). Although it contains little or no starch, it is an excellent high-fiber energy source (82% TDN). Citrus pulp is also rich in Ca. Brewer’s grain More than 195 million barrels of beer were produced in the United States in 2005 [10]. Brewer’s grains may be dried and used as a supplement for grazing cattle; however, most of the product is used in its wet form by dairies and feedlots located within a few miles of the various sites of production. Brewer’s grains, considered an intermediate protein source (29.2%), are relatively low in energy (66% TDN) among the grain and food-processing by-products. Soybean hulls Soybean hulls are the seed coat of the soybean kernel. Soybean hulls represent approximately 5% of the original weight of the soybean kernel, and yield about 3 pounds per bushel of whole, raw soybeans [11]. In 2005, 61.7 billion bushels of soybeans were processed in the United States. [12]. Soybean meal may or may not be dehulled in the oil extraction process. Generally, soybean meal that has not been dehulled is marketed as 44% CP soybean meal; so-called ‘‘high-protein soybean meal’’ (48% CP) has been dehulled. Soybean hulls are an excellent source of highly digestible fiber for forage-based beef cattle diets. On average, soybean hulls contain 77% TDN and 12.1% CP. Wheat middlings During the wheat milling process, 70% to 75% of the grain, by weight, is converted to wheat flour. The remainder of the kernel is used as animal feed
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[13]. This animal feed by-product can be referred to by several names; the most common is ‘‘wheat middlings.’’ In 2005 and 2006, 914 million bushels of wheat were processed for food, yielding approximately 8.5 million tons of wheat middlings [14]. The nutritional character of wheat middlings may vary according to wheat variety and processing method but, in general, they are moderate in energy (69% TDN) and moderate in protein (18.7%). Molasses Molasses is produced as a by-product of the beet and cane sugar-processing industries. Sugarcane production in 2005 and 2006 totaled nearly 25 million tons, according to the US Department of Agriculture. Sugarcane production, coupled with the 33.6 million tons of sugar beets processed that year, produced as much as 2.3 million tons of molasses. Molasses contains very little protein (O5% CP) but is rich in sugar-based energy (72%–75% TDN). In the feed industry, most molasses is used as a basis for dietary supplements that contain natural protein, urea, minerals, vitamins, ionophores, and antibiotics (ie, micro ingredients). Often, molasses is a major ingredient in free-choice, intake-limited supplements specifically developed for grazing cattle. Corn gluten feed Corn gluten feed is a by-product of the wet corn–milling industry. Wet corn milling is designed to facilitate the extraction of starch from the corn kernel and its conversion to dextrose. Subsequently, the dextrose is converted to high fructose corn syrup, or fermented to produce ethanol. Livestock feeds produced from the wet-milling process include corn gluten meal, corn germ meal, condensed steepwater solubles, and corn gluten feed. Corn gluten feed is a combination of corn bran and the condensed steepwater solubles. The wet-milling process yields roughly 13.5 lb (DM) of corn gluten feed, 2.5 lb of corn gluten meal, 1.6 lb of corn oil, and 33 gallons of corn sweetener per bushel of corn [15]. Alternatively, 2.3 gallons of ethanol may be produced in lieu of the corn sweetener. In 1999, 9.45 million tons of corn gluten feed (DM) were produced in the United States. Corn gluten feed is moderate in protein (16%–20%) and a good energy source for cattle (80% TDN). Dried distillers grains with solubles The ethanol industry is undergoing a rapid expansion in the United States. According to the Renewable Fuels Association, 3.9 billion gallons of ethanol were produced in the United States in 2005. The National Corn Growers Association estimated that production could swell to 15.9 billion gallons by 2015. Distillers grains and distillers solubles are the primary by-products of ethanol production by way of dry corn milling. In this process, the grain
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
49
(primarily corn or sorghum) is ground, and ethanol is fermented from starch in the grain. Because corn is approximately two thirds starch, the remaining nutrients (eg, protein, fiber, trace minerals) are concentrated in the thin stillage, the material remaining after extraction of alcohol. The solid components of thin stillage become distillers grains, and the liquid becomes distillers solubles. Solubles are condensed through heating and evaporation and may be sold for livestock feed as condensed distillers solubles. Most solubles, however, are added to the distillers grains to create distillers grains with solubles. Distillers grains with solubles may be available locally in the wet form (approximately 70% moisture), as modified or partially dried distillers grains (DDG) with solubles (approximately 50% moisture), or as dried distillers grains with solubles (DDGS; 10% moisture). DDGS are an excellent protein source (30% CP) for grazing cattle. Because the fiber and oil in the original grain are concentrated in the DDGS, it is also an excellent source of fiber-based energy (90% TDN). Rapid expansion in the ethanol industry is changing world perspectives on corn. The grain is no longer chiefly a source of feed energy for livestock. It has become a raw material for both food and fuel. Competition with the renewable fuels industry for feed grains represents a new era for the livestock industry. Fortunately, by-products of corn processing are excellent protein and energy supplements for grazing cattle. Prospects for an increased supply of these feeds, along with dwindling availability of feed grains, suggest that economic use of grain processing by-products will increase in the future.
Response to supplementation The response of cattle to various supplementation programs can be assessed in different ways. The response of stocker cattle to supplementation may be measured in terms of improved daily gain or increased carrying capacity of the forage base. Similarly, the response of cows to supplementation may influence pasture carrying capacity, cow body condition, cow reproductive performance, and calf growth. By-products of the grain processing industry typically contain little or no starch. Rather, the energy that they contain is a highly digestible form of fiber or cellulose. Cellulose is also the major carbohydrate in forages. Several studies have been conducted that demonstrate a depression in digestion of plant cellulose when feeds that are rich in starch (ie, grains) are fed with forage-based diets [16]. Fiber-rich by-product feeds do not interfere with ruminal digestion of plant cellulose. As a result, numerous studies of forage-fed cattle report that by-product supplements support production that is equal to, or greater than, grain supplements, even though by-products appear to have lesser energy yields than grains [17]. The response of forage-fed animals to supplementation with by-product feeds and grains is influenced by forage
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quality [18]. By-product feeds and grains tend to promote similar levels of animal response when added to high-quality forage diets. Conversely, byproduct feeds tend to promote a much more vigorous response, in terms of intake and digestion, than do grains, when added to low-quality forage diets. Kunkle and coworkers [19] reviewed a number of studies in which the performance of forage-fed cattle supplemented with high-fiber by-products was compared to that of grain-fed cattle. It was noted that gain responses were similar to grain when cattle were fed soy hulls, corn gluten feed, beet pulp, or citrus pulp. In a 3-year summary of studies, a combination of wheat middlings and soybean hulls were compared with grain-based supplementation fed to stocker cattle grazing wheat pasture (ie, a high-quality forage) [20]. Daily gains were 2.03, 2.32, and 2.38 lb per day for unsupplemented, grainsupplemented, and by-product-supplemented cattle, respectively. The supplemented cattle were fed at a rate of 0.75% of body weight per day; stocking rates on wheat pasture were increased 22% to 44% by supplementation. Supplementation of high-quality forages generally reduces forage intake, allowing the supply of forage to be extended, or stocking rates to be increased. Conversely, supplementation of low-quality forages with high-protein supplements (ie, O30% CP) generally increases forage intake. This change in forage consumption associated with supplemental feeding has been termed the ‘‘substitution coefficient,’’ and has been reviewed [21]. The substitution coefficient is defined as the change in basal forage intake divided by supplement intake. The extent of the change in forage intake with supplementation is related to the quantity of supplemental energy intake, the energy-to-protein ratio of the forage, and the level of voluntary forage intake [22]. Responses of beef cows to supplementation may include improved cow condition and reproductive performance [16]. Corn gluten feed has been used effectively to maintain cow condition for cows grazing stockpiled winter forage [23]. Distillers grains are also effective in this role. In a Nebraska study, DDG were supplemented at rates of 0, 1.0, 2.1, 3.1, or 4.2 lb per day to heifers grazing bromegrass (ie, a high quality forage) [24]. For each pound of DDG that was fed, 1.72 fewer pounds of forage were consumed (DM basis). Heifer weight gains increased 0.06 lb per day for each pound of DDG consumed. The economic relationship between DDG and forage value was calculated as DDG value ð$=TÞ ¼ 5:07 forage value ð$=animal unit month ½AUMÞ þ 66:08 Standardized Performance Analysis summaries reported an average economic value of pasture grazing in the Midwest for cow–calf producers of $14.26 per AUM [25]. The value for corn stalk grazing was $3.43 per AUM. Using these values and the Nebraska equation, DDG have a value of $138.38 and $83.47 in terms of pasture and cornstalk grazing, respectively.
BY-PRODUCT FEED UTILIZATION BY GRAZING CATTLE
51
Knowledge of pasture productivity and quality is the first step in developing a sound supplementation program. Once the patterns of pasture growth and nutrient yield have been established, animal requirements and production goals can be determined. An assessment of the expected response to supplementation, taking into account the anticipated value of additional gain, is essential in determining the potential returns on investment in a supplementation program. Pasture systems are complex, and each operation has a different set of costs and constraints. Knowledge of the basic animal requirements and forage-supplement tradeoffs lead to better, more informed decisions.
References [1] National Research Council. Nutrient requirements of beef cattle. Washington, DC: National Academy Press; 1996. [2] Boggs DL, Smith EF, Schalles RR, et al. Effects of milk and forage intake on calf performance. J Anim Sci 1980;51:550–3. [3] Corah LR, Dargatz D. Forage analyses from cow/calf herds in 18 states. Beef cow/calf health and productivity audit. Fort Collins (CO): National Animal Health Monitoring System. USDA; 1996. [4] Barnhart SK. Guide to year round forage supply. Iowa State University Extension Publication Pm 1771; 1998. [5] Strohbehn D, Loy D, Morrical D, et al. A summary of monthly nutrient values for research pastures in growing months. Iowa State University Animal Industry Report. ASL R187; 2004. [6] Smith E. Growing cattle on grass. Kansas State University Agricultural Experiment Station Bulletin 638; 1981. [7] Ali B. Characteristics and production costs of U.S. sugarbeet farms. United States Department of Agriculture Statistical Bulletin SB974-8; 2004. [8] USDA. Citrus-fruits (2006 summary). Agricultural Statistical Bulletin Fr Nt 3-1(06); 2006. [9] Goodrich RM, Braddoah RJ. Major by-products of the Florida citrus-processing industry. University of Florida IFAS Extension FSHN05-22; revised 2006. [10] Robinson M. Brewers almanac. Washington, DC: Beer Institute; 2006. [11] Blasi D, Drouillard J, Titgemeyer EC, et al. Soyhulls: composition and feeding value for beef and dairy cattle. Kansas State University Agricultural Experiment Station Mt-2438; 2000. [12] USDA. Agricultural Outlook - Statistical Indicators. Economic Research Service, 2006. Available at: http://www.ers.usda.gov/publications/aotables/2006/12Ded/Ao1206.pdf. [13] Blasi DA, Reed CL, Dionisia M, et al. Wheat middlings: composition, feeding value and storage guidelines. Kansas State University Agricultural Experiment Station MF-2353; 1998. [14] USDA. Wheat Outlook. ERS WHS-06; 2006. [15] Loy D, Miller W. Ethanol coproducts for cattle: the process and products. Iowa State University Extension Publication IBC-18; 2002. [16] Winger H, ZoBell DR, Olson KC. Supplementation of energy and protein for beef cattle, a literature review. Utah State University Extension Publication. AG/Beef/2006-01; 2006. [17] Merchen NR. Digestion, absorption and excretion in ruminants. In: Clark DC, editor. The ruminant animal. Englewood Cliffs (NJ): Prentice Hall; 1988. p. 172–201. [18] Summer P, Trenkle A. Effects of supplementing high or low quality forages with corn or corn coproducts upon digestibility of dry matter and energy by steers. Iowa State University Beef Research Report. ASL-R1540; 1998.
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[19] Kunkle WE, Johns JT, Poore MH, et al. Designing supplement programs for beef cattle fed forage-based diets. Proceedings American Society of Animal Science 1999. Available at: http://www.asas.org/jas/symposia/proceedings/0912.pdf. [20] Horn GW, Beck PA, Andrae JG, et al. Designing supplementation for stocker cattle grazing wheat pasture. J Anim Sci 2005;83(E Suppl):E69–78. [21] Caton JS, Dhuyvetter DV. Influence of energy supplementation on grazing ruminants: requirements and responses. J Anim Sci 1997;75:533–42. [22] Moore JE, Brant MH, Kunkle WE, et al. Effects of supplementation on voluntary forage intake, diet digestibility and animal performance. J Anim Sci 1999;77(Suppl 2):122–35. [23] Russell J, Driskoll R, Moricall D, et al. Effects of stocking rate and corn gluten feed supplementation on performance of two-year old cows grazing stockpiled forage during winter. Iowa State University Animal Industry Report. ASL-R2065; 2006. [24] MacDonald JC, Klopfenstein T. Dried distillers grains as a grazed forage supplement. Nebraska Beef Report; 2004. p. 25–7. [25] Miller A, Knipe R. SPA. Summary beef cow business record final report. Available at: http: www.iowabeefcenter.org/content/SPASummary.pdf. 2004.