For cattle to successfully finish on pasture, abundant high-quality forage must be available to the grazing animals. Eighty-eight steers were finished on pasture with grain supplementation ranging from 0 to 75 % of the dietary energy supplied by grain. Pastures were intensively managed, cool-season, grass-legume pastures. Forage dry matter availability increased throughout the grazing season. The quality of the pastures also improved through the season, with crude protein (CP) content increasing and acid detergent fiber (ADF) content decreasing. Forage intake decreased at an average rate of 1 lb for each lb of grain fed. Observed average daily gains (ADG) were consistent with predicted ADG based on forage plus grain intake levels.
Introduction: To successfully finish cattle on pasture, forage quality must be high and forage availability maintained at adequate levels to ensure optimal intake. Blaser et al. (1977) suggest that energy intake will limit performance of ruminants grazing cool-season forages before protein or other nutrients. Energy content of perennial cool-season forages is most affected by maturity of the plant. Management of high energy potential pastures must focus on maintaining plants in a high quality, vegetative state. In this research we examined trends in forage availability, pasture quality, and voluntary forage intake.
Materials and Methods: A pasture-based, beef finishing project was conducted at the University of Missouri – Forage Systems Research Center in north-central Missouri in 1995. Eighty-eight steers were assigned to four grain feeding levels on pasture with each treatment replicated twice. Grain feeding levels were expressed as the percent of their total dietary energy intake supplied by grain and were 0, 25, 50 and 75 % with the remaining nutrients supplied by pasture. The pasture with steers receiving no grain was stocked at 1.0 steer to the acre. The pasture with steers receiving 25 % of their energy from grain was stocked at 1.25 steers per acre. The pasture with steers receiving 50 % of their energy from grain was stocked at 1.5 steers per acre. The pasture with steers receiving 75 % of their energy from grain was stocked at 1.75 steers per acre (Table 1).
During the first phase of the experiment, April 22 to August 22, the supplement was cracked corn. During the second phase, August 23 to October 30, the supplement contained 70 % cracked corn and 30 % corn gluten feed.
Each treatment consisted of 8 acres which were divided into six permanent paddocks (Fig 1). During the grazing season, these were further divided with temporary fences and animals were allowed to back graze the paddock in order to access the water supply. Each subdivision within a paddock provided the animals with 1 to 3 days of feed depending on the season. Rest periods ranged from 10 to 35 days depending upon season and subdivision within paddock. Pastures were clipped for seedhead control in early June after cool-season grasses had headed.
Table 1. Supplementation level, stocking rate, and number of steers per treatment group in pasture-based finishing study. ================================================================ Supplementation Stocking Number of level rate steers ---------------------------------------------------------------- % of diet steers/acre no. 0 1.00 8 25 1.25 10 50 1.50 12 75 1.75 14 ----------------------------------------------------------------
Within each treatment, individual paddocks were gridded into 900 ft2 blocks for pasture sampling purposed. Prior to the allocation of a new grazing strip, one 2.7 ft2 quadrat was cut from each grid block in that pasture allocation strip. Samples were oven dried and CP, ADF, and neutral detergent fiber (NDF) were determined using near infra-red reflectance spectroscopy. Forage samples were collected from May 16 to October 18, 1995. Species composition data were collected from these paddocks as well. Forage dry matter intake (DMI) was calculated by the difference method using quadrats clipped prior to and immediately following grazing of an individual paddock.
Results and Discussion: Forage quality of these cool-season grass-legume pastures increased during the season, with CP increasing and ADF decreasing. Crude protein was not limiting for the animals at any time during the grazing season. Crude protein requirement for medium-framed 700-pound steers gaining 2.0 lb/day is approximately 10%, according to the National Research Council (1984). Crude protein levels of the forage exceeded this requirement throughout the season (Fig. 2). This result is in concurrence with the claims of Blaser et al. (1977) that protein would not limit performance on cool-season pastures. Linear regression was used to determine trend in forage quality through the season using day of year as the independent variable. For CP the relationship of CP to day of year was significant (P=.05) for the 0%- and the 75%- grain groups and a strong trend held true for the 25% and 50% groups (P<.10). For ADF the relationship to day of year was also significant (P=.05) for the 0%- and 75%- grain levels, and again the strong trend held true for the 25% and 50% groups. Cool-season pastures are often cited as being low quality during the summer months. Results of this research indicate that cool-season pastures managed to maintain vegetative forage are quite high quality even through the summer months.
Forage dry matter availability also increased during the season. Accurate forage sampling was hampered in the early part of the season due to extremely wet weather so forage availability data is presented for only Phase II. Forage intake by grazing animals during the Phase I period also appeared to be depressed due to heat stress and excessive rainfall. Rainfall during the Phase I period was 18 in. above normal for the research location. Phase II forage availability at turn-in and daily forage intake are in Table 1. The observed intake indicates that the steers in the 0 grain treatment were consuming adequate forage dry matter to maintain the expected ADG of 2.0 pounds per day.
A concern about feeding high levels of grain on pasture is the substitution of grain intake for forage intake. Based on the intake data in Table 2, it appears that the first increment of grain fed has the greatest negative impact on forage intake. The substitution coefficients for 25-, 50-, and 75%-grain feeding levels were 1.26, 1.00, and .74, respectively. Steer performance in this study as reported by Martz et al. (1996) indicates very little difference in ADG between the 0- and 25%-grain groups. The lacck of response to grain supplementation at the 25% level may be the result of decreased forage intake in the presence of added grain in a quantity that was high enough to affect rumen performance but not high enough to increase ADG. Average forage availability was very similar between the 0- and 25%-grain supplemented pastures, suggesting that forage availability was probably not limiting intake. Mean forage availability in the 50- and 75%- grain supplemented pastures was significantly lower than the 0 and 25% grain pastures. As the steers receiving higher levels of supplementation increased body weight, their forage consumption in terms of pounds of dry matter per head likely increased more rapidly than the steers growing at a slower rate. More forage was, therefore, consumed in each grazing cycle and the residual following grazing was reduced. The lower residual dry matter resulted in slower regrowth and lower dry matter yield at turn-in on each subsequent grazing cycle. The availability was low enough that forage intake may have been limited on these pastures explaining why steer performance on the 75%-grain pastures was not as high as what would have been predicted.
In summary, forage quality tended to increase throughout the grazing season on all treatments. Forage availability at the beginning of each rotation remained near constant or slightly increased for the 0- and 25%-grain groups while availability tended to decrease slightly through the season for the steers receiving 50 and 75% grain levels. It appears that forage availability was more likely to limit steer performance than would forage quality on these mixed cool- season grass-legume pastures.
Blaser, R.E., W.C. Stringer, E.B. Rayburn, J.P. Fontenot, R.C. Hammes, Jr., and H.T. Bryant. 1977. Forage-Fed Beef, Production and Marketing Alternatives in the South. Southern Cooperative Series, Bull. 220.
Martz, F.A., J.R. Gerrish, and V.G. Tate. 1996. Performance of steers finished on pasture with four levels of grain supplementation. IN: M.J. Williams (Ed.) Proc. Amer. Forage Grassl. Council, Vol. 5. June 13-16, 1996, Vancouver, B.C., Canada. AFGC, Georgetown, TX. (In Press)
National Research Council (NRC). 1984. Nutrient requirements of beef cattle, sixth revised edition. National Research Council, National Academy Press, Washington, D.C.
Table 2. Forage dry matter availability and voluntary dry matter intake of steers grazing pasture at four levels of grain supplementation. ============================================================ Grain Available Voluntary supplementation dry matter forage level at turn-in intake ____________________________________________________________ % of diet -- lb/A -- - lb/hd/day - 0 2659 21.5 25 2583 12.7 50 1983 9.6 75 2161 8.2 _____________________________________________________________
1Research Assistant Professor, Research Professor of Animal Sciences and Superintendent, and Research Associate, respectively; University of Missouri-Forage Systems Research Center (FSRC), Route 1 Box 80, Linneus, MO 64653
This paper was published in the Proceedings 1996 AFGC Annual Conference Vancouver BC, June 12-16, 1996.