The effects of corn silage feeding level on steer growth performance, feed intake and carcass composition. Summary The influence of corn silage feeding level was examined on eighty-three Charolais crossbred steers (389+30 kg BW). Each steer was randomly assigned to one of four diets which contained either 0, 15, 50 or 85 percent corn silage (CS) dry matter. Each ration was balanced to a 13.6 percent crude protein level. Mineral, vitamin and ionophore fortifications (monensin @ 33 ppm) were similar for each ration. The cattle were fed ad libitum with DM intakes for individual steers recorded weekly. The steers were slaughtered in three groups after achieving a 6 millimetre backfat thickness (ultrasound) or a 600 kg body weight. The cattle were individually fed their assigned rations for either 119, 139 or 168 days before being transported to an abattoir, where they were processed and carcass data was collected. P.L. McEwen Ridgetown College, University of Guelph yardage) were greater (per kilogram of gain) for cattle fed the high grain or concentrate diet. A trend towards higher gain costs as grain level increased was observed. In contrast, hot carcass weights, marbling grade scores and ribeye area measurements were unaffected by diet while subcutaneous fat measurements were reduced for the high versus intermediate silage rations. A similar level of marbling was achieved on the high silage cattle while grade fat thicknesses were reduced. The present results are consistent with a previous trial (Beef Research Update, 1998/99) initiative where ribeye area and marbling grades were also unaffected by silage feeding level. More research is needed however, to determine if silage feeding level will influence the taste and other quality attributes of the end product, beef. Introduction Silage level influenced steer growth rate and days to market. Steers fed diets with 85 percent silage grew at a slower rate than the 50 percent silage group while other daily gain estimates were similar. Days to market were also increased for the high silage cattle when compared to the two intermediate groups (15 and 50 percent forage dry matter), while cattle fed the high concentrate ration (0% corn silage) were on-feed for a similar number of days. Steers fed the 50 percent silage diet consumed more daily dry matter while dry matter to gain estimates were similar for each diet. As a result, cost estimates (feed + Historically, corn silage varieties have been selected on their grain and dry matter yield capabilities. However there is evidence that corn silages can also differ in digestibility. Previous trial results at Ridgetown College (1995-7) indicated that steer growth rates could be influenced by differences in silage grain content and estimates of digestibility. The results were based on high levels of corn silage feeding (85 to 92 percent of diet dry matter). In a more recent initiative (Beef Research Update, 1998/99) four silage feeding levels (20, 40, 60 or 80 percent silage DM) were compared based on measurements of steer performance and carcass quality. In this trial,
roughage feeding level influenced steer growth rate and days to market. Steers fed diets with 80 percent silage grew at a slower rate and required more days to finish. Growth rates and days on feed were similar for the other three diets, however. Total dry matter intake was also greater for the high silage diet while average dry matter intakes per day were similar for each group. Feed efficiency estimates were therefore increased (kilograms DM/kg gain) for the high silage cattle. However, all carcass traits were similar for each silage level with the exception of marbling score. The two intermediate rations produced cattle with higher marbling scores than the low silage-high concentrate diet. Therefore the primary objective for the present trial was to further investigate the effects of corn silage feeding level on steer growth rate, feed intake and carcass composition. A second objective was to (partially) evaluate the effect of silage feeding level on economic returns. Materials and Methods The corn hybrid, Pioneer 37M81, was planted to evaluate the effect of corn silage feeding level on beef steer growth performance and carcass quality. Seven hectares of the variety were planted at 79,330 seeds per hectare. The planting was completed on the same day, in 76 centimetre rows, to minimize environmental differences within the field. After a very dry growing season, the silage was harvested over a two day period. Silage samples and load weights were recorded during the harvesting process. The samples were then identified and frozen for a later nutritional (laboratory) analysis. The silage was stored in an upright stave silo located at the Ridgetown Beef Research Facility. Eighty-four Charolais crossbred steers were purchased from one source and location. After arrival, the cattle were weighed, divided into groups of six and assigned to their respective pens. A three week adjustment period was needed for the steers to adapt to the Calan feeding system. The steers were also slowly introduced to their respective diets over this time period. Four diets were formulated for the feeding period. Each ration contained either 0, 15, 50 or 85 percent corn silage dry matter basis (table #1). A constant level (DM basis) of supplement (56% CP) was incorporated into the three diets which contained corn silage. A high fibre supplement (40% CP) was a feed ingredient for the whole corn diet. Supplement inclusion rates were standardized to ensure that ionophore (monensin @ 33 ppm) and vitamin fortifications were similar for each ration. A custom premix (one for each diet) was also added to ensure that mineral and crude protein levels (13.6 percent crude protein) were consistent for all rations. Silage samples were taken daily and mixed weekly. Silage moisture levels were then determined and monitored on a weekly basis. Rations were adjusted (if needed) to reflect changes in silage dry matter content. Weekly composite samples were also mixed monthly and stored for a later nutritional analysis which was completed by staff at the University of Guelph. The cattle were gradually introduced to their respective diets over a three week period and were being fed ad libitum before the trial officially began. Each animal remained on their assigned diet for the duration of their feeding period. The amount of feed offered at each feeding was automatically recorded with feed refusals (weighbacks) recorded weekly. At ninety day intervals, the cattle were also implanted with an estradiol implant to improve
measurements of steer performance. Steer initial weights were determined by averaging two consecutive day weights for each animal. The cattle were then weighed at 14 days intervals to assess biweekly weight gains. Fat cover (ultrasound) measurements (between the 12th and 13th rib) were also periodically taken to help determine a market endpoint for each steer. Each steer was sent to a federally inspected abattoir (3 groups) after achieving a 6 mm backfat (ultrasound) thickness or a 600 kg body weight. All carcasses were federally inspected and graded. Hot carcass weights, fat cover thicknesses (between 12 th and 13 th rib), marbling scores and ribeye area (cm 2 ) measurements were also recorded for use in the later analyses. Results and Discussion Covariables were included in each final model if they were found to significantly influence the dependent variable of interest (Table #2). Significant covariables were identified and included in each final analysis to more precisely compare the diets based on animal performance, feed intake and carcass measurements. The average starting weight for the trial cattle was 389±30 kg (table 2). Silage level influenced steer growth rate, days to market, daily dry matter intake, percent dry matter intake, grade fat, dressing percentage and carcass yield index. Cattle fed the high silage diet (85 percent corn silage DM) grew (kg/day) at a slower rate than the 50 percent silage group while other growth rate comparisons were similar. Days to market were also increased for the high silage cattle when they were compared to the two intermediate diet groups (15 and 50 percent forage DM). The high silage and whole corn fed cattle were marketed at a similar number of days (154.8 versus 145.2). Total dry matter intakes (per steer for trial) were similar for each dietary group while steers fed the 50 percent silage diet consumed more daily dry matter and had a higher dry matter intake percentage (% of body weight). In contrast, feed efficiency estimates were similar for each group due to a slight growth rate advantage for the 50 percent silage cattle. As noted in table 2, cost of gain estimates (feed + yardage) were directly related to the diet s concentrate level. As grain levels increased, cost of gain estimates also increased. A similar trend was also observed in a previous trial initiative (Beef Research Update 1998/99). The carcass measurements, grade fat thickness, dressing percentage and yield index were significantly influenced by diet. Subcutaneous fat (grade fat) estimates were reduced for the high versus intermediate silage rations (15 or 50 % corn silage DM). This observation was supported by a numeric advantage in yield index for the high silage cattle, while ribeye area (cm 2 ) measurements were similar for each dietary group. Marbling grades (from carcass) were also comparable for each ration indicating that sufficient energy levels were always present for adequate levels of intramuscular fat deposition. Dressing percentages were also similar for three of the four diets with an increased percentage observed for the 15 percent silage diet. An increased rumen capacity for the higher silage fed cattle might be a possible reason for this observed difference. Significant silage feeding level differences were therefore observed for steer growth rate, feed intake, days to market, grade fat thickness, dressing percentage and carcass yield index. In contrast, ribeye area and marbling estimates were unaffected by
roughage feeding level. Significance to the Industry Scott Leeson for their continued Ridgetown Beef Facility service. The trial indicated that silage feeding level influenced steer growth rate, feed intake and days to market. Cattle fed the high silage diet were marketed at a similar weight to other groups with less external fat cover. Days to market were also increased for the high silage group when compared to the two intermediate diets while ribeye area and marbling estimates were unaffected by roughage level. A preliminary cost analysis indicated that feed (will depend on ingredient costs) and overhead costs (cost/kg gain) could be reduced if the dietary silage content of feedlot rations were increased. More research is needed however, to determine if a higher roughage feeding level (for cattle) would be deleterious to the end product s taste, tenderness and overall acceptability. Acknowledgments The author would like to thank Agribrands Purina Ltd. for their financial assistance and help. Data collection by University of Guelph and BIO staff was also very much appreciated. I would also like to thank Gary Brien and
Table 1. Ration formulations for the trial on an as fed basis (36% corn silage DM). 85%Corn Silage (CS) 15% Concentrate 50% CS 50% Conc. 15% CS 85% Conc. 100% Conc. Ingredient Inclusion Rates (per tonne basis) Corn silage (kg) 934 708 299 - Cracked corn 11 218 607 - Whole corn - - - 810 56% Supplement 33 42 60-40% Supplement - - - 141 Premix 1, 2, 3 or 4 22 32 34 49
Table 2. EFFECT OF CORN SILAGE LEVEL ON BEEF STEER GROWTH PERFORMANCE AND CARCASS QUALITY. 85%Corn Silage (CS) 15% Concentrate 50% CS 50% Conc. 15% CS 85% Conc. 100% Conc. Significant Model Variables Growth Performance Number of Calves 21 21 21 20 Ave. Initial Weight Ave. Final Weight 381.6 382.1 395.2 398.7 595.9 592.6 596.7 605.6 Days to Market 154.8 a 134.0 b 136.6 b 145.2 ab initial weight Average Daily Gain 1.4 a 1.6 b 1.5 ab 1.4 ab ration Feed Intake Measurements Total Dry Matter Intake Average Dry Matter Intake (kg) Percent Dry Matter Intake (%) Feed Efficiency (DM/kg Gain) 1372.1 1407.4 1258.5 1269.8 8.8 a 10.3 b 9.3 a 8.8 a initial weight 1.8 a 2.1 b 1.9 a 1.8 a initial weight 6.4 6.7 6.3 6.2 Cost($)/kg gain 0.94 1.05 1.17 1.30 Carcass Measurements Grade Fat (mm) 6.2 a 8.3 b 7.9 b 7.1 ab initial fat thickness Hot Carcass wt. 333.4 332.0 343.8 339.9 Dressing Percentage 55.7 a 55.9 a 57.9 b 56.4 a Initial weight Yield Index (%) 61.4 a 60.0 b 60.4 ab 60.4 ab Initial weight Ribeye Area (cm 2 ) 83.0 81.6 83.3 80.9 Initial ribeye area Marbling Score (4 point scale) 2.2 2.1 2.5 2.4 a and b LS means within row that do not share a common superscript differ significantly (p < 0.05). - LS means without a superscript are similar within row (p < 0.05).