Proportion of corn silage in diets of feedlot steers fed to achieve stepwise increases in growth.

Two experiments were conducted to investigate the effects of proportion of dietary corn silage during periods of feed restriction on performance of steers. In Exp. 1, Simmental x Angus steer calves (n = 107; initial BW = 273 +/- 3.8 kg) were allotted to 12 pens with eight or nine steers/pen and four pens/treatment. Periods of growth were 273 to 366 kg BW (Period 1), 367 to 501 kg BW (Period 2), and 502 to 564 kg BW (Period 3). In two of the dietary regimens, steers were given ad libitum access to feed throughout the experiment and were fed either a 15% corn silage diet in each period or an 85, 50, and 15% corn silage diet in Periods 1, 2, and 3; respectively. In the third feeding regimen, a programmed intake feeding regimen was used. Steers were fed a 15% corn silage diet in each period. However, feed intake was restricted to achieve a predicted gain of 1.13 kg/d in Period 1 and 1.36 kg/d in Period 2, and feed was offered for ad libitum consumption in Period 3. For the entire experiment, ADG was similar (P = 0.41) among treatments and feed efficiency was lower (P < 0.10) for steers in the corn silage regimen than for steers in the programmed intake and ad libitum regimens. In Exp. 2, Simmental x Angus steer calves (n = 106; initial BW = 233 +/- 2 kg) were allotted by BW to 12 pens (three pens/treatment) and fed in three periods similar to those described in Exp. 1. Four feeding regimens were investigated: 1) AL; steers were offered a 15% corn silage diet for ad libitum consumption in all three periods; 2) PI; DMI was programmed to achieve gains as described in Exp. 1; 3) CS-HLL; programmed intake as described above except diets contained 85, 15, and 15% corn silage in Periods 1, 2, and 3, respectively; and 4) CS-HIL; same feeding regimens as CS-HLL, except diets contained 85, 50, and 15% corn silage in Periods 1, 2, and 3, respectively. Steers were given ad libitum access to feed in Period 3. Overall ADG was lower (P < 0.05) for steers in the CS-HLL and CS-HIL feeding regimens than for steers in the AL and PI regimens; feed efficiency was greatest for steers in the PI regimen. Few effects of feeding regimen on carcass characteristics were observed.     

Residual feed intake of purebred Angus steers: effects on meat quality and palatability

Relationships between residual feed intake (RFI) and other performance variables were determined using 54 purebred Angus steers. Individual feed intake and BW gain were recorded during a 70-d post-weaning period to calculate RFI. After the 70-d post-weaning test, steers were fed a finishing ration to a similar fat thickness (FT), transported to a commercial facility, and slaughtered. A subsample of carcasses (n = 32) was selected to examine the relationships among RFI, meat quality, and palatability. Steers were categorized into high (> 0.5 SD above the mean; n = 16), medium (mid; +/- 0.5 SD from the mean; n = 21), and low (< 0.5 SD below the mean; n = 17) RFI groups. No differences were detected in ADG, initial BW, and d 71 BW among the high, mid, and low RFI steers. Steers from the high RFI group had a greater DMI (P = 0.004) and feed conversion ratio (FCR; DMI:ADG; P = 0.002) compared with the low RFI steers. Residual feed intake was positively correlated with DMI (r = 0.54; P = 0.003) and FCR (r = 0.42; P = 0.002), but not with initial BW, d 71 BW, d 71 ultrasound FT, initial ultrasound LM area, d 71 ultrasound LM area, or ADG. The FCR was positively correlated with initial BW (r = 0.46; P = 0.0005), d 71 BW (r = 0.34; P = 0.01), and DMI (r = 0.40; P = 0.003) and was negatively correlated with ADG (r = -0.65; P = 0.001). There were no differences among RFI groups for HCW, LM area, FT, KPH, USDA yield grade, marbling score, or quality grade. Reflectance color b* scores of steaks from high RFI steers were greater (P = 0.02) than those from low RFI steers. There was no difference between high and low RFI groups for LM calpastatin activity. Warner-Bratzler shear force and sensory panel tenderness and flavor scores of steaks were similar across RFI groups. Steaks from high RFI steers had lower (P = 0.04) off-flavor scores than those from low RFI steers. Cook loss percentages were greater (P = 0.005) for steaks from low RFI steers than for those from mid RFI steers. These data support current views that RFI is independent of ADG, but is correlated with DMI and FCR. Importantly, the data also support the hypothesis that there is no relationship between RFI and beef quality in purebred Angus steers.     

Grass silage intake, rumen and blood variables, ultrasonic and body measurements, feeding behavior, and activity in pregnant beef heifers differing in phenotypic residual feed intake

The objectives of this study were to quantify the phenotypic variation in residual feed intake (RFI) in pregnant beef heifers offered a grass silage diet and to characterize their productivity. Seventy-three pregnant (mean gestation d 198, SD = 27 d) Simmental and Simmental × Holstein-Friesian heifers (mean initial BW 548, SD = 47.5 kg) were offered grass silage ad libitum. Heifer DMI, BW, BCS, skeletal measurements, ultrasonic fat and muscle depth, visual muscularity score, rumen fermentation, total tract digestibility, blood metabolite and hematology variables, feeding, and activity behavior were measured during an 84-d feed intake study. After parturition calf birth weight, calving difficulty, cow serum IgG, hematology variables, and calf humoral immune status were measured. In a subset of cows (n = 28), DMI, milk yield and various body composition variables were also measured approximately 3 wk postpartum. Phenotypic RFI was calculated for each animal as the difference between actual DMI and expected DMI. Expected DMI was computed for each animal by regressing average daily DMI on conceptus-adjusted mean BW(0.75) and conceptus-adjusted ADG over an 84-d period. Within breed, heifers were ranked by RFI into low (efficient), medium, and high (inefficient) groups by dividing them into thirds. Heifers with high RFI had 8.8 and 17.1% greater (P < 0.001) DMI than medium and low RFI groups, respectively. The RFI groups did not differ in ADG or BW (P > 0.05). Residual feed intake was positively correlated with DMI (r = 0.85) but not with feed conversion ratio, ADG, or BW. The RFI groups did not differ (P > 0.05) in skeletal size, BCS, ultrasonic fat depth, total tract digestibility, calf birth weight, calving difficulty, serum IgG concentrations, or milk yield. Visual muscularity scores, initial test and postpartum ultrasonic muscle depth were negatively correlated with RFI (P < 0.05). Including mean ultrasonic muscle depth into the base RFI regression model increased its R(2) (0.29 to 0.38). Pearson rank correlation between RFI and muscle-adjusted RFI was 0.93. The results show that efficient RFI heifers consumed less feed without any compromise in growth, body composition, or maternal traits measured.     

Effects of feeding level on efficiency of high- and low-residual feed intake beef steers

Comparing heat production after ad libitum (ADLIB) and restricted (RESTRICT) feeding periods may offer insight into how residual feed intake (RFI) groups change their energy requirements based on previous feeding levels. In this study, the authors sought to explain the efficiency changes of high- and low-RFI steers after feed restriction. To determine RFI classification, 56 Angus-cross steers with initial body weight (BW) of 350 ± 28.7 kg were individually housed, offered ad libitum access to a total mixed ration, and daily intakes were recorded for 56 d. RFI was defined as the residual of the regression of dry matter intake on mid-test BW^0.75 and average daily gain. High- and low-RFI groups were defined as >0.5 SD above or below the mean of zero, respectively. Fourteen steers from each high and low groups (n = 28) were selected for the subsequent 56-d RESTRICT period. During the RESTRICT period, intake was restricted to 75% of previous ad libitum intake on a BW^0.75 basis, and all other conditions remained constant. After the RESTRICT period, both RFI groups had decreased maintenance energy requirements. However, the low-RFI group decreased maintenance energy requirements by 32% on a BW^0.75 basis, more (P < 0.05) than the high-RFI group decreased maintenance requirements (18%). Thus, the low-RFI steers remained more efficient after a period of feed restriction. We conclude that feed restriction decreases maintenance energy requirement in both high- and low-RFI groups that are restricted to the same degree.     

Phenotypic and genetic relationships among feeding behavior traits, feed intake, and residual feed intake in steers fed grower and finisher diets

Data from a 3-yr feeding trial of crossbred steers (n = 331) were used to examine the relationship between feeding behavior traits and feed efficiency in steers fed grower and finisher diets, successively. There were 2 feeding periods each year whereby the steers were fed a grower diet in the first feeding period (P1) and a finisher diet in the second feeding period (P2). Each feeding period lasted for a minimum of 10 wk, ad libitum. In addition to feed intake, records on 3 measures of feeding behavior [feeding duration (FD), head-down time (HDT), and feeding frequency (FF)] were collected using the GrowSafe feeding system. Residual feed intake (RFI) was calculated by regression, after which the steers were classified as low (<0.5 SD), medium (±0.5 SD), or high (>0.5 SD) from the mean. The steers had greater (P < 0.001) FD, HDT, and FF when the grower diet was fed but greater feeding rate (FR) when the finisher diet was fed. Including the measures of feeding behavior as covariates to the feed intake prediction model containing ADG, metabolic midweight, and ultrasound backfat accounted for more variation in DMI than models that did not contain these additional parameters. The FD and HDT were significantly different (P < 0.05) among the RFI classes regardless of the feeding period, whereas no differences (P > 0.90) were found for FR among the RFI classes. For the growing period and finishing period, respectively, FD had phenotypic correlations with HDT (0.79, 0.83), FF (0.14, 0.55), DMI (0.38, 0.34), and FR (-0.34, -0.21). Heritability estimates in P1 and P2 for FD, HDT, and FF were 0.25 ± 0.16, 0.14 ± 0.11; 0.14 ± 0.15, 0.09 ± 0.10; and 0.56 ± 0.19, 0.59 ± 0.18, respectively. Genetic correlations between P1 and P2 were 0.91 ± 0.26, 0.93 ± 0.37, and 0.94 ± 0.11 for FD, HDT, and FF, respectively. The results suggest that it may be appropriate to include feeding behavior traits as covariates to indicate measure(s) of animal activity in the calculation of RFI. Feeding behavior phenotypes were greater during the grower-fed period than the finisher-fed period. During these feeding periods, efficient steers exhibited fewer FF, shorter FD, and shorter HDT than inefficient steers.     

Effect of source of energy and rate of growth on performance,carcass characteristics,ruminal fermentation,and serum glucose and insulin of early-weaned steers

Seventy-three crossbred steers (initial BW = 170.5 +/- 5.5 kg) from The Ohio State University (Exp. 1) and 216 crossbred steers (initial BW 135.4 +/- 4.4 kg) from the University of Illinois (Exp. 2) were used to determine the effect of source of energy and rate of growth on performance, carcass characteristics, and glucose and insulin profiles on early-weaned steers. Effects of the diets used in Exp. 1 and 2 on ruminal pH and VFA concentrations were quantified using ruminally fistulated steers (Exp. 3). Cattle were weaned at an average age of 119 d in all experiments and were allotted by age, BW, and breed to one of four diets: high-concentrate, fed ad libitum (ALCONC), high-concentrate fed to achieve a gain of either 1.2 kg/d (1.2CONC) or 0.8 kg/d (0.8CONC), or high-fiber, fed ad libitum (ALFIBER). At 218 d of age, all steers were placed on the ALCONC diet until slaughter. Steers were implanted with Compudose at the initiation of all experiments and with Revalor-S when they were estimated to be 100 d from slaughter. When steers in Exp. 1 averaged 181 and 279 d of age, serum samples were collected to determine glucose and insulin concentrations. Steers were slaughtered when a fat thickness of 1.27 cm was reached (Exp. 1) or after 273 d on feed (Exp. 2). In Exp. 1, days in the feedlot (P < 0.01) and age at slaughter (P < 0.01) were lowest for ALCONC and ALFIBER steers, and greatest for 0.8CONC steers. Overall, ADG was greatest for ALCONC and lowest for 0.8CONC steers; feed efficiency was lowest (P < 0.01) for ALFIBER steers. Final BW did not differ (P > 0.57) among treatments. At 181 and 218 d of age, serum insulin was increased (P < 0.10) and intramuscular fat percentage was greatest (P < 0.07), respectively, for ALCONC steers. In Exp. 2, overall ADG (P < 0.06) and final BW (P < 0.04) were greatest for ALCONC and lowest for 1.2CONC and 0.8CONC steers. Overall feed efficiency was greatest for 0.8CONC and lowest for ALFIBER (P < 0.01). Growing phase diet did not affect marbling score at 218 d of age or at slaughter (P > 0.81). In Exp. 3, differences in ruminal pH after feeding may have been a consequence of increasing acetate (ALFIBER), propionate (ALCONC), or a combination of VFA (0.8CONC and 1.2CONC), respectively (diet x time after feeding, P < 0.10). Controlling growth by limit-feeding a high-concentrate diet for only 100 d does not extend the growth curve of early-weaned steers or enhance intramuscular fat deposition at slaughter compared to ad libitum intake of a high-concentrate or high-fiber diet.     

Cattle selected for lower residual feed intake have reduced daily methane production

Seventy-six Angus steers chosen from breeding lines divergently selected for residual feed intake (RFI) were studied to quantify the relationship between RFI and the daily rate of methane production (MPR). A 70-d feeding test using a barley-based ration was conducted in which the voluntary DMI, feeding characteristics, and BW of steers were monitored. The estimated breeding value (EBV) for RFI (RFI(EBV)) for each steer had been calculated from 70-d RFI tests conducted on its parents. Methane production rate (g/d) was measured on each steer using SF(6) as a tracer gas in a series of 10-d measurement periods. Daily DMI of steers was lower during the methane measurement period than when methane was not being measured (11.18 vs. 11.88 kg; P = 0.001). A significant relationship existed between MPR and RFI when RFI (RFI(15d)) was estimated over the 15 d when steers were harnessed for methane collection (MPR = 13.3 x RFI(15d) + 179; r(2) = 0.12; P = 0.01). Animals expressing lower RFI had lower daily MPR. The relationship established between MPR and RFI(15d) was used to calculate a reduction in daily methane emission of 13.38 g accompanied a 1 kg/d reduction in RFI(EBV) in cattle consuming ad libitum a diet of 12.1 MJ of ME/kg. The magnitude of this emission reduction was between that predicted on the basis of intake reduction alone (18 g x d(-1) x kg of DMI(-1)) and that predicted by a model incorporating steer midtest BW and level of intake relative to maintenance (5 g x d(-1) x kg of DMI(-1)). Comparison of data from steers exhibiting the greatest (n = 10) and lowest (n = 10) RFI(15d) showed the low RFI(15d) group to not only have lower MPR (P = 0.017) but also reduced methane cost of growth (by 41.2 g of CH(4)/kg of ADG; P = 0.09). Although the opportunity to abate livestock MPR by selection against RFI seems great, RFI explained only a small proportion of the observed variation in MPR. A genotype x nutrition interaction can be anticipated, and the MPR:RFI(EBV) relationship will need to be defined over a range of diet types to account for this.     

Circulating ghrelin concentrations fluctuate relative to nutritional status and influence feeding behavior in cattle

The objective of these experiments was to establish the relationship of plasma ghrelin concentrations with feed intake and hormones indicative of nutritional state of cattle. In Exp.1, 4 steers (BW 450 +/- 14.3 kg) were used in a crossover design to compare plasma ghrelin concentrations of feed-deprived steers with those of steers allowed to consume feed and to establish the relationship of plasma ghrelin concentrations with those of GH, insulin (INS), glucose (GLU), and NEFA. After adaptation to a once-daily feed offering (0800), 2 steers continued the once-daily feeding schedule (FED), whereas feed was withheld from the other 2 steers (FAST). Serial blood samples were collected via indwelling jugular catheter from times equivalent to 22 h through 48 h of feed deprivation. Average plasma ghrelin concentrations were greater (P < 0.001) in FAST compared with FED (690 and 123 +/- 6.5 pg/mL) steers. Average plasma ghrelin concentrations for FED steers prefeeding were elevated (P < 0.001) when compared with those postfeeding (174 and 102 +/- 4.2 pg/mL, respectively). Average plasma GH concentration was elevated (P < 0.05) for FAST steers compared with FED steers. Plasma GLU concentrations were not different; however, for FAST steers, NEFA concentrations were elevated (P < 0.001) and INS concentrations were decreased (P < 0.001). In Exp. 2, 4 steers (BW 416 +/- 17.2 kg) were used in a crossover design to determine the effects of i.v. injection of bovine ghrelin (bGR) on plasma GH, INS, GLU, and NEFA concentrations; length of time spent eating; and DMI. Steers were offered feed once daily (0800). Serial blood samples were collected from steers via indwelling jugular catheter. Saline or bGR was injected via jugular catheter at 1200 and 1400. A dosage of 0.08 microg/kg of BW bGR was used to achieve a plasma ghrelin concentration similar to the physiological concentration measured in a FAST steer in Exp. 1 (1,000 pg/mL). Injection of bGR resulted in elevated (P < 0.005) plasma GH concentrations after the 1200 but not the 1400 injection. Plasma INS, GLU, and NEFA concentrations were not affected by bGR injection. For the combined 1-h periods postinjection, length of time spent eating was greater (P = 0.02) and DMI tended to be increased (P = 0.06) for bGR steers. These data are consistent with the hypothesis that ghrelin serves as a metabolic signal for feed intake or energy balance in ruminants.     

Effects of programmed growth rate and days fed on performance and carcass characteristics of feedlot steers.

Two experiments were conducted to investigate a feeding regimen in which a programmed amount of feed was offered daily to control growth rate of steers. In Exp. 1, steers (n = 107, 309 +/- 3 kg) were used to determine effects of offering ad libitum access to feed (AL) vs a programmed intake feeding regimen (PI) and the number of days steers were fed (168 vs 203) on performance and carcass characteristics. Steers in the programmed intake feeding regimen were fed to achieve a predicted gain of 1.13 kg/d for the first 78 kg of gain, 1.36 kg/d for the next 124 kg of gain, and were given ad libitum access to feed for the final 54 or 103 kg of gain before slaughter (for steers fed for 168 d or 203 d, respectively). Feed efficiency was greater (P < 0.02) for steers in the PI than for those in the AL feeding regimen (0.193 vs 0.183 kg gain/kg feed, respectively). From d 169 to 203, steers in the PI feeding regimen had greater (P < 0.06) ADG (1.60 vs 1.38 kg/d) and similar (P = 0.38) feed efficiency than steers in the AL regimen. In Exp. 2, steers (n = 96; 308 +/- 3 kg BW) were offered feed ad libitum throughout the experiment (AL) or were programmed to gain at a high (PI-H) or low (PI-L) growth rate. For the first 78 kg of gain, intake was restricted to achieve predicted gains of 1.13 kg/d (PI-L) or 1.25 kg/d (PI-H). For the next 124 kg of gain, intake was restricted to achieve predicted gains of 1.36 kg/d (PI-L) or 1.47 kg/d (PI-H). Feed was offered ad libitum for the final 58 kg of gain. Overall ADG was similar (P > 0.37) among feeding regimens despite lower DMI for the steers in the PI-L and PI-H feeding regimens than for those in the AL regimen. Feeding regimen did not affect (P < 0.22) carcass characteristics. Programmed intake feeding regimens sustained growth rate and feed efficiency for an extended period of time without detrimental effects on carcass characteristics.     

Influence of clostridial vaccines and injection sites on performance, feeding behavior, and lesion size scores of beef steers.

Several clostridial vaccines are currently being used in the beef cattle industry. Of greatest concern is altering the location and route of administration of these vaccines to reduce injection-site lesions while maintaining seroconversion. Two experiments were conducted to determine the effect of clostridial vaccines and injection sites on the performance, feeding behavior, and lesion size scores of beef steers. In Exp. 1, 80 crossbred beef steers (BW 237 +/- 3.2 kg) were allotted randomly into five groups and given 14 d to adapt to the feed and individual feed intake-monitoring devices (Pinpointer devices) before starting the study. Each group was assigned randomly to one of the following vaccination treatments: 1) control (sterile saline water), 2) Alpha-7 Ear (A7E), 3) Alpha-7 Prescapula (A7P), 4) Vision-7 Prescapula (V7P), and 5) Ultrabac-7 Prescapula (U7P). All vaccines were injected s.c. in the ear or prescapular region, and injection sites were palpated on d 0 and 28 (Exp. 1) and on d 63 and 91 (Exp. 2). The protocol for Exp. 2 was exactly the same as for Exp. 1 except treatments included control, A7P, Alpha-CD Ear (ACDE), Alpha-CD Prescapula (ACDP), Fortress-7 Prescapula (F7P), and V7P. Also, control and steers receiving F7P and V7P were revaccinated on d 63 and palpated on d 91. Results of Exp. 1 indicated that the A7E and U7P steers had a feed intake lower (P < 0.01) than all other treatment groups. The ADG of the A7P and A7E steers were not different (P > 0.05) from those of the control steers. The gain:feed ratio of the A7E steers was 41% higher (P < 0.01) than that of the V7P steers (Exp. 1). The results of Exp. 2 indicated that the control, ACDP, and V7P steers had greater (P < 0.01) ADG than all other treatment groups, but the gain:feed ratios were not different (P > 0.05) among all treatment groups. Lesion sizes differed by vaccine and injection site in both experiments. These data suggest that vaccinating beef steers s.c. in the ear produced gain:feed ratios and lesion size scores that were similar to prescapular vaccinations. However, more research is required to determine the immune response of vaccinating cattle in the ear.     

Performance and carcass quality of steers fed whole raw soybeans at increasing inclusion levels

Two experiments were conducted to determine the efficacy of whole raw soybeans as a partial or whole replacement for soybean meal in a corn/soybean meal-based feedlot diet. In Exp. 1, 80 crossbred steers (average BW = 441.3 kg) and, in Exp. 2, 96 Angus-sired steers (average BW = 413.7 kg) were blocked by weight and assigned randomly to one of four dietary treatments. Treatments were 0, 8, 16, and 24% dietary inclusions of whole raw soybeans. Diets within experiments were isonitrogenous. Across experiments, diets were similar, differing only in amount of corn silage (8 vs. 15% DM) at the expense of whole, shelled corn for Exp. 1 and Exp. 2, respectively. No treatment differences were observed for ADG or final BW. Dry matter intake from d 0 to d 58 decreased linearly (P < 0.05) with increased inclusion of whole raw soybeans in Exp. 1, with no effect on feed efficiency. In Exp.2 from d 0 to 72, whole raw soybean inclusion had no effect on DMI or feed efficiency. There tended (P < 0.10) to be a linear reduction in hot carcass weight when whole raw soybeans were included in Exp. 1. Unexpectedly, longissimus muscle area tended (P < 0.10) to respond quadratically (P < 0.10) to the increased inclusion of whole raw soybeans in Exp.1. No differences were detected in marbling score, 10th-rib backfat, or yield grade for Exp. 1 and 2 steers. In Exp. 2, inclusion of whole raw soybeans had no effect on hot carcass weight or longissimus muscle area. Incrementally increasing the inclusion of whole raw soybeans in the diet of feedlot steers had little overall effect on weight gain, feed efficiency, or carcass quality in Exp. 1 and 2. There were subtle differences in the treatment responses observed for hot carcass weight and longissimus muscle area between Exp. 1 and Exp. 2 for the 24% inclusion level. These noted differences may indicate that inclusion levels above 24% might not be beneficial.     

The effect of residual feed intake classification on forage intake by grazing beef cows

Although feed intake and efficiency differences in growing cattle of low and high residual feed intake (RFI) classification have been established, little is known about the difference in grazed forage intake between beef cows of known RFI classification. Two experiments were conducted using Hereford cows for which RFI had been determined as heifers using the GrowSafe 4000E feed intake system, after which heifers had been divided into thirds as low RFI, mid RFI, and high RFI. During Exp. 1, 2 replicates of low and high RFI cows (n = 7/replicate) in mid- to late-gestation were blocked to 1 of 4 non-endophyte-infected tall fescue paddocks (1.8 to 2.4 ha), which they grazed continuously for 84 d during summer. Using grazing exclosures, weekly rising plate meter readings, and forage harvests every 21 d, average forage DMI was calculated. Low and high RFI groups did not differ (P > 0.05) in BW change or BCS change over the trial (19.5 vs. 22.1 kg of BW gain and 0.11 vs. 0.10 BCS gain), but low RFI cows had a 21% numerically lower DMI than high RFI cows (12.4 vs. 15.6 kg/d; P = 0.23). The average area needed per paddock over the trial was similar for low and high RFI cows (1.71 vs. 1.82 ha; P = 0.35), and the average DM on offer over the trial was less for low RFI than for high RFI cows (4,215 vs. 4,376 kg; P = 0.06). During Exp. 2, 3 replicates of low and high RFI cows with their calves (n = 4 pair/replicate) strip-grazed stockpiled and early spring growth tall fescue paddocks (0.7 to 0.9 ha) for 60 d in late winter and early spring. Because of limiting forage availability and quality at trial initiation, cow-calf pairs were also fed 3.31 kg/pair of pelleted soyhulls daily. Pre- and post-grazed forage samples were harvested for 4 grazing periods, and forage growth was estimated using a growing degree days calculation and on-site weather station data. Performance did not differ (P > 0.05) between low and high RFI cows throughout the experiment (18.4 vs. 26.6 kg of BW gain and -0.04 vs. 0.15 BCS gain). Despite the utilization of forage offered being similar for low and high RFI cow-calf pairs (P > 0.05), low RFI cows and their calves had an 11% numerically lower DMI than high RFI pairs (12.5 vs. 14.1 kg/d; P = 0.12). We concluded that either no intake differences existed between low and high RFI cows or that current methodology and small animal numbers limited our ability to detect differences.     

Relationships of feedlot feed efficiency, performance, and feeding behavior with metabolic rate, methane production, and energy partitioning in beef cattle

Residual feed intake (RFI) is the difference between the actual and expected feed intake of an animal based on its BW and growth rate over a specified period. The biological mechanisms underlying the variation in feed efficiency in animals with similar BW and growth rate are not well understood. This study determined the relationship of feedlot feed efficiency, performance, and feeding behavior with digestion and energy partitioning of 27 steers. The steers were selected from a total of 306 animals based on their RFI following feedlot tests at the University of Alberta Kinsella Research Station. Selected steers were ranked into high RFI (RFI > 0.5 SD above the mean, n = 11), medium RFI (RFI +/- 0.5 SD above and below the mean, n = 8), and low RFI (RFI < -0.5 SD below the mean, n = 8). The respective BW +/- SD for the RFI groups were 495.6 +/- 12.7, 529.1 +/- 18.6, and 501.2 +/- 15.5 kg. Digestibility and calorimetry trials were performed on a corn-or barley-based concentrate diet in yr 1 and 2, respectively, at 2.5 x maintenance requirements. Mean DMI (g/kg of BW(0.75)) during the measurements for high-, medium-, and low-RFI groups, respectively, were 82.7 +/- 2.0, 78.8 +/- 2.6, and 81.8 +/- 2.5 and did not differ (P > 0.10). Residual feed intake was correlated with daily methane production and energy lost as methane (r = 0.44; P < 0.05). Methane production was 28 and 24% less in low-RFI animals compared with high- and medium-RFI animals, respectively. Residual feed intake tended to be associated (P < 0.10) with apparent digestibilities of DM (r = -0.33) and CP (r = -0.34). The RFI of steers was correlated with DE (r = -0.41; P < 0.05), ME (r = -0.44; P < 0.05), heat production (HP; r = 0.68; P < 0.001), and retained energy (RE; r = -0.67; P < 0.001; energy values are expressed in kcal/kg of BW(0.75)). Feedlot partial efficiency of growth was correlated (P < 0.01) with methane production (r = -0.55), DE (r = 0.46), ME (r = 0.49), HP (r = -0.50), and RE (r = 0.62). With the exception of HP (r = 0.37; P < 0.05), feed conversion ratio was unrelated to the traits considered in the study. Feeding duration was correlated (P < 0.01) with apparent digestibility of DM (r = -0.55), CP (r = -0.47), methane production (r = 0.51), DE (r = -0.52), ME (r = -0.55), and RE (r = -0.60). These results have practical implications for the selection of animals that eat less at a similar BW and growth rate and for the environmental sustainability of beef production.     

The relationship between mitochondrial function and residual feed intake in Angus steers

The objective of this study was to examine the relationship between mitochondrial function and residual feed intake in Angus steers. Individual feed intakes were recorded for a contemporary group of 40 steers via the GrowSafe feed intake system. Intakes were then used to calculate residual feed intake (RFI), a measure of efficiency. Based on these calculations, 9 low (RFI = -0.83) and 8 high (RFI = 0.78) RFI animals were selected for further study. Blood samples were collected via jugular venipuncture 1 wk before slaughter for the determination of plasma glucose and insulin concentrations. Tissue samples were taken from the LM from both the high and low RFI animals and mitochondria were isolated for measurement of oxygen consumption and hydrogen peroxide production. Average daily gain and carcass composition were not different between the high and low RFI steers; however, ADFI by the high RFI animals was 1.54 kg/d greater (P < 0.001) than for the low RFI animals. Low RFI steers exhibited a greater (P < 0.05) rate of state 2 and 3 respiration, respiratory control ratio, and hydrogen peroxide production than high RFI steers when provided with glutamate or succinate as a respiratory substrate. The acceptor control and adenosine diphosphate:oxygen ratios were not different between the 2 groups for either substrate. When hydrogen peroxide production was expressed as a ratio to respiration rate there was no difference between groups, signifying that electron leak was similar for both groups. Plasma glucose concentration was greater (P < 0.05) in the high RFI steers than in the low RFI steers; however, plasma insulin concentration was not different (P = 0.22) between the 2 groups. The ratio between plasma glucose and insulin concentration was similar (P = 0.88) between the 2 groups indicating no difference in glucose metabolism. The increased plasma glucose concentration observed in the high RFI steers was presumed to be the result of a greater feed intake by these animals. It seems that mitochondrial function is not different between the high and low RFI groups but rather the rate of mitochondrial respiration is increased in low RFI steers compared with high RFI steers.     

Effects of ractopamine hydrochloride on performance, rate and variation in feed intake, and acid-base balance in feedlot cattle

Two experiments evaluated effects of ractopamine hydrochloride (RAC) on performance, intake patterns, and acid-base balance of feedlot cattle. In Exp. 1, 360 crossbred steers (Brangus, British, and British x Continental breeding; initial BW = 545 kg) were used in a study with a 3 x 3 factorial design to study the effects of dose [0, 100, or 200 mg/(steer x d) of RAC] and duration (28, 35, or 42 d) of feeding of RAC in a randomized complete block design (9 treatments, 8 pens/treatment). No dose x duration interactions were detected (P > 0.10). As RAC dose increased, final BW (FBW; P = 0.01), ADG (P < 0.01), and G:F (P < 0.01) increased linearly. As duration of feeding increased, ADG increased quadratically (P = 0.04), with tendencies for quadratic effects for FBW (P = 0.06), DMI (P = 0.07), and G:F (P = 0.09). Hot carcass weight increased linearly (P = 0.02) as dose of RAC increased. Thus, increasing the dose of RAC from 0 to 200 mg/(steer x d) and the duration of feeding from 28 to 42 d improved feedlot performance, although quadratic responses for duration of feeding indicated little improvement as the duration was extended from 35 to 42 d. In Exp. 2, 12 crossbred beef steers (BW = 593 kg) were used in a completely random design to evaluate the effects of RAC [0 or 200 mg/(steer x d) for 30 d; 6 steers/treatment] on rate of intake, daily variation in intake patterns, and acid-base balance. To assess intake patterns, absolute values of daily deviations in feed delivered to each steer relative to the total quantity of feed delivered were analyzed as repeated measures. There were no differences (P > 0.10) in feedlot performance, urine pH, blood gas measurements, or variation in intake patterns between RAC and control cattle, but steers fed RAC had increased (P = 0.04) LM area, decreased (P = 0.03) yield grade, and increased (P < 0.10) time to consume 50 and 75% of daily intake relative to control steers. Our results suggest that feeding RAC for 35 d at 200 mg/(steer x d) provided optimal performance, and no effects on acid-base balance or variation in intake patterns of finishing steers were noted with RAC fed at 200 mg/(steer x d) over a 30-d period.     

Effects of buffer or DL-methionine with different amounts of supplemental corn on feed intake and nutrient digestion by Holstein steers consuming bermudagrass hay

Two latin squares were conducted to determine the effects on feed intake and nutrient digestion of adding a ruminal buffer or DL-methionine to supplemental ground corn for Holstein steers (avg BW of 286 and 222 kg in Exp. 1 and 2, respectively) with ad libitum access to bermudagrass hay. In Exp. 1, steers were not supplemented (control) or were given .5 (LC) or 1.0% BW/d (HC) of ground corn without or with .021% BW of sodium bicarbonate (B). Total DMI was 2.39, 2.71, 2.79, 2.81 and 2.98% BW (effects of supplementation [P less than .05], level of corn [P less than .05] and buffer [P less than .06]), and OM digested was 3.56, 4.65, 4.65, 4.96 and 5.34 kg/d for control, LC, LCB, HC and HCB, respectively (effects of supplementation and corn level, P less than .05). In Exp. 2, corn levels were .24 and .74% BW/d and .0022% BW of DL-methionine (M) replaced B. Total DMI was 2.85, 3.00, 2.99, 3.22 and 3.34% BW (effects of supplementation and corn level, P less than .05), and digestible OM intake was 3.78, 4.24, 4.30, 4.84 and 5.12 kg/d for control, LC, LCM, HC and HCM, respectively (effects of supplementation and corn level, P less than .05). Overall, changes in feed intake and digestion with additions of a ruminal buffer and DL-methionine to corn supplements were not marked; however, buffer addition increased DMI intake to the greatest degree with 1.0% BW/d of corn.     

Effects of restricted versus conventional dietary adaptation on feedlot performance,carcass characteristics,site and extent of digestion,digesta kinetics,and ruminal metabolism

Three experiments were conducted to determine effects of restricting intake of the final finishing diet as a means of dietary adaptation compared with diets increasing in grain over a period of 20 to 22 d on overall cattle performance, carcass characteristics, digestibility, digesta kinetics, and ruminal metabolism. In Exp. 1, 84 Angus x Hereford yearling steers (initial BW = 418 +/- 29.0 kg) were fed for 70 d. Restricting intake during adaptation had no effect (P > 0.10) on overall ADG:DMI, but decreased (P < 0.05) DMI compared with ad libitum access to adaptation diets, which resulted from differences during the initial 28 d of the experiment. In Exp. 2, 150 mixed crossbred steer calves (initial BW = 289 +/- 22.9 kg) were fed for an average of 173 d. Restricting intake decreased (P < 0.01) overall daily gain (1.51 vs 1.65 kg/d) and DMI (8.68 vs 9.15 kg/d) compared with ad libitum fed steers; however, ADG:DMI was not influenced (P > 0.10) by adaptation method. Experiment three used eight ruminally and duodenally fistulated steers (initial BW = 336 +/- 20 kg) in a completely random design. Total tract digestibility, digesta kinetics and ruminal metabolism were determined. Restricting intake reduced (P < 0.10) daily DMI variation from d 1 through 7, 8 through 14, and 22 through 28 compared with ad libitum feeding of three adaptation diets. Restricted steers had reduced (adaptation method x period interaction, P < 0.05) intakes and fecal excretions of ADF and greater OM digestibilities on d 4 through 7, 11 through 14, and 18 through 21. Digesta kinetics and ruminal metabolism were generally not affected (P > 0.10) by adaptation method. Our results suggest that restricted-feeding of the final diet as a means of dietary adaptation can be used in finishing cattle with few problems from acidosis or related intake variation. In light-weight steers (Exp. 2), disruptions in intake during the adaptation period might have resulted in restriction for an extended period, which decreased (P < 0.01) hot carcass weight compared with calves fed ad libitum. Effects of limit feeding during the initial 28 d of the feeding period on site and extent of digestion, digesta kinetics, and ruminal metabolism were minimal, supporting few differences in performance across the finishing period for yearling cattle.     

Effects of supplemental alfalfa hay on feed intake and digestion by Holstein steers consuming high-quality bermudagrass or orchardgrass hay

Effects of level and frequency of supplementation with alfalfa (A) on feed intake and digestion by steers fed bermudagrass (B) or orchardgrass (O) were determined in two Latin square experiments. In Exp. 1, six Holstein steers (224 kg) were fed B (2.25% N; 71.4% NDF) or O (2.52% N; 64.3% NDF) with 0, 15 or 30% (DM) A (2.70% N; 44.0% NDF). Total DMI was 2.43, 2.72 and 2.85% BW for B and 2.98, 3.00 and 2.87% BW for O with 0, 15 and 30% A, respectively. Total DMI was affected by forage (P less than .05), A level (linear; P less than .06) and a forage x A level (linear) interaction (P less than .05). Digestible OM intake increased .42 (15%) and .67 kg (24%) with feeding of 15 and 30% A, respectively, for B, but for O, only dietary inclusion of 30% A elevated digestible OM intake (.14 kg and 4% increases). In Exp. 2, five Holstein steers (165 kg) were fed B (1.81% N; 78.6% NDF) alone or with A (2.76% N; 52.8% NDF). Morning meals consisted of ad libitum B (OA), .3% BW of A daily (.3A), .6% BW of A every 2nd d (.6A), .9% BW of A every 3rd d (.9A) or 1.2% BW of A every 4th d (1.2A). All steers received B in the afternoon ad libitum, and B was given in the morning when A was not fed. Total DMI was 2.31, 2.12, 2.12, 2.26 and 2.29% BW for OA, .3A, .6A, .9A and 1.2A, respectively (SE .049). Grass characteristics affected response in feed intake to legume supplementation. Frequency of dietary legume addition may alter feed intake.     

Effect of management strategies on reducing heat stress of feedlot cattle: feed and water intake

Three experiments were conducted to evaluate management strategies designed to decrease heat stress of cattle finished during the summer. In Exp. 1, 144 Angus crossbred yearling steers were assigned to three treatments: 1) ad libitum access to feed at 0800 (ADLIB); 2) fed at 1600 with feed amount adjusted so that no feed was available at 0800 (BKMGT); and 3) fed at 1600 at 85% of predicted ad libitum levels (LIMFD). Treatments were imposed for 23 d of an 82-d study, after which all steers were fed ad libitum at 0800. Treatment did not affect (P > 0.10) overall DMI, although ADLIB cattle tended to consume less feed. Overall water intake was decreased (P < 0.05) by 6.8 L x animal(-1) x d(-1) for LIMFD vs. ADLIB steers. In Exp. 2, 96 Angus crossbred yearling steers were assigned to three treatments: 1) control, no water application; 2) water applied to the pen surfaces between 1000 and 1200 (AM); and 3) water applied to pen surfaces between 1400 and 1600 (PM). Water intake and DMI did not differ among treatments; however, feed efficiency of AM steers was superior (P = 0.06) to that of PM steers. Conversely, marbling scores of PM steers were higher (P = 0.06) than those of AM steers. In Exp. 3, 192 crossbred steers were used to determine the effects of feeding time (0800 [AMF] vs. 1400 [PMF]), with (WET) and without (DRY) sprinkling (20 min every 1.5 h between 1000 and 1750). Feed DMI did not differ among treatments; however, water intake and marbling scores were highest (P < 0.05) for AMF/DRY steers. During these experiments, bunk scores (0 = <10% of feed delivered remaining; 1 = 10 to 50% of feed remaining; 2 = >50% of feed remaining) were assigned to each pen at various times during the day. In Exp. 1, bunk scores of BKMGT pens remained similar (P > 0.20) under varying environmental conditions, whereas LIMFD steers had lower scores (P < 0.05) as days on feed increased, even under hot environmental conditions. In Exp. 3, bunk scores of PMF/WET steers tended to be lower (P < 0.10) at 1700 and 2000 compared with PMF/ DRY pens under mild heat stress but not under severe heat stress. Alternative feeding regimens and sprinkling can alter the feed intake pattern of steers. Heat stress management strategies imposed in these experiments had minimal effects on cattle performance. Such strategies would be most useful for decreasing the susceptibility of cattle to hyperthermia and reducing related feedlot cattle deaths without adversely affecting performance.