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.     

Relation of growth hormone response to growth hormone-releasing hormone before weaning and postweaning growth performance in beef calves

Previously, GH response to GHRH challenge at weaning has been shown to be indicative of ADG during a standard postweaning growth performance test in Angus cattle. In this study, we tested the hypothesis that GH response to GHRH before weaning would predict postweaning ADG. Bulls with the highest and lowest GH responses to GHRH over a 3-yr period, relative to their contemporaries, were used as sires, to allow for examination of the persistence of GH response to GHRH through selection. The selected calves in this study were sired by one of four Angus bulls chosen based on their GH response to GHRH (high response, n = 2; low response, n = 2). Forty-nine Angus calves (bulls, n = 24; heifers, n = 25) were challenged with GHRH at approximately 60, 105, and 150 d of age and at weaning (219 d; SD = 25). Blood samples were taken immediately prior to and 10 min following an i.v. clearance dose of 4.5 microg of GHRH/100 kg BW and, 2 h later, immediately prior to and 10 min following a challenge dose of either 1.5 or 4.5 microg of GHRH/100 kg BW. Two hours later, the procedure was repeated, with each calf receiving the other challenge dose. Body weight was measured every 28 d and ADG was calculated over a 140-d growth performance test (heifers and bulls maintained separately). Data were log-transformed for statistical analyses. In the selected bulls and heifers, response of GH to 1.5 microg of GHRH/100 kg BW at 60 and 105 d of age was positively related (P < 0.05) to postweaning ADG. Response to 4.5 microg of GHRH/100 kg BW at 105 d of age and at weaning was positively related (P < 0.01) to postweaning ADG. Inclusion of sire in the analysis improved the relationship between GH response and ADG for calves of sires with high GH responses from R2 = 0.18 (P = 0.01) to R2 = 0.33 (P = 0.02). When the GH response to GHRH of the unselected calves at weaning was added to the data from the selected animals and analyzed, the GH response of the bulls was related to postweaning ADG (R2 = 0.09; P = 0.04). In conclusion, GH response to GHRH as early as 60 d of age is indicative of postweaning ADG in beef cattle. In addition, the relationship between GH response to GHRH and postweaning ADG is improved with selection for greater GH response to GHRH.     

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.     

Buffalo heifers selected for lower residual feed intake have lower feed intake,better dietary nitrogen utilisation and reduced enteric methane production

This study was conducted to evaluate the utilisation of the residual feed intake (RFI ) as a feed efficiency selection tool and its relationship with methane emissions. Eighteen Murrah buffalo (Bubalus bubalis ) heifers were fed ad libitum with total mixed ration (TMR ) for 120 days. Based on linear regression models involving dry matter intake (DMI ), average daily gain (ADG ) and mid‐test metabolic body size (MBW ^0.75), heifers were assigned into low and high RFI groups. The RFI varied from ?0.09 to +0.12 kg DM /day with average RFI of ?0.05 and 0.05 kg DM /day in low and high RFI heifers respectively. Low RFI heifers ate 11.6% less DM each day, yet average daily gain (ADG ) and feed utilisation were comparable among low and high RFI groups. Low RFI heifers required significantly (p  < .05) less metabolizable energy for maintenance (ME m) compared to high RFI heifers. Apparent nutrient digestibility showed non‐significant difference (p >  .05) among low and high RFI groups. Although the nitrogen balance was similar among heifers of low and high RFI groups, nitrogen metabolism was significantly higher (p  > .05) in high RFI heifers. Comparison of data from heifers exhibiting the low (n  = 9) and high (n  = 9) RFI showed that the low RFI heifers have lower enteric methane production and methane losses than high RFI heifers. In conclusion, results of this study revealed that selection of more efficient buffalo heifers has multiple benefits, such as decreased feed intake and less emission of methane.     

Characterization of feed efficiency traits and relationships with feeding behavior and ultrasound carcass traits in growing bulls

The objectives of this study were to characterize feed efficiency traits and to examine phenotypic correlations between performance and feeding behavior traits, and ultrasound measurements of carcass composition in growing bulls. Individual DMI and feeding behavior traits were measured in Angus bulls (n=341; initial BW=371.1+/-50.8 kg) fed a corn silage-based diet (ME=2.77 Mcal/kg of DM) for 84 d in trials 1 and 2 and for 70 d in trials 3 and 4 by using a GrowSafe feeding system. Meal duration (min/d) and meal frequency (events/d) were calculated for each bull from feeding behavior recorded by the GrowSafe system. Ultrasound measures of carcass 12th-rib fat thickness (BF) and LM area (LMA) were obtained at the start and end of each trial. Residual feed intake (RFIp) was computed from the linear regression of DMI on ADG and midtest BW(0.75) (metabolic BW, MBW), with trial, trial by ADG, and trial by midtest BW(0.75) as random effects (base model). Overall ADG, DMI, and RFIp were 1.44 (SD=0.29), 9.46 (SD=1.31), and 0.00 (SD=0.78) kg/d, respectively. Stepwise regression analysis revealed that inclusion of BW gain in BF and LMA in the base model increased R(2) (0.76 vs. 0.78) and accounted for 9% of the variation in DMI not explained by MBW and ADG (RFIp). Residual feed intake and carcass-adjusted residual feed intake (RFIc) were moderately correlated with DMI (0.60 and 0.55, respectively) and feed conversion ratio (FCR; 0.49 and 0.45, respectively), and strongly correlated with partial efficiency of growth (PEG; -0.84 and -0.78, respectively), but not with ADG or MBW. Gain in BF was weakly correlated with RFIp (0.30), FCR (-0.15), and PEG (-0.11), but not with RFIc. Gain in LMA was weakly correlated with RFIp (0.17) and FCR (-0.19), but not with PEG or RFIc. The Spearman rank correlation between RFIp and RFIc was high (0.91). Meal duration (0.41), head-down duration (0.38), and meal frequency (0.26) were correlated with RFIp and accounted for 35% of the variation in DMI not explained by MBW, ADG, and ultrasound traits (RFIc). These results suggest that adjusting residual feed intake for carcass composition will facilitate selection to reduce feed intake in cattle without affecting rate or composition of gain.     

Residual feed intake, body composition, and fertility in yearling beef heifers

One hundred thirty-seven spring-born yearling beef heifers of British breed types were used to determine the relationships between residual feed intake (RFI) and growth rate, body composition, mature size, and fertility. Heifers were housed in a dry lot facility during the experimental period, and data were collected over a 2-yr period (yr 1, n = 67; yr 2, n = 70). Individual feed intake, BW, BCS, hip height, and ultrasonic measurements [subcutaneous rib fat (UBF), rump fat (URF), LM area (LMA), and intramuscular fat (IMF)] of body composition were recorded. Individual feed intakes (kg of TDN consumed/d) were used to calculate RFI combining both years of data. Heifers averaged 387.0 ± 19.4 d of age and 337.1 ± 29.9 kg of BW at initiation of the experiment. Mean ADG was 1.14 ± 0.21 kg/d during the trial. Based on RFI, with year of test and farm of origin included in the model as covariates, heifers were classified into groups: positive (POS; 0.74 kg of TDN/d) or negative (NEG; -0.73 kg TDN/d) for first analysis and high (HI), medium (MED), or low (LO; mean RFI = 1.06, -0.01, and -1.13 kg of TDN/d, respectively) subsequently. An initial phenotypic relationship (P < 0.05) between RFI and both UBF and URF (r = 0.19 and 0.17, respectively) was sustained (P < 0.01) with UBF (r = 0.27) and URF (r = 0.24) to trial conclusion. No other correlations with RFI were significant. Heifers classified as POS reached puberty earlier than those classified as NEG (414 ± 3.83 vs. 427 ± 4.67 d of age, P = 0.03), and possessed greater LMA per 100 kg of BW (LMACWT) at conclusion of the trial (P < 0.01). Medium heifers exhibited less URF (P < 0.05) compared with either HI or LO heifers at trial initiation. Low heifers possessed less LM area (cm(2)) per 100 kg of BW (P < 0.05) than HI but did not differ (P > 0.10) from MED heifers at either the beginning or the end of test. Additionally, a negative linear relationship was observed between RFI and age at puberty (P < 0.05). Each 1-unit increase in RFI corresponded to a decrease of 7.5 d in age at puberty, but did not affect pregnancy or conception rates (P > 0.10). Differences in body fat and rate of metabolism associated with RFI could delay reproductive maturity.     

Relationships between feed efficiency and puberty in Bos taurus and Bos indicus-influenced replacement beef heifers

The objective of this experiment was to investigate the relationship between residual feed intake (RFI) and parameters associated with reproductive efficiency in growing beef heifers. One hundred and seventy-nine replacement beef heifers (Bos taurus and Bos indicus-influenced) were enrolled in a retrospective cohort design. Heifers were assigned to a 70-d feed efficiency test, and results were utilized to retrospectively classify heifers into low (n = 51), medium (n = 66), or high (n = 62) RFI groups based on their individual RFI values. Blood samples were collected weekly throughout the feed efficiency test and breeding season, which were analyzed for plasma concentration of progesterone to determine age at puberty (PUB). By design, residual feed intake differed among RFI groups where high (1.21 ± 0.08 kg/d) had greater (P < 0.01) RFI than medium (0.00 ± 0.08 kg/d) and low RFI groups (–1.49 ± 0.09 kg/d), and medium had greater (P < 0.01) RFI than low RFI heifers. Yet, no differences were observed between breed types for RFI (P = 0.53). Average daily gain (ADG) differed between breed types where Bos taurus heifers had greater ADG (P = 0.02) than B. indicus-influenced heifers. Furthermore, ADG tended (P = 0.08) to differ among RFI groups, where medium RFI heifers tended to have reduced ADG compared with low and high RFI heifers. No differences were determined between breed types for gain-to-feed ratio (G:F; P = 0.20; however, G:F differed among RFI groups and was greater (P < 0.01) in low vs. high, and low vs. medium RFI heifers. The percentage of cycling heifers by the start of the breeding season differed (P = 0.03) by RFI group, where a greater percentage of low RFI heifers were cycling compared with high RFI heifers. A difference was determined on PUB between breed types (P = 0.03), where Bos taurus (393.40 ± 4.64 d) heifers had a reduced PUB compared with B. indicus-influenced (406.90 ± 5.07 d) heifers. In addition, PUB differed by RFI group where low (385.96 ± 6.20 d; P < 0.01) and medium (398.47 ± 5.47 d; P = 0.02) RFI heifers had a reduced PUB compared with High RFI heifers (416.03 ± 5.58 d). In conclusion, more feed efficient heifers attained PUB earlier than less feed efficient heifers. Therefore, selection of heifers for feed efficiency may positively impact reproductive performance of replacement beef heifers.     

Effect of castration technique on beef calf performance, feed efficiency, and inflammatory response

The objective of this experiment was to examine the effect of castration technique on daily feed intake (DFI), daily water intake (DWI), growth performance, residual feed intake (RFI), and inflammatory response in weaned beef calves. Seventy-five beef calves (214 ± 3.2 kg; 200 ± 26 d of age) were housed in a GrowSafe 4000 feed intake facility 7 d post weaning (15 calves/pen). Calves were offered a total mixed ration (TDN = 67.3% and CP = 12.2%, DM = 89%) for ad libitum consumption. On d 0, calves were assigned to 1 of 5 treatments (n = 15 calves/treatment): 1) steers castrated surgically pre-weaning (52 d of age; CON); 2) intact bulls (BULL); 3) bulls castrated by the Callicrate Bander on d 0 (No-Bull Enterprises LLC.; BAN); 4) bulls castrated by the Henderson Castrating Tool on d 0 (Stone Mfg & Supply Co.; HEN); and 5) bulls castrated surgically utilizing an emasculator on d 0 (SUR). Average daily gain, DFI, and DWI were recorded over 84 d. Blood was collected from a sub-sample of calves (n = 45) on d 0, 2, 6, 9, 12, and 15 relative to castration. Castration decreased (P = 0.06) ADG for castrates compared with CON from d 0 to 14 but not d 0 to 84. Daily feed intake and DWI were similar (P > 0.10) among treatments during d 0 to 84. Gain:feed was not affected by castration technique; however, RFI tended (P = 0.09) to be negative for CON and BULL compared with castrates on d 0 to 14 but not d 0 to 84. Acute phase protein analyses indicated that surgical castration (SUR or HEN) elicited a short-term inflammatory response in calves, whereas calves castrated with BAN elicited a delayed response. Calves castrated pre-weaning had improved d 0 to 14 ADG, feed intake, and inflammation response compared with calves castrated at weaning. Banding elicited a delayed negative response in ADG, DWI, and inflammation. In weaned calves, castration method did not affect performance, DFI, DWI, or inflammatory response during the 84-d trial.     

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.     

Genetic and phenotypic relationships of feed intake and measures of efficiency with growth and carcass merit of beef cattle

Feed intake and efficiency of growth are economically important traits of beef cattle. This study determined the relationships of daily DMI, feed:gain ratio [F:G, which is the reciprocal of the efficiency of gain (G:F) and therefore increases as the efficiency of gain decreases and vice versa, residual feed intake (RFI), and partial efficiency of growth (efficiency of ADG, PEG) with growth and carcass merit of beef cattle. Residual feed intake was calculated from phenotypic regression (RFIp) or genetic regression (RFIg) of ADG and metabolic BW on DMI. An F1 half-sib pedigree file containing 28 sires, 321 dams, and 464 progeny produced from crosses between Alberta Hybrid cows and Angus, Charolais, or Alberta Hybrid bulls was used. Families averaged 20 progeny per sire (range = 3 to 56). Performance, ultrasound, and DMI data was available on all progeny, of which 381 had carcass data. Phenotypic and genetic parameters were obtained using SAS and ASREML software, respectively. Differences in RFIp and RFIg, respectively, between the most and least efficient steers (i.e., steers with the lowest PEG) were 5.59 and 6.84 kg of DM/d. Heritabilities for DMI, F:G, PEG, RFIp, and RFIg were 0.54 +/- 0.15, 0.41 +/- 0.15, 0.56 +/- 0.16, 0.21 +/- 0.12, and 0.42 +/- 0.15, respectively. The genetic (r = 0.92) and phenotypic (r = 0.97) correlations between RFIp and RFIg indicated that the 2 indices are very similar. Both indices of RFI were favorably correlated phenotypically (P < 0.001) and genetically with DMI, F:G, and PEG. Residual feed intake was tendentiously genetically correlated with ADG (r = 0.46 +/- 0.45) and metabolic BW (r = 0.27 +/- 0.33), albeit with high SE. Genetically, RFIg was independent of ADG and BW but showed a phenotypic correlation with ADG (r = -0.21; P < 0.05). Daily DMI was correlated genetically (r = 0.28) and phenotypically (r = 0.30) with F:G. Both DMI and F:G were strongly correlated with ADG (r > 0.50), but only DMI had strong genetic (r = 0.87 +/- 0.10) and phenotypic (r = 0.65) correlations with metabolic BW. Generally, the phenotypic and genetic correlations of RFI with carcass merit were not different from zero, except genetic correlations of RFI with ultrasound and carcass LM area and carcass lean yield and phenotypic correlations of RFI with backfat thickness (P < 0.01). Daily DMI had moderate to high phenotypic (P < 0.01) and genetic correlations with all the ultrasound and carcass traits. Depending on how RFI technology is applied, adjustment for body composition in addition to growth may be required to minimize the potential for correlated responses to selection in cattle.     

Selection response and genetic parameters for residual feed intake in Yorkshire swine

Residual feed intake (RFI) is a measure of feed efficiency defined as the difference between the observed feed intake and that predicted from the average requirements for growth and maintenance. The objective of this study was to evaluate the response in a selection experiment consisting of a line selected for low RFI and a random control line and to estimate the genetic parameters for RFI and related production and carcass traits. Beginning with random allocation of purebred Yorkshire littermates, in each generation, electronically measured ADFI, ADG, and ultrasound backfat (BF) were evaluated during a approximately 40- to approximately 115-kg of BW test period on approximately 90 boars from first parity and approximately 90 gilts from second parity sows of the low RFI line. After evaluation of first parity boars, approximately 12 boars and approximately 70 gilts from the low RFI line were selected to produce approximately 50 litters for the next generation. Approximately 30 control line litters were produced by random selection and mating. Selection was on EBV for RFI from an animal model analysis of ADFI, with on-test group and sex (fixed), pen within group and litter (random), and covariates for interactions of on- and off-test BW, on-test age, ADG, and BF with generations. The RFI explained 34% of phenotypic variation in ADFI. After 4 generations of selection, estimates of heritability for RFI, ADFI, ADG, feed efficiency (FE, which is the reciprocal of the feed conversion ratio and equals ADG/ ADFI), and ultrasound-predicted BF, LM area (LMA), and intramuscular fat (IMF) were 0.29, 0.51, 0.42, 0.17, 0.68, 0.57, and 0.28, respectively; predicted responses based on average EBV in the low RFI line were -114, -202, and -39 g/d for RFI (= 0.9 phenotypic SD), ADFI (0.9 SD), and ADG (0.4 SD), respectively, and 1.56% for FE (0.5 SD), -0.37 mm for BF (0.1 SD), 0.35 cm(2) for LMA (0.1 SD), and -0.10% for IMF (0.3 SD). Direct phenotypic comparison of the low RFI and control lines based on 92 low RFI and 76 control gilts from the second parity of generation 4 showed that selection had significantly decreased RFI by 96 g/d (P = 0.002) and ADFI by 165 g/d (P < 0.0001). The low RFI line also had 33 g/d lower ADG (P = 0.022), 1.36% greater FE (P = 0.09), and 1.99 mm less BF (P = 0.013). There was not a significant difference in LMA and other carcass traits, including subjective marbling score, despite a large observed difference in ultrasound-predicted IMF (-1.05% with P < 0.0001). In conclusion, RFI is a heritable trait, and selection for low RFI has significantly decreased the feed required for a given rate of growth and backfat.     

Evaluation of serum insulin-like growth factor binding proteins (IGFBP) in Angus cattle divergently selected for serum IGF-I concentration

Postweaning serum insulin-like growth factor-I (IGF-I) concentrations and serum IGF binding proteins (IGFBP) were investigated in 68 (1992 Fall-born) and 84 (1999 Fall-born) Angus cattle selected for either high or low serum IGF-I concentrations since 1989. Relative serum levels of IGFBP were determined by [¹²⁵I]IGF-I Western ligand blotting. IGFBP species of 38–42, 34, 30, and 24 kDa were identified. The 34 kDa species was identified as IGFBP-2 by immunoblot analysis. No significant line effects were observed for any of the IGFBP. In both 1992 and 1999, heifers had higher IGFBP-2 levels than bulls (P<0.0005). In 1992 calves, relative levels of the 38–42 and 24 kDa species were significantly correlated with serum IGF-I concentration. In 1999 calves, none of the IGFBP were correlated with serum IGF-I, although IGFBP-2 was negatively correlated with several measures of body weight. No significant line effects were observed for growth or serum IGF-I traits in 1992 calves. However, 1999 high line calves had higher serum IGF-I concentrations and body weights than low line calves (P<0.05). In both 1992 and 1999 calves, bulls had higher serum IGF-I concentrations and body weights than heifers (P<0.05). Thus, while selection for high versus low serum IGF-I concentrations has resulted in divergence between the selection lines and also in changes in body weights, it has not resulted in changes in serum IGFBP levels. Furthermore, circulating IGFBP-2 appears to be higher in heifers than in bulls, and also appears to be negatively correlated with body weights.     

Genetic and phenotypic parameter estimates for feed intake and other traits in growing beef cattle, and opportunities for selection

Growth, feed intake, and temperament indicator data, collected over 5 yr on a total of 1,141 to 1,183 mixed-breed steers, were used to estimate genetic and phenotypic parameters. All steers had a portion of Hereford, Angus, or both as well as varying percentages of Simmental, Charolais, Limousin, Gelbvieh, Red Angus, and MARC III composite. Because the steers were slaughtered on various dates each year and the animals thus varied in days on feed, BW and feed data were adjusted to a 140-d feeding period basis. Adjustment of measures of feed efficiency [G:F or residual feed intake (RFI), intake adjusted for metabolic body size, and BW gain] for body fatness recorded at slaughter had little effect on the results of analyses. Average daily gain was less heritable (0.26) than was midtest BW (MBW; 0.35). Measures of feed intake had greater estimates of heritability, with 140-d DMI at 0.40 and RFI at 0.52; the heritability estimate for G:F was 0.27. Flight speed (FS), as an indicator of temperament, had an estimated heritability of 0.34 and a repeatability of 0.63. As expected, a strong genetic (0.86) correlation was estimated between ADG and MBW; genetic correlations were less strong between DMI and ADG or MBW (0.56 and 0.71). Residual feed intake and DMI had a genetic correlation of 0.66. Indexes for phenotypic RFI and genotypically restricted RFI (no correlation with BW gain) were compared with simple economic indexes incorporating feed intake and growth to elucidate expected selection responses under different criteria. In general, few breed differences were detected across the various measurements. Heterosis contributed to greater DMI, RFI, and MBW, but it did not significantly affect ADG, G:F, or FS. Balancing output (growth) with input costs (feed) is needed in practicing selection, and FS would not be recommended as an indicator trait for selection to change feed efficiency. An index including BW gain and RFI produced the best economic outcome.     

Effects of restricted feeding of beef heifers during the postweaning period on growth, efficiency, and ultrasound carcass characteristics

Traits used for identification of replacement beef heifers and feeding levels provided during postweaning development may have major financial implications due to effects on maintenance requirements and level of lifetime production. The current study evaluated the effects of 2 levels of feeding during the postweaning period on growth, G:F, and ultrasound carcass measurements of heifers, and the associations among these traits. Heifers (1/2 Red Angus, 1/4 Charolais, and 1/4 Tarentaise) born in 3 yr were randomly assigned to a control (fed to appetite; n = 205) or restricted (fed at 80% of that consumed by controls adjusted to a common BW basis; n = 192) feeding during a 140-d postweaning period. Heifers were individually fed a diet of 68% corn silage, 18% alfalfa, and protein-mineral supplement (DM basis) in pens equipped with Calan gates. Ultrasound measurements of LM area, intramuscular fat, and subcutaneous fat thickness over the LM were made on d 140 (382 +/- 0.8 d of age). Average daily DMI was 4.1 and 5.6 kg/d for restricted and control heifers, respectively (P < 0.001). Feed restriction decreased (P < 0.001) BW (292 vs. 314 kg), ADG (0.52 vs. 0.65 kg/d), LM area (55 vs. 59 cm2), intramuscular fat (3.2 vs. 3.5%), and subcutaneous fat thickness over the LM (3.2 vs. 3.9 mm), but increased G:F (0.12 vs. 0.11) when compared with control at the end of the 140-d study. The magnitude of the associations of DMI with ADG (r = 0.32 vs. 0.21), 140-d BW (r = 0.78 vs. 0.36), hip height (r = 0.57 vs. 0.17), LMA (r = 0.30 vs. 0.18), and BCS (r = 0.17 vs. 0.11) was greater in restricted- than control-fed heifers. Variance of residual feed intake, calculated within each treatment, was greater (P < 0.01) in control (0.088) than restricted (0.004) heifers, and magnitude of association between residual feed intake and average DMI was greater in control (r = 0.88) than restricted (r = 0.41) heifers. Pregnancy rate tended (P = 0.11) to be reduced in heifers that had been developed on restricted feeding (86.3 +/- 2.3 vs. 91.5 +/- 2.3%). However, ADG was greater (P < 0.001) in restricted than control heifers (0.51 vs. 0.46 kg/d) while grazing native range in the 7 mo after restriction. In summary, restricted heifers consumed 22% less feed on a per-pregnant-heifer basis during the development period and had a greater magnitude of association between DMI and several growth-related traits at the end of the 140-d postweaning feeding period, which is indicative of improved efficiency.     

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.     

Genetic parameters for residual feed intake in growing pigs, with emphasis on genetic relationships with carcass and meat quality traits

Data were collected over the first 4 generations of a divergent selection experiment for residual feed intake of Large White pigs having ad libitum access to feed. This data set was used to obtain estimates of heritability for residual feed intake and genetic correlations (r(a)) between this trait and growth, carcass, and meat quality traits. Individual feed intake of group-housed animals was measured by single-space electronic feeders. Upward and downward selection lines were maintained contemporarily, with 6 boars and 35 to 40 sows per line and generation. Numbers of records were 793 for residual feed intake (RFI1) of boar candidates for selection issued from first-parity (P1) litters and tested over a fixed BW range (35 to 95 kg) and 657 for residual feed intake (RFI2) and growth, carcass, and meat quality traits of castrated males and females issued from second-parity (P2) litters and tested from 28 to 107 kg of BW. Variance and covariance components were estimated using REML methodology applied to a series of multitrait animal models, which always included the criterion for selection as 1 of the traits. Estimates of heritability for RFI1 and RFI2 were 0.14 +/- 0.03 and 0.24 +/- 0.03, respectively, whereas the estimate of r(a) between the 2 traits was 0.91 +/- 0.08. Estimates of r(a) indicated that selection for low residual feed intake has the potential to improve feed conversion ratio and reduce daily feed intake, with minimal correlated effect for ADG of P2 animals. Estimates of r(a) between RFI2 and body composition traits of P2 animals were positive for traits related to the amount of fat depots (r(a) = 0.44 +/- 0.16 for carcass backfat thickness) and negative for carcass lean meat content (r(a) = -0.55 +/- 0.14). There was a tendency for a negative genetic correlation between RFI2 and carcass dressing percent (r(a) = -0.36 +/- 0.21). Moreover, selection for low residual feed intake is expected, through lower ultimate pH and lighter color, to decrease pork quality (r(a) = 0.77 +/- 0.14 between RFI2 and a meat quality index intended to predict the ratio of the weight of ham after curing and cooking to the weight of defatted and boneless fresh ham).     

Effect of fat type and forage level on performance of finishing cattle.

Four trials were conducted to determine the effects of adding various levels and types of fat to dry-rolled corn (DRC) finishing diets containing 0 or 7.5% forage. In Trial 1, 88 yearling steers (mean BW = 352 +/- 38 kg) and 176 heifers (mean BW 316 +/- 15 kg) were blocked by sex and weight into four replications. Treatments were 0, 2, 4, or 6% (DM basis) bleachable fancy tallow (BT) fed with 0 or 7.5% (DM basis) forage. Addition of BT to the 7.5% forage diet had no effect on ADG or gain/feed (G/F). However, adding BT to the all-concentrate diet decreased ADG (linear, P < .01) and G/F (linear, P = .08). In Trial 2, 184 yearling steers (mean BW = 347 +/- 21 kg) and 144 heifers (mean BW 322 +/- 8 kg) were blocked by sex and weight into six replications. Fat treatments were 0% fat, 4% BT, or 4% animal-vegetable oil blend (A-V); each fat treatment was fed with 0 or 7.5% forage. Across forage levels, the addition of fat increased (P < .01) ADG and G/F for cattle fed DRC. In Trial 3, 18 crossbred wether lambs (mean BW = 44.4 +/- 2.5 kg) were fed DRC and 7.5% forage and allotted randomly to the same fat treatments fed in Trial 2. Apparent total tract fat digestibility increased (P < .01) with the addition of BT or A-V. In Trial 4, 40 crossbred wethers (mean BW = 25 +/- 4.1 kg) and 16 ewes (mean BW = 23 +/- 2.7 kg) were individually fed 7.5% forage diets containing 0, 1, 2, or 4% BT. Addition of BT increased (linear, P = .10) G/F. In summary, fat addition to DRC finishing diets fed to yearling cattle did not consistently affect gain/feed, feed intake, and ADG.     

Use of growth hormone response to growth hormone-releasing hormone to determine growth potential in beef heifers.

The response of GH to GHRH at weaning is known to predict postweaning growth and body composition in beef bulls. The objective of this study was to determine whether GH response to a challenge of GHRH and plasma IGF-I can predict growth rate and body composition in the beef heifer. Growth hormone response to a challenge with two doses of GHRH was measured in 67 Angus heifers averaging 225 d of age (SD = 21) and 217 kg BW (SD = 32). Blood samples were collected at 0 and 10 min relative to an initial "clearance dose" (4.5 micrograms GHRH/100 kg BW) and again, 3 h later, relative to a challenge dose (1.5 or 4.5 micrograms GHRH/100 kg BW). Each animal received each of the two challenge doses, which were randomly assigned across 2 d of blood collection. Serum GH concentration was measured by RIA. Plasma was collected every 28 d during a 140-d growth test and assayed for IGF-I by RIA. Body weight was measured every 28 d and hip height was measured at weaning and at the end of a 140-d growth test. Average daily gain was calculated on d 140 of the growth test and body composition measurements were estimated by ultrasound 2 wk after completion of the growth test. Responses to the two GHRH challenges were dose-dependent (P < 0.05). Average daily gain tended to be related to GH response to the 1.5 micrograms GHRH/100 kg BW dose (R2 = 0.05; P = 0.06), but no relationship was observed at the 4.5 micrograms GHRH/100 kg BW dose (R2 = 0.00; P = 0.93). An inverse relationship (R2 = 0.06; P = 0.02) was observed between response to the 1.5 micrograms GHRH/100 kg BW dose and intramuscular fat percentage. Mean plasma IGF-I concentration was positively associated with ADG (R2 = 0.06; P < 0.01). Growth hormone response to GHRH is modestly related to body composition but not to ADG in weanling beef heifers and likely has limited use in evaluation of growth performance in replacement beef heifers.     

Morbidity effects on productivity and profitability of stocker cattle grazing in the Southern Plains

Effects of bovine respiratory disease (BRD) on stocker cattle systems are unknown under extensive rangeland environments. Three experiments were conducted to test the hypothesis that BRD-based morbidity is a major factor affecting the productivity and profitability of stocker cattle grazing Southern Plains rangelands. In Exp. 1 (658 male calves; average BW = 231 kg), 17% of the cattle were treated for BRD <8 d, 6% for 8 to 14 d, and 8% for >14 d. Morbid cattle had lower ADG than did healthy cattle (P < 0.10). Cattle requiring 14 d of pharmaceutical therapy gained less than cattle having <14 d therapy (P < 0.01). In Exp. 2, (279 steers and bulls; average BW = 216 kg), the ADG by steers (0.74 kg x animal(-1) x d(-1)) was greater (P < 0.05) than by bulls castrated after arrival (0.64 kg x animal(-1) x d(-1)). Castration after arrival led to a 13.5% loss in daily gain and a 10.3% loss in season-long gain. More (P < 0.05) bulls castrated after arrival (60%) were morbid compared with steers (28%). In Exp. 3, 633 heifers (average BW = 251 kg) were used to test the effects of morbidity on weight gain and reproduction. Heifers with lower initial weights exhibited increased (P < 0.05) morbidity. Heifers requiring two or more antibiotic treatments gained 0.03 kg/d less (P < 0.10) than did healthy heifers and had lower (P < 0.05) conception rates (66 vs. 81%). Conception rate in twice-treated heifers was 19% less than healthy heifers. Morbid heifers conceived 0.6 mo later (P < 0.05) than healthy heifers. Under the conditions of Exp. 1 and Exp. 2, morbidity decreased net returns 9.7 to 21.3% per animal. Adjusted gross returns per animal in Exp. 3 for replacement heifers were 3 to 7.8% less for morbid heifers.     

Evidence of residual feed intake reranking in crossbred replacement heifers

The objective of the study was to examine whether residual feed intake (RFI) reranking exists between 2 consecutive periods in replacement heifers fed the same diet. The study collected feed intake and BW data from 190 crossbred heifers over a 3-yr period (61 in 2007, 68 in 2008, and 61 in 2009) during the winter-spring season. The heifers were fed a roughage-based diet (90% barley silage and 10% rolled barley grain) throughout the feeding trial, which was broken down into 2 feeding periods with ADG of 0.94 and 0.90 kg?d(-1) in the first and second periods, respectively. The RFI was calculated for the entire period using different models, which included ADG, mid-metabolic BW, body composition, and feeding activity. Gain:feed ratio and Kleiber ratio were also calculated. Rank correlations among the RFI calculated from different models were obtained, as well as rank correlations between the 2 feeding periods for the feed efficiency measures. Including body composition and feeding activity only improved the R(2) by 1 to 5%. The rank correlations among the different models were high (90 to 95%) for RFI calculated for the entire feeding period. However, the RFI calculated within the second feeding period had greater rank correlation than the RFI calculated from the entire feeding period. Compared with G:F and Kleiber ratio, RFI gave lesser reranking between periods 1 and 2. About 49% of the heifers maintained their RFI class, whereas 51% of the heifers had a different RFI class in period 2. Furthermore, 41% of the heifers changed their RFI in period 2 by <0.5 SD, whereas the rest of the heifers changed by ≥0.5 SD. These results indicate that reranking exists in heifers despite receiving the same diet in the 2 feeding periods and that the reranking may be more serious in heifers (28%) with extreme RFI performances in each period.