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J. Anim Sci. 2008. 86:2296-2309. doi:10.2527/jas.2008-0978
© 2008 American Society of Animal Science
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ANIMAL NUTRITION

Effects of supplementation frequency on performance, reproductive, and metabolic responses of Brahman-crossbred females1

R. F. Cooke*,{dagger}, J. D. Arthington*,2, D. B. Araujo*,{dagger}, G. C. Lamb{ddagger} and A. D. Ealy{dagger}

* University of Florida–IFAS, Range Cattle Research and Education Center, Ona 33865; and {dagger} University of Florida–Animal Sciences, Gainesville 32611; and {ddagger} University of Florida–IFAS, North Florida Research and Education Center, Marianna 32446


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Two experiments were conducted to compare performance and metabolic responses of beef females consuming low-quality forages and offered an energy supplement based on fibrous byproducts daily (S7) or 3 times per week (S3) at similar weekly rates. In Exp. 1, BW gain, reproductive performance, mRNA expression of hepatic and skeletal muscle genes associated with nutritional metabolism and growth, and concentrations of blood urea nitrogen (BUN), plasma glucose, insulin, and IGF-I were assessed in 56 Brahman x Angus heifers supplemented at a daily rate of 1.0% of BW. Mean BW gain was greater (P = 0.03) for S7 compared with S3 heifers. Treatment x sampling day interactions were detected (P < 0.01) for all blood measurements. Heifers provided S7 had less daily variation in concentrations of BUN, glucose, and insulin, and frequently had greater (P < 0.05) concentrations of IGF-I compared with S3 heifers. Expression of liver IGF-I mRNA was greater (P = 0.04) for S7 heifers compared with S3 heifers. Treatment x day interactions were detected (P ≤ 0.05) for mRNA expression of liver IGFBP-3, gluconeogenic enzymes, and muscle myostatin because the expression of these transcripts was greater (P < 0.05) for S3 heifers when both treatment groups were supplemented, but was similar or greater (cytosolic phosphoenolpyruvate carboxykinase; P = 0.04) for S7 heifers when only these were supplemented. Attainment of puberty and pregnancy were hastened (P = 0.03 and 0.02, respectively) in S7 heifers compared with S3 heifers. In Exp. 2, 12 Brahman x British mature cows received S3 or S7 for a 3-wk period at a daily rate of 0.5% of BW. Concentrations of BUN were greater for S7 compared with S3 cows (P < 0.03). A treatment x time interaction was detected (P = 0.01) for insulin concentrations because a time effect was significant (P < 0.01) for S3 but not S7 cows. With the advance of the experiment, concentrations of IGF-I increased for S7 (P < 0.01) but not S3 cows (treatment x week interaction; P = 0.02). The combined expression of gluconeogenic enzymes mRNA tended to be greater (P = 0.09) for S3 cows when both treatment groups received supplements, but was greater (P = 0.03) for S7 cows when only these were supplemented (treatment x day interaction; P < 0.01). In conclusion, offering an energy supplement based on fibrous byproducts daily instead of 3 times weekly enhanced the nutritional and metabolic status of forage-fed Brahman-crossbred females, resulting in improved growth and reproductive performance of developing heifers.

Key Words: beef female • gene expression • metabolism • performance • reproduction • supplementation frequency


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Energy supplementation is a common practice in cow-calf systems because BW gain and reproductive function of beef females are influenced positively by energy intake (Schillo et al., 1992Go; Roberts et al., 1997Go). The labor expenses associated with supplement feeding contribute significantly to the fixed costs of cattle operations (Miller et al., 2001Go); therefore, offering supplements 3 times or once weekly instead of daily are typical strategies to decrease costs of production. However, reducing the supplementation frequency of energy feeds to cattle consuming low-quality forages can be detrimental to their performance (Kunkle et al., 2000Go).

Supplementation frequency can affect performance of beef females by many mechanisms, including the modulation of blood concentrations of hormones and metabolites. Infrequent feed intake reduces circulating progesterone (P4) concentrations (Vasconcelos et al., 2003Go) and consequently may be detrimental to puberty attainment and pregnancy establishment (Gonzalez-Padilla et al., 1975Go; Spencer and Bazer, 2002Go). Blood concentrations of glucose, insulin, and IGF-I are affected positively by increased supplementation frequency (Cooke et al., 2007aGo), and these substances are associated with BW gain and reproductive function of cattle (Schillo et al., 1992Go; Spicer and Echternkamp, 1995Go). Based on these observations, we hypothesized that beef females consuming low-quality forages would benefit if supplemented daily instead of 3 times weekly with an energy supplement.

Two experiments were conducted to investigate the effects of supplementation frequency on Brahman-crossbred females. Experiment 1 evaluated BW gain, concentrations of plasma metabolites and hormones, mRNA expression of liver and muscle genes associated with metabolism and growth, and reproductive performance of heifers. Experiment 2 assessed plasma metabolites and hormone concentrations, and mRNA expression of hepatic genes associated with nutritional metabolism of mature cows.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 LITERATURE CITED
 
The animals utilized in these experiments were cared for in accordance with acceptable practices as outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999Go), and the experimental protocols were reviewed and approved by the University of Florida, Institutional Animal Care and Use Committee.

Both experiments were conducted at the University of Florida–IFAS, Range Cattle Research and Education Center, Ona. The first experiment was conducted from September to December 2006, and was divided into a sampling phase (September and October) and a breeding phase (November and December). The second experiment was conducted during the months of October and November 2006.

Animals

Experiment 1. Fifty-six Brahman x Angus heifers (initial BW ± SD = 228 ± 28 kg; initial age ± SD = 293 ± 29 d) were utilized in this experiment. For the sampling phase (d 0 to 45), heifers were stratified by initial BW and age, and randomly allocated to 14 pens (4 heifers/pen) on d –11. Pens were assigned randomly to receive an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 18.2 kg of DM per heifer. Pen was considered the experimental unit (7 pens/treatment), and each pen consisted of 2 ha of bahiagrass (Paspalum notatum) pasture. Heifers were adapted to assigned treatments from d –11 to –1. For the breeding phase (d 46 to 107), heifers were reallocated by treatment into 2 bahiagrass pastures and exposed to Angus bulls. During both sampling and breeding phase, heifers were not rotated among pastures.

Experiment 2. Twelve nonlactating, nonpregnant multiparous Brahman x British cows (BW ± SD = 553 ± 50 kg; average age = 6 ± 2 yr) were stratified by BW and age, housed in individual pens, and randomly assigned to receive S3 or S7 at a weekly rate of 20.3 kg of DM per cow. Cow was considered the experimental unit (6 cows/treatment). Before the beginning of the experiment, with the purpose of acquiring cows with similar and substantial plasma P4 concentrations on d 0 of the study, cows received a 100-µg treatment of GnRH (Cystorelin, Merial Ltd., Duluth, GA) and received a controlled internal drug releasing device containing 1.38 g of P4 (CIDR, Pfizer Animal Health, New York, NY) on d –18, PGF2{alpha} treatment (25 mg Lutalyse, Pfizer Animal Health) and CIDR removal on d –12, and a second GnRH treatment (100 µg) on d –10. On d –4, cows received another PGF2{alpha} treatment (25 mg) and received 2 CIDR that remained in cows throughout the experimental period (d 0 to 17). Transrectal ultrasonography examinations (5.0-MHz transducer, 500V, Aloka, Wallingford, CT) were performed immediately and 48 h after second GnRH (d –8) and PGF2{alpha} (d –4) treatments to verify ovulation and corpus luteum regression, respectively. All cows utilized in this experiment responded to the hormonal treatment, and were adapted to assigned treatments from d –8 to 0.

Diets

Experiment 1. Forage and supplement samples were analyzed for nutrient content by a commercial laboratory (Dairy One Forage Laboratory, Ithaca, NY). All samples were analyzed by wet chemistry procedures for concentrations of CP, ADF, and NDF, whereas TDN was calculated using the equation proposed by Weiss et al. (1992)Go. Pasture quality was estimated at 54% TDN and 8.8% CP (DM basis) from samples collected at the beginning and during the experiment. The pastures utilized in this experiment were not fertilized before or during the experimental period. Stargrass (Cynodon nlemfuensis) hay was offered in amounts to ensure ad libitum access when pasture availability was limited. Hay quality was estimated at 53% TDN and 7.7% CP (DM basis) from samples collected at the beginning of the experiment. A complete commercial mineral/vitamin mix (14% Ca, 9% P, 24% NaCl, 0.20% K, 0.30% Mg, 0.20% S, 0.005% Co, 0.15% Cu, 0.02% I, 0.05% Mn, 0.004% Se, 0.3% Zn, 0.08% F, and 82 IU/g of vitamin A) and water were offered for ad libitum consumption throughout the experiment. Random samples of the supplement were also collected during the experiment. Composition and nutritional profile of the supplement are described in Table 1Go. Heifers were offered supplement at 0700 h daily (S7) or on Mondays, Wednesdays, and Fridays (S3).


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  		Table 1. Ingredient composition and nutrient profile of the supplement offered in Exp. 1 and 2
 
Experiment 2. Forage and supplement samples were analyzed for nutrient content as in Exp. 1. Stargrass hay was offered for ad libitum consumption throughout the entire experiment, and hay quality was estimated at 51% TDN and 6.0% CP (DM basis) from samples collected at the beginning of the experiment. Cows had free access to a complete mineral mix (similar to Exp. 1) and water. A sample of the supplement was also collected at the beginning of the experiment. Composition and nutritional profile of the supplement are described in Table 1Go. Cows were offered supplement at 0800 h daily (S7) or on Mondays, Wednesdays, and Fridays (S3).

Sampling

Experiment 1. Heifers were weighed on 2 consecutive days to determine both full and shrunk (after 16 h of feed and water restriction) BW before the start (d –12 and –11) and at the end of the experiment (d 107 and 108). Blood samples were collected weekly (Wednesday) throughout the entire experiment to determine onset of puberty using plasma P4 concentrations. Heifers were considered pubertal once plasma P4 concentrations were >1.5 ng/mL for 2 consecutive weeks (Cooke et al., 2007bGo).

During the sampling phase, in addition to the weekly collections, blood samples were obtained once per day during 4 consecutive days, every other week, starting at 4 h after supplement was offered to determine concentrations of glucose, blood urea nitrogen (BUN), insulin, IGF-I, and P4. These samples were collected from d 0 to 3, d 14 to 17, d 28 to 31, and d 42 to 45, which were classified as periods (PR1, PR2, PR3, and PR4, respectively). Periods began on Monday and ended on Thursday.

Heifers within pen were assigned randomly for either muscle or liver biopsying on d 35 or 36 of the experiment (Monday or Tuesday). Biopsy procedures began 4 h after supplement was offered. As a result, 2 liver and 2 muscle samples were obtained from each pen for quantitative real-time reverse transcription (RT)-PCR assessment of IGF-I, IGFBP-3, pyruvate carboxylase (PC), cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), mitochondrial PEPCK (PEPCK-M), and cyclophilin mRNA expression in liver samples, and IGF-I, IGFBP-3, IGFBP-5, myostatin, and cyclophilin mRNA expression in muscle samples.

During the breeding phase, each treatment group was exposed to 2 mature Angus bulls at the same time (1:14 bull:heifer ratio), and bulls were rotated weekly between groups to account for potential bull effects. Heifer pregnancy status was verified by detecting a fetus with transrectal ultrasonography (5.0-MHz transducer, Aloka 500V) 70 d after the end of the experiment. Date of conception was estimated retrospectively by subtracting gestation length (286 d; Reynolds et al., 1980Go) from the calving date.

Experiment 2. During a 3-wk period, blood samples were collected immediately before and 4, 8, 24, 28, and 32 h after the first supplement feeding of the week in which cows from both treatments were offered supplements (Mondays; d 1, 8, and 15). Blood samples were analyzed for concentrations of glucose, BUN, insulin, IGF-I, and P4.

Liver samples were collected on d 15 and 16 via needle biopsy, concurrently with blood samplings at 4 and 28 h after first supplement feeding of wk 3, to determine the mRNA expression of IGF-I, IGFBP-3, PC, PEPCK-C, PEPCK-M, and cyclophilin via quantitative real-time RT-PCR.

Blood Analysis

Blood samples were collected via jugular venipuncture during Exp. 1 and from coccygeal vein or artery during Exp. 2 into commercial blood collection tubes (10 mL Vacutainer, Becton Dickinson, Franklin Lakes, NJ) containing sodium heparin, placed on ice immediately, and centrifuged at 2,400 x g for 30 min for plasma collection. Plasma was frozen at –20°C on the same day of collection.

Glucose and BUN concentrations were determined using quantitative colorimetric kits G7521 and B7551, respectively (Pointe Scientific Inc., Canton, MI). A double-antibody RIA was used to determine concentrations of insulin (Malven et al., 1987Go; Badinga et al., 1991Go) and IGF-I (Badinga et al., 1991Go). The extraction procedure used in the IGF-I assay was modified from Badinga et al. (1991)Go by using an ethanol:acetone:acetate ratio of 6:3:1. Concentrations of P4 were determined using Coat-A-Count solid-phase 125I RIA kit (DPC Diagnostic Products Inc., Los Angeles, CA). The intra- and inter-assay CV for Exp. 1 were, respectively, 4.1 and 2.7% for glucose, 3.8 and 5.3% for BUN, 8.8 and 7.9% for insulin, 8.9 and 11.8% for IGF-I, and 4.8 and 5.9% for P4. The intra- and interassay CV for Exp. 2 were, respectively, 4.8 and 6.4% for glucose, 3.8 and 9.5% for BUN, 12.3 and 12.0% for insulin, 8.6 and 5.1% for IGF-I, and 6.7 and 6.1% for P4. For both experiments, the minimum detectable concentrations of insulin, IGF-I, and P4 were 0.02, 10, and 0.1 ng/mL, respectively.

Tissue Analysis

Tissue Collection and RNA Extraction. Liver and LM biopsies were performed by trained personnel following the techniques described by Arthington and Corah (1995)Go. Immediately after collection, liver and muscle samples (average 100 mg of tissue, wet weight) were placed in 1 mL of RNA stabilization solution (RNAlater, Ambion Inc., Austin, TX), maintained at 4°C for 24 h, and stored at –20°C.

Total RNA was extracted from tissue samples using TRIzol Plus RNA Purification Kit (Invitrogen, Carls-bad, CA). Quantity and quality of isolated RNA were assessed via UV absorbance at 260 nm and 260/280 nm ratio, respectively (GeneQuant spectrophotometer, Amersham Pharmacia Biotech, Cambridge, UK). Extracted RNA was stored at –80°C until further processing.

Real-Time RT-PCR. Extracted RNA from liver and muscle samples (2.5 and 1.0 µg, respectively) were incubated at 37°C for 30 min in the presence of RNase-free DNase (New England Biolabs Inc., Ipswich, MA) to remove contaminant genomic DNA. After inactivating the DNase (75°C for 15 min), samples were reverse-transcribed using the High Capacity cDNA Reverse Transcription Kit with random hexamers (Applied Biosystems, Foster City, CA). Real-time PCR was completed using the SYBR Green PCR Master Mix (Applied Biosystems) and specific primer sets (25 ng/mL; Table 2Go), with a 7300 Real-Time PCR System (Applied Biosystems). Following incubation at 95°C for 10 min, 40 cycles of denaturation (95°C for 15 s) and annealing/synthesis (60°C for 2 min) were completed. Each RNA sample was analyzed in triplicate, and the absence of genomic contamination was verified by including a fourth reaction lacking exposure to reverse transcriptase. At the end of each PCR, amplified products were subjected to a dissociation gradient (95°C for 15 s, 60°C for 30 s, and 95°C for 15 s) to verify the amplification of a single product by denaturation at the anticipated temperature. A portion of the amplified products was purified with the QIAquick PCR purification kit (Qiagen Inc., Valencia, CA) and sequenced at the University of Florida DNA Sequencing Core Facility to verify the specificity of amplification. All amplified products represented only the genes of interest.


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  		Table 2. Primer sets used for quantitative real-time reverse transcription-PCR
 
Responses were quantified based on the threshold cycle (CT), the number of PCR cycles required for target amplification to reach a predetermined threshold. All CT responses from genes of interest were normalized to cyclophilin CT examined in the same sample and assessed at the same time as the targets. Results are expressed as relative fold change (2–{Delta}{Delta}CT), as described by Ocón-Grove et al. (2008)Go.

Internal RNA standard samples were included in each assay (PCR plate). The interassay CV for Exp. 1 was 0.7% for liver IGF-I, 0.6% for liver IGFBP-3, 0.6% for PC, 0.6% for PEPCK-C, 0.8% for PEPCK-M, 0.7% for muscle IGF-I, 0.5% for muscle IGFBP-3, 0.9% for IGFBP-5, and 0.7% for myostatin. The interassay CV for Exp. 2 was 0.9% for liver IGF-I, 1.0% for liver IG-FBP-3, 0.9% for PC, 1.1% for PEPCK-C, and 1.2% for PEPCK-M.

Statistical Analysis

Experiment 1. Performance, physiological, and gene expression data were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC) and Satterthwaite approximation to determine the denominator degrees of freedom for the tests of fixed effects. Gene expression data were further tested for normality with the Shapiro-Wilk test from the UNIVARIATE procedure of SAS, and results indicated that all data were distributed normally (W ≥0.90). The model statement used for hormone and metabolite analysis contained the effects of treatment, period, day(period), and the interactions of treatment x period and treatment x day(period). Data were analyzed using heifer(pen) and pen(treatment) as random variables. The model statement used for ADG contained only the effect of treatment. Data were analyzed using pen(treatment) as the random variable. The model statement used for gene expression analysis contained the effects of treatment, day, and the interaction. Results are reported as least squares means and were separated using LSD. Puberty and pregnancy data were analyzed with survival analysis (LIFETEST procedure of SAS) by regressing the proportion of pre-pubertal or nonpregnant heifers on week of the study or breeding season, respectively. Differences between treatment survival curves were determined by the Wilcoxon test. For all analysis, significance was set at P ≤ 0.05, tendencies were determined if P > 0.05 and ≤0.10, and results are reported according to treatment effects if no interactions were significant, or according to the highest-order interaction detected.

Experiment 2.

Data were analyzed using the MIXED procedure of SAS and Satterthwaite approximation to determine the denominator degrees of freedom for the tests of fixed effects. Gene expression data also were tested for normality with the Shapiro-Wilk test from the UNIVARIATE procedure of SAS, and results indicated that all data were distributed normally (W ≥0.90). The model statement used for hormone and metabolite analysis contained the effects of treatment, week, time(week), and the interactions of treatment x week and treatment x time(week). Data were analyzed using cow(treatment) and cow(treatment) x week as random variables. The model statement used for gene expression analysis contained the effects of treatment, day, and the interaction. The random variable was cow(treatment). Additionally, mRNA expression of PEPCK-C, PEPCK-M, and PC were analyzed jointly. This model statement contained the effects of treatment, day, enzyme, and the resultant interactions, whereas cow(treatment) was the random variable. Significance was set at P ≤ 0.05, tendencies were determined if P > 0.05 and ≤ 0.10. Results reported are least squares means, were separated using LSD, and are reported according to treatment effects if no interactions were significant, or according to the highest-order interaction detected.


   RESULTS AND DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
LITERATURE CITED
 
Experiment 1

Heifers provided S7 had greater (P = 0.03) ADG compared with S3 heifers (0.41 vs. 0.33 kg/d, respectively; SEM = 0.02; data not shown), concurring with studies reporting increased BW gain of cattle fed low-quality forages and offered energy supplements daily vs. 3 times weekly (Kunkle et al., 2000Go; Cooke et al., 2007aGo). Reproductive performance was also affected by treatments. Attainment of puberty (Figure 1Go) and pregnancy (Figure 2Go) were hastened (P = 0.03 and 0.02, respectively) in S7 heifers compared with S3 heifers. In a review article, Kunkle et al. (2000)Go indicated that decreased supplementation frequency of energy supplements containing high-starch ingredients is detrimental to cattle performance because of impaired rumen function, forage intake, and digestibility. However, different outcomes were observed in cattle supplemented with low-starch energy byproducts. Loy et al. (2007)Go reported similar mean forage intake, rumen pH, and in situ forage NDF disappearance of beef heifers supplemented with distillers grains daily or on alternate days. Cooke et al. (2007a)Go reported that beef steers offered citrus pulp-based supplements daily had similar mean forage DMI but improved ADG compared with steers offered the same supplement 3 times weekly, and attributed the differences in ADG to beneficial effects of daily supplementation on concentrations of hormones and metabolites associated with energy intake.


Figure 1
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  		Figure 1. Proportion of prepubertal heifers by week during Exp. 1. Survival curves represent heifers offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 18.2 kg of DM per heifer. Heifers were considered pubertal once plasma progesterone concentrations were greater than 1.5 ng/mL for 2 consecutive weeks, and puberty attainment was declared at the first week of elevated progesterone. A treatment effect was detected (P = 0.03).

 

Figure 2
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  		Figure 2. Proportion of nonpregnant heifers by week during the 60-d breeding season of Exp. 1. Survival curves represent heifers offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 18.2 kg of DM per heifer. Date of conception was estimated retrospectively by subtracting gestation length (286 d; Reynolds et al., 1980Go) from the calving date. A treatment effect was detected (P = 0.02).

 
A treatment x day(period) interaction was detected (P < 0.01) for BUN analysis (Table 3Go). During the initial half of PR2, and throughout PR3 and PR4, S3 heifers had greater (P ≤ 0.05) BUN concentrations compared with S7 heifers during the days that only S7 heifers were offered supplements, but decreased (P < 0.01) BUN concentrations during the days that both treatment groups were offered supplements. Concentrations of BUN are positively associated with intake of rumen-degradable protein, levels of ruminal ammonia, and ruminal protein:energy ratio (Hammond, 1997Go). The reductions in BUN concentrations detected in S3 heifers on days that both treatment groups were supplemented reflected a decreased ruminal protein:energy ratio created in these heifers because they were offered a greater amount of supplemental energy compared with S7 heifers. Conversely, increased BUN concentrations of S3 heifers during days that only S7 heifers were supplemented reflected an increased ruminal protein:energy ratio because S3 heifers did not receive any supplemental energy but likely maintained a significant ruminal N supply due to urea recycling (Lapierre and Lobley, 2001Go). This biphasic pattern of BUN concentrations detected in S3 heifers was also reported by others (Bohnert et al., 2002Go) and illustrates the greater daily variation of energy and protein intake of S3 heifers compared with S7 heifers. Heifers from both treatments frequently had BUN concentrations above the optimal level (11 and 15 mg/dL; Byers and Moxon, 1980Go), indicating that rumen-degradable protein and thus CP were consumed in excess by these animals. Further, the sharp increases in BUN detected for S3 heifers during nonsupplementation days may have influenced negatively their performance because excessive rumen ammonia concentrations requires additional energy to be metabolized into urea by the liver (Reynolds, 1992Go).


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  		Table 3. Blood urea N (BUN) and plasma concentrations of glucose, insulin, and IGF-I of heifers (Exp. 1) offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 18.2 kg of DM per heifer1,2
 
A treatment x day(period) interaction was detected (P < 0.01) for glucose and insulin analysis (Table 3Go). A day(period) effect was detected (P < 0.01) for glucose concentrations of S3 heifers throughout the experimental period, whereas a tendency (P = 0.06) for a day(effect) in glucose concentrations of S7 heifers was detected only during PR4. Similarly, a day(period) effect was detected (P < 0.01) for insulin concentrations of S3 heifers throughout the experimental period, whereas it was significant (P = 0.02) for S7 heifers only during PR4. In addition, glucose and insulin concentrations were typically greater for S3 heifers compared with S7 heifers on the days that only S7 heifers were supplemented, but not on days that both treatment groups were supplemented (Table 3Go). Significant differences (P ≤ 0.05) were detected on Wednesday during PR1, Tuesday during PR2, Tuesday and Thursday during PR3, and Tuesday during PR4 for glucose, and on Tuesday during PR2, PR3, and PR4 for insulin. Concentrations of plasma glucose and insulin are influenced positively by rate of nutrient intake (Vizcarra et al., 1998Go); therefore, the day(period) effects observed in this experiment reflected the differences in nutrient intake pattern between treatments. During the experimental period, S3 heifers received and readily consumed (within 2 h) bolus amounts of supplements on 3 d of the week, but had low-quality forage as the only source of feed available on the remaining days. Conversely, S7 heifers received and consumed within 1 h smaller portions of supplements on a daily basis.

A treatment x day interaction was detected for mRNA expression of liver PC (P = 0.02) and PEPCK-C (P < 0.01; Table 4Go). These interactions were detected because a day effect (P < 0.01) was significant for these transcripts in S3 heifers but not in S7 heifers. Additionally, the mRNA expressions of PC and PEPCK-C were greater (P < 0.01 and P = 0.02, respectively) for S3 heifers compared with S7 heifers when both treatment groups were supplemented (d 35), but were similar or greater (PEPCK-C; P = 0.04) for S7 heifers when only these were supplemented (d 36). Treatment effects on liver PEPCK-M mRNA expression differed from the other gluconeogenic transcripts because S7 heifers tended (P = 0.08) to have a greater mRNA expression of liver PEPCK-M compared with S3 heifers (Table 4Go). The treatment effects detected on mRNA expression of liver PC and PEPCK-C also reflect differences in the nutrient intake pattern between S3 and S7 heifers. Expression of these enzymes was associated positively with enzymatic activity and consequent glucose synthesis in cattle (Greenfield et al., 2000Go; Agca et al., 2002Go; Bradford and Allen, 2005Go). Given that the availability of nutrients originating from ruminal fermentation increases rapidly in forage-fed ruminants after supplementation (Seoane and Moore, 1969Go; Rihani et al., 1993Go; Farmer et al., 2001Go) and that expression of liver enzymes associated with gluconeogenesis is quickly altered (She et al., 1999Go; Massillon et al., 2003Go) and increased when precursors are available (Greenfield et al., 2000Go; Karcher et al., 2007Go), the greater mRNA expression of PC and PEPCK-C detected in S3 heifers compared with S7 heifers when both treatment groups were offered supplements can be attributed to their greater supplement consumption on that day. Accordingly, mRNA expression of these enzymes was significantly reduced in S3 heifers but not in S7 heifers when only S7 heifers received supplements. The reason why PC and PEPCK-C expression was altered by 4 h postsupplementation, whereas plasma glucose and insulin responses to supplement consumption were not detected until the next day in S3 heifers is likely because of the time required for PC and PEPCK-C mRNA to be translated into active enzymes and substantially change the magnitude of glucose synthesis and release by the liver. Previous studies also reported that plasma glucose concentrations of forage-fed developing heifers (Cooke et al., 2007bGo) and yearling steers (Cooke et al., 2007aGo) offered supplements based on low-starch energy by-products 3 times weekly were greater at 28 vs. 4 h after supplementation. Liver PEPCK-M may be responsible for as much as 61% of glucose synthesis in ruminant hepatocytes (Aiello and Armentano, 1987Go), although it is considered constitutive and not responsive to hormones and nutritional state (Greenfield et al., 2000Go; Agca et al., 2002Go). Nevertheless, the tendency for greater mRNA expression of liver PEPCK-M in S7 heifers compared with S3 heifers may be an indicator of their improved energy status and have contributed to their increased performance.


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  		Table 4. Expression of hepatic genes associated with nutritional metabolism and growth of heifers (Exp. 1) offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 18.2 kg of DM per heifer1,2
 
Concentrations of IGF-I were typically increased for S7 heifers compared with S3 heifers, and there was a treatment x day(period) interaction (P < 0.01; Table 3Go). Significant differences (P ≤ 0.05) were detected on Mondays (PR1, PR2, PR3, and PR4), and Wednesday of PR4. Furthermore, mean IGF-I concentrations tended (P = 0.10) to be greater for S7 vs. S3 heifers (190 and 161 ng/mL, respectively; SEM = 12). Nutritional status, growth, and reproductive performance of beef cattle are influenced positively by circulating IGF-I (Roberts et al., 1997Go; Diskin et al., 2003Go; Cooke et al., 2007bGo). Therefore, the treatment effects detected for ADG and reproductive performance in this experiment can be attributed, at least to some degree, to the greater concentrations of IGF-I frequently detected for S7 compared with S3 heifers. Although several studies reported a positive association between IGF-I concentrations and nutrient intake (Ellenberger et al., 1989Go; Bossis et al., 1999Go; Lapierre et al., 2000Go), treatment differences in IGF-I concentrations were detected despite the similarity in overall supplement intake between S3 and S7 heifers. Thus, it remains possible that such differences resulted from treatment effects on forage intake, but this explanation is unlikely because previous research efforts reported similar mean forage intake of cattle offered low-starch supplements daily or 3 times weekly (Cooke et al., 2007aGo; Loy et al., 2007Go). In addition, increases in plasma IGF-I concentrations due to enhanced nutrient or feed intake are usually accompanied by increased mean glucose and insulin concentrations (Bossis et al., 1999Go; Lapierre et al., 2000Go; Hersom et al., 2004Go), and nutrients originating from low-quality forages do not contribute significantly to hepatic IGF-I synthesis and release (Cooke et al., 2007bGo).

Heifers provided S7 had greater (P = 0.04) expression of liver IGF-I mRNA compared with S3 heifers (Table 4Go). A treatment x day interaction was detected (P = 0.04) for liver IGFBP-3 because a day effect (P < 0.01) was detected for IGFBP-3 mRNA expression in S3 but not in S7 heifers, whereas IGFBP-3 mRNA expression was greater (P = 0.03) for S3 heifers compared with S7 heifers when both treatment groups were supplemented (d 35) but similar when only S7 heifers were supplemented (d 36). The major source of circulating IGF-I synthesis is the liver (D’Ercole et al., 1984Go). Blood concentrations of IGF-I are associated with IGF-I mRNA expression in hepatic cells (Thissen et al., 1994Go), and this relationship is supported by the present experiment. The availability of energy substrates has a positive influence on the expression of liver IGF-I mRNA and consequent translation into the circulating protein (McGuire et al., 1992Go; Thissen et al., 1994Go). Increased hepatic IGF-I mRNA expression and plasma IGF-I concentrations detected in S7 heifers may be attributed to a greater availability of substrates for metabolic and physiologic processes in these heifers because of an improved efficiency of nutrient metabolism. By consuming small quantities of supplement on a daily basis, S7 heifers were perhaps capable of retaining and processing these nutrients more efficiently than S3 heifers, although further research efforts are required to evaluate this assumption. Treatment effects on liver IGFBP-3 mRNA expression differed from those detected for IGF-I, although IGFBP-3 expression and synthesis are stimulated by circulating IGF-I (Thissen et al., 1994Go). Nevertheless, the day effect detected for expression of liver IGFBP-3 mRNA only in S3 heifers may be an additional indicator of the daily variation of nutrient intake and consequent availability of substrates for metabolic processes in these heifers.

A treatment x day interaction was detected (P = 0.05) for muscle myostatin mRNA expression (Figure 3Go). A day effect (P < 0.01) was detected in S3 heifers but not in S7 heifers. Further, myostatin mRNA expression was greater (P = 0.04) for S3 heifers compared with S7 heifers when both treatment groups were supplemented (d 35) but similar when only S7 heifers were supplemented (d 36). Myostatin is a growth factor expressed in skeletal muscle of developing and mature animals that negatively influences muscle growth (Lee and McPherron, 2001Go; Dayton and White, 2008Go). Myostatin is believed to impair glucose uptake in muscle tissues by decreasing the activity of insulin-dependent glucose transporter 4, resulting in insulin resistance and impaired muscle tissue development (Strassman et al., 2002Go; Antony et al., 2007Go). The treatment effects detected for myostatin mRNA expression indicate that muscle tissues of S7 heifers were developing in a more constant pattern compared with those of S3 heifers, possibly due to treatment differences on nutrient intake and availability. No further treatment differences were detected within mRNA expression of muscle genes (data not shown). Nevertheless, the lack of treatment effects on muscle IGF-I and IGFBP may indicate that within the IGF-I sources, circulating IGF-I was likely the major contributor for heifer body growth. Supporting our findings, Johnson et al. (1996)Go reported that muscle growth is significantly stimulated by circulating IGF-I. Conversely, IGF-I synthesized by skeletal muscle in growing cattle is implicated as an important and often essential autocrine and paracrine mediator of tissue development and differentiation (McGuire et al., 1992Go; Johnson et al., 1998Go), and research with mice suggested that IGF-I synthesized in muscle tissues exerts a more important role in muscle growth compared with hepatic-originated IGF-I (Sjogren et al., 1999Go; Yakar et al., 1999Go).


Figure 3
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  		Figure 3. Expression of muscle myostatin, as relative fold change (Ocón-Grove et al., 2008Go), of heifers (Exp. 1) offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 18.2 kg of DM per heifer. Heifers from both S3 and S7 groups were supplemented on d 35, whereas only S7 heifers were supplemented on d 36 of the study. Tissue collection started 4 h after supplement feeding time. A treatment x day interaction was detected (P = 0.05). A day effect was detected for S3 heifers (P < 0.01), but not for S7 heifers. Treatment comparison within days: *P = 0.04.

 
In summary, offering an energy supplement based on fibrous byproducts daily instead of 3 times weekly to developing heifers resulted in a normalized mRNA expression pattern of genes associated with nutritional metabolism and growth, reduced daily variation in plasma concentrations of BUN, glucose, and insulin, and improved heifer nutritional status as reflected by increased hepatic mRNA expression and elevated plasma concentrations of IGF-I. These beneficial metabolic effects were translated into greater BW gain and hastened attainment of puberty and pregnancy; therefore, daily supplementation of high-energy byproducts enhances development and performance of Brahman-crossbred heifers consuming low-quality forages.

Experiment 2

A treatment effect and a treatment x time(week) interaction were detected (P = 0.03 and P < 0.01, respectively) for BUN (Table 5Go). Cows receiving S7 had increased BUN concentrations compared with S3 cows during wk 2 and 3 (Table 5Go), with significant differences (P < 0.05) detected at 4 and 8 h after the first supplement feeding of wk 2, and at 0, 4, 8, 24 and 32 h after the first supplement feeding of wk 3. Additionally, S7 cows had greater mean BUN concentrations compared with S3 cows (9.2 vs. 7.9 mg/dL, respectively; SEM = 0.36). These results differed from Exp. 1 because S7 cows likely had greater mean ruminal ammonia concentrations compared with S3 cows, BUN concentrations increased in a similar pattern for S3 and S7 cows after supplement consumption, and were typically at adequate levels for both treatments (8 mg/dL; Hammond, 1997Go). The differences in BUN responses between Exp. 1 and 2 may be explained by the reduced amount of supplement and thus rumen-degradable protein consumed by cows vs. heifers in relation to their BW (0.5 and 1.0% of BW on a daily basis, respectively). Cows were offered supplements at a daily rate of 0.5% of BW to avoid energy overfeeding that may hinder the detection of treatment effects (Cooke et al., 2007bGo), whereas this rate is adequate to maintain brood cows at a moderate positive energy balance (NRC, 1996Go).


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  		Table 5. Blood urea nitrogen (BUN) and plasma insulin concentrations of cows (Exp. 2) offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 20.3 kg of DM per cow1
 
A treatment x time(week) interaction was detected (P = 0.01) for insulin (Table 5Go) because a time(week) effect was detected for insulin concentrations of S3 cows (P < 0.01) but not S7 cows, and treatment differences were detected at 0 h (P = 0.03) and 48 h (P = 0.05) after the first supplement feeding of wk 2 (Table 5Go). Similarly, a significant time(week) effect was detected for glucose concentrations of S3 cows (P < 0.01) but not S7 cows, although a treatment x time(week) interaction was not significant (P = 0.12; data not shown). Within the gluconeogenic enzymes, a treatment x day interaction was detected (P < 0.01) only for PC mRNA expression. Similar to Exp. 1, PC mRNA expression was numerically greater for S3 cows compared with S7 cows when both treatment groups were supplemented (d 15), but numerically greater for S7 cows when only these were offered supplements (d 16; Table 6Go). When data from all 3 gluconeogenic enzymes are analyzed jointly, S3 cows tended (P = 0.09) to have increased mRNA expression of these transcripts on d 15 (2.49 vs. 2.07 relative fold change, respectively; SEM = 0.17), but reduced (P = 0.03) mRNA expression of these transcripts on d 16 (1.48 vs. 2.03 relative fold change, respectively; SEM = 0.17) compared with S7 cows (treatment x day interaction, P < 0.01). Further, a day effect was observed in the combined mRNA expression of gluconeogenic enzymes for S3 cows (P < 0.01), but not for S7 cows. These results support those reported in Exp. 1, particularly because supplement intake behavior was similar in both experiments, and indicate that concentrations of glucose and insulin and expression of gluconeogenic enzymes transcripts varied significantly within 32 h after supplementation in S3 cows, but remained generally constant for S7 cows during the same period because of treatment differences in nutrient intake pattern. Different from Exp. 1, treatment effects detected for PEPCK-M mRNA expression were similar to those detected for PC and PEPCK-C, indicating that mRNA expression of this transcript was also influenced by nutrient intake pattern.


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  		Table 6. Expression of hepatic genes associated with nutritional metabolism and status of cows (Exp. 2) offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 20.3 kg of DM per cow1,2
 
A treatment x week interaction was detected (P = 0.02) for IGF-I (Table 7Go) because concentrations of this hormone increased for S7 cows (P < 0.01) but not for S3 cows with the advance of the experiment. As a consequence, S7 cows tended (P = 0.09) to have greater IGF-I concentrations compared with S3 cows on wk 3 (Table 7Go), indicating that nutritional status was improving for S7 cows compared with S3 cows with the advance of the experiment. No treatment effects were detected in the expression of liver IGF-I and IGFBP-3 mRNA (Table 6Go). However, liver IGF-I mRNA expression was numerically increased for S7 cows compared with S3 cows, and the lack of statistical significance is most likely related to the reduced number of animals utilized in this experiment.


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  		Table 7. Plasma IGF-I concentrations (ng/mL) of cows (Exp. 2) offered an energy supplement based on fibrous byproducts daily (S7) or 3 times weekly (S3), at a weekly rate of 20.3 kg of DM per cow1
 
No differences were detected for plasma P4 concentrations (data not shown) between treatments. A week effect was detected (P < 0.01) because P4 concentrations decreased linearly for both treatments from the first week to the last week of the experiment, and this effect can be associated with the decrease in P4 release from CIDR with advancing time (6.62, 4.54, and 2.68 ng/mL for wk 1, 2, and 3, respectively; P < 0.01; SEM = 0.37; data not shown). Infrequent intake of large amounts of feed decreases circulating concentrations of P4 in cattle (Vasconcelos et al., 2003Go; Cooke et al., 2007bGo), and therefore may impair reproductive performance because P4 is required for adequate attainment of puberty (Gonzalez-Padilla et al., 1975Go) and establishment of pregnancy (Spencer and Bazer, 2002Go). Cooke et al. (2007b)Go reported that beef heifers and cows consuming low-quality forages and offered supplements based on low-starch energy byproducts 3 times weekly at a daily rate of 1.0% of BW had reduced plasma P4 concentrations on days that supplements were offered vs. days that supplements were not offered. In the present experiment, cows were offered moderate amounts of supplement (0.5% of BW on a daily basis) to avoid energy overfeeding; therefore, this lower level of supplementation likely prevented plasma P4 concentrations from changing significantly. Conversely, heifers from Exp. 1 were supplemented at 1.0% of BW on a daily basis and treatment effects on P4 concentrations were not evaluated due to the reduced number of pubertal heifers during the sampling phase. Consequently, further research is required to address the effects of supplementation frequency on circulating P4 metabolism of developing heifers.

In summary, offering an energy supplement based on fibrous byproducts daily instead of 3 times weekly to mature cows resulted in a normalized pattern of gluconeogenic enzymes mRNA expression, reduced variation in plasma concentrations of glucose and insulin, and improved cow nutritional status as observed by increasing plasma IGF-I concentrations. Therefore, it could be postulated that Brahman-crossbred cows consuming low-quality forages and supplemented daily with high-energy byproducts would experience improved performance and reproductive efficiency compared with cows supplemented 3 times weekly, although further research is required to address this matter.


   Footnotes
 
1 Appreciation is expressed to Austin Bateman, Joyce Hayen, Idania Alvarez, Mehmet Gulay, and Flavia Cooke (University of Florida-IFAS, Gainesville) for their assistance during this study. Back

2 Corresponding author: jarth{at}ufl.edu

Received for publication February 22, 2008. Accepted for publication April 23, 2008.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 


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