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1 Department of Animal Sciences, University of Illinois, Urbana 61801
2 USDA-ARS, BFGL, Beltsville, MD 20705
Corresponding author: Geoffrey E. Dahl; e-mail: gdahl{at}uiuc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: photoperiod • protein • heifer • growth
Abbreviation key: EBF = empty body fat, EBM = empty body mineral, EBP = empty body protein, EBW = empty body water, EBWT = empty body weight, HG = heart girth, HH = hip height, LDPP = long-day photo-period, PRL = prolactin, SDPP = short-day photoperiod, WH = withers height.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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We hypothesized that the potential negative effects of accelerating growth and hastening puberty of dairy heifers through exposure to LDPP could be overcome by providing greater supply of metabolizable protein through RUP feeding. The objectives of the slaughter study were to determine the influence of photoperiod on mammary growth and to compare the slaughter (i.e., direct chemical analysis of tissue composition) and deuterium oxide (D2O) dilution methods for assessment of body composition in prepubertal heifers exposed to LDPP or short-day photoperiod (SDPP). A second growth study was conducted to determine whether LDPP in combination with RUP could promote lean growth without limiting body size in the prepubertal heifer.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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. Animals were fed to ad libitum intake at 1000 h each day. Calves were housed in identical pens within the Photoperiod Research Barn at the University of Illinois Dairy Center. Each pen was lit by metal halide bulbs to an average illumination intensity of 450 lx at a height of 1 m above the stall floor. Although only 2 pens were used for this initial study, heifer was used as the experimental unit, as each animal received light individually, albeit from the same overall source. It can be argued that pen per se may influence the response, and photo-period and pen are confounded, yet in previous studies we have not observed an effect of pen on photoperiodic responses in this (Auchtung et al., 2004, 2005) or other facilities (Auchtung et al., 2003; Kendall et al., 2003).
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For the main growth experiment, an additional 32 weaned heifers (86 ± 2 d old; BW = 106.2 ± 17.3 kg) were assigned to 1 of 4 treatments in a randomized complete block design and 2 x 2 factorial arrangement until the onset of puberty, assessed by progesterone concentration in plasma. The treatments were photoperiod (LDPP or SDPP) and diet (control or RUP-supplemented diet). Heifers were housed similarly to those in the slaughter study except that there were 3 pens of each dietary treatment within a photoperiodic treatment, for a total of 12 pens. Each pen contained 2 or 3 animals, all on the same dietary by photoperiodic treatment. As in the slaughter study, animal was considered the experimental unit. Heifers were assigned to treatments in 3 groups on October 18, 2002 (n = 8; group 1), December 5, 2002 (n = 12; group 2), and January 24, 2003 (n = 12; group 3).
The RUP-supplemented diet contained fish meal (2% on a DM basis) to increase the RUP level. The protein content of each diet was balanced to 16% according to NRC (2001) recommendations. Control animals received an identical forage diet with the addition of soybean meal instead of fish meal to fulfill the protein requirement. The diet was adjusted every 14 d to account for changes in the DM content of the ingredients. Feed was offered ad libitum once daily after refusals were collected. Weekly samples of each TMR were analyzed for chemical composition (Table 1
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In the growth experiment BW, withers height (WH), hip height (HH), and heart girth (HG) were recorded on d –1 of the experiment and every 14 d thereafter during the experimental period until the onset of puberty, and during posttreatment until heifers reached the breeding target of BW (375 kg) and WH (132 cm). After confirmation of puberty, heifers were removed individually from experimental treatments, commingled among treatments, and housed under natural photoperiod (spring–summer season) in heifer facilities at the University of Illinois Dairy Center. Animals received regular nutritional management for growing heifers according to NRC (2001) recommendations. Collection of skeletal growth data (i.e., BW, WH, HH, and HG) continued during the posttreatment period until pregnancy was confirmed, and a final measurement was made at parturition.
D2O Dilution
Seven days before each slaughter, a D2O study was conducted to assess the water content of the heifers. Deuterium oxide dilution is a noninvasive technique that provides an economical and repeatable method to describe the body composition of animals throughout their growth period (Shipley and Clark, 1972; Byers, 1979). Because of the association of body water and body composition, a stable isotope such as D2O can be used to estimate empty body water (EBW). The positive relationship between EBW, empty body protein (EBP), and empty body mineral (EBM) is used to estimate the protein and mineral content of the empty body weight (EBWT). However, there is an inverse relationship between empty body fat (EBF) and EBW. Blood samples (10 mL) were collected via jugular venipuncture using a Vacutainer tube and 20-gauge needle (Becton Dickinson Vacutainer Systems, Franklin Lakes, NJ) and immediately chilled in ice. The baseline sample was immediately followed by injection of 300 mg of D2O/kg of BW in sterile physiological saline (0.9%). The syringes containing the D2O in physiological saline were weighed before and after administration to record the actual dose of D2O injected gravimetrically. The D2O was allowed to equilibrate with body water for 1 h. Six blood samples (10 mL) were taken over 6 d (1/d) between 0800 and 1000 h before feeding to estimate the dilution of the tracer. Blood was centrifuged (2000 x g for 30 min, 4°C), and plasma was harvested and stored at – 20°C until D2O analysis. This analysis consisted of detection of D2O by infrared spectroscopy (Miran Fixed Infrared Filter; The Foxboro Co., Bridgewater, MA), using the procedures of Byers (1979) modified for automated sample delivery. The disappearance curve of D2O provided the statistic for estimation of EBW.
Two models were used to analyze data for the D2O dilution, the 1-pool and 2-pool models; however, we were unable to estimate the statistical parameters using the 2-pool model. The 1-pool model was used to estimate EBW content in 3- and 5-mo-old heifers; however, we were unable to estimate the kinetics of EBW in 7-mo-old heifers.
For the 1-pool model:
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where Q(t) is the D2O concentration, and Q0 is the concentration at time 0, e is the natural log base, k is the constant decay rate; and t is time in min.
Blood Sampling and Hormone Assays
In the growth experiment, blood samples were collected every 2 wk to monitor circulating progesterone concentrations in a sequential manner. The frequency of sampling increased to once a week when each heifer was approximately 60 d from its expected puberty date (i.e., 7 mo old). Plasma was separated by centrifugation at 2000 x g for 30 min and was stored at –20°C until assayed. Heifers were considered pubertal at the first of 2 consecutive samples containing progesterone concentrations greater than 1.0 ng/mL. In both experiments, blood samples were also collected to determine prolactin (PRL) concentrations and confirm the effect of photoperiod treatment. Plasma samples were analyzed in duplicate using samples from 3 consecutive days at d 70 to 72 and d 140 to 142 of the experimental period using a previously validated PRL radioimmunoassay (Miller et al., 1999).
Growth, Development, and Composition
Slaughter periods were selected to allow investigation of the effect of photoperiod treatment between weaning and before the predicted onset of puberty (Moallem et al., 2004a). Protein, fat, and water were determined in carcass and inedible samples in duplicate (Moallem et al., 2004b). Samples (10 g) for carcass DM determinations were placed in crucibles in an oven (105°C) for 24 h. Crucibles were removed from the oven and placed in a desiccator at room temperature for 30 min, and dry weight was recorded. Dry matter was estimated from the final weight subtracted from the initial weight. Total nitrogen was determined and CP was calculated as N x 6.25. Ether extract was determined by AOAC (1984) procedures. Ash content was measured by combustion at 600°C for 16 h. Empty body composition of water, protein, fat, and ash for each individual animal was estimated using the weight of the carcass and inedible fractions multiplied by the percentage of each fraction. Udder halves obtained at slaughter were sliced into ~34-mm sections and parenchyma was dissected from mammary fat. Each fraction was weighed and parenchyma was ground for DNA content determination by Hoechst 33258 dye binding (Labarca and Paigen, 1980). Parenchyma lipid percentage was determined gravimetrically by chloroform-methanol extraction (Folch et al., 1957).
Statistical Analyses
The software of SAS System v. 9.1 (SAS Inst. Inc., Cary, NC) for the ANOVA in the MIXED procedure provided the statistics for the dependent variables. In the preliminary study, because of the variation in BW between baseline and experimental animals, heifers were blocked for BW. The MIXED procedure from SAS was used to analyze estimated values of EBW, EBP, EBM, and EBF. For the growth study, the analysis of body growth included the covariates of BW, WH, HH, and HG at d –1. The model included time in the REPEATED statement, the fixed effects of photoperiod and diet, and the random effects of heifer and all possible interactions. Each heifer was a subject of the repeated measures statement and was nested within treatment. The variance covariance structure used in the model was autoregressive 1 (AR 1) based on best-fit statistic using Akaike’s information criterion (AIC), AIC for small samples, and Bayesian information criterion for the body growth analysis. Further, linear regression and correlation coefficients were determined between age at puberty and different body size parameters (BW, WH, HH, and HG). Puberty as well as plasma PRL concentrations were analyzed using the MIXED procedure of SAS and the model included the fixed effects of photoperiod and diet, and the random effect of heifer. Student’s t-test was used to evaluate effects of photoperiod on mammary growth parameters.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Body Composition
In the slaughter study, comparisons of body composition determined by a technique using carcass slaughter balance chemical analysis and by the D2O technique were made for the baseline and 5 mo-old heifers; however, the third evaluation of body composition, at 7 mo of age, was determined only by chemical analysis. Although there were no significant treatment differences observed for EBWT, EBW, or EBP during the experimental period, SDPP heifers at 7 mo of age accrued less body fat (P = 0.01) relative to LDPP heifers (Table 2). Across time, although not significant, body water content decreased on average from 65 to 62%, CP content remained similar at approximately 19%, and fat content increased with age from 11 to 16%, and mean ash declined from 3.5 to 2.5%.
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). However, measurements of fat content using the 2 methods were not correlated in 5-mo-old heifers (Table 3).
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; Tables 4 and 5). This reduced rate of gain was apparent regardless of treatment and may have resulted from cold stress due to low ambient temperature, low stocking density, and sand bedding. These conditions were especially severe during the early stages of the experimental period for group 2 (i.e., December and January) relative to groups 1 and 3 that began treatments under milder ambient temperatures in October and late January. Because of this potential limitation in the design and increase in variability, results of growth variables were analyzed with and without the second block of heifers. Exclusion of group 2 revealed significant treatment effects on growth variables that were masked when all calves were included in the analysis, likely because of the reduced overall variability among animals. Therefore, data from group 2 were removed from the final statistical analyses, leaving 20 heifers as reported below. However, a summary of group 2 data is presented separately in Table 5.
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). There was no effect of diet on age of puberty (Table 4). Further, the correlation coefficients between pubertal age and BW were similar for both LDPP and SDPP heifers (r = 0.71). However, the correlation between pubertal age and WH was higher in SDPP (r = 0.83) relative to LDPP heifers (r = 0.52).
Body Growth and Mammary Development
Relative to SDPP, LDPP promoted BW gain (P = 0.02; Figure 2). Exposure to LDPP increased average daily gain compared with SDPP (Table 4). Relative to SDPP, LDPP heifers gained significantly (P = 0.009) more height at the withers. Heifers supplemented with RUP tended to be taller (P = 0.06) than control animals. There was a significant interaction between photoperiod and diet, in which LDPP and RUP increased (P = 0.001) HH accumulation during the experimental period (Figure 3). Long-day photoperiod (P = 0.03) and RUP supplementation (P = 0.005) increased HG.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Body composition, assessed by direct chemical balance analysis, followed a similar pattern as expected from previous studies (Petitclerc et al., 1983; Zinn et al., 1986). In general, the protein content of the carcass remained relatively constant during development, whereas the fat content increased with age in growing animals. However, at 7 mo of age, there was no significant effect of photoperiod in body composition. Estimation of EBW dynamics by the D2O methodology allowed for estimation of body composition across time in 3- and 5-mo-old heifers and the results were consistent with direct chemical analyses. The 2-pool model described by Shipley and Clark (1972) was not used due to failure to estimate the second pool. In mature animals, it is crucial to harvest an appropriate number of samples (Woodford et al., 1984; Andrew et al., 1995) for accurate estimation of pool sizes, and ultimately body composition. In an earlier study using dairy cows, Woodford et al. (1984) harvested blood every 10 min for the first 4 h after injection of the tracer and during the equilibration phase of the tracer with water in the body. However, in a study conducted in our laboratory using preweaned calves (Auchtung et al., 2002), a smaller number of samples were sufficient to obtain an accurate D2O dilution curve. Consistent with that study, 3-mo-old heifers in the present study showed a small error term of total body water during the first D2O collection period, probably because the gastrointestinal tract in young animals is not yet fully developed. The D2O method had greater variance in estimation of total body water in heifers at 5 and 7 mo of age, again suggesting that greater sampling frequency is required to accurately predict EBW as animals mature and the gastrointestinal tract develops more fully. Overall, the estimation of carcass composition by the D2O method was similar to the chemical analyses for water, protein, and minerals, but not fat content. These results provide evidence that the D2O dilution technique is appropriate to estimate water, protein, and ash (i.e., lean body components) in young heifers.
Reported effects of photoperiod treatment on prepubertal mammary growth are inconsistent. Studies have shown that long days either have no effect on mammary parenchymal growth (Petitclerc et al., 1984; Ringuet et al., 1989) or increase parenchymal growth (Petitclerc et al., 1985). These discrepancies may be the result of separate influences of photoperiod on rate of mammary growth and duration of allometric mammary growth (which terminates peripubertally). In the present study, there were no differences in mass of mammary parenchyma or extraparenchymal fat between LDPP-and SDPP-treated heifers. Furthermore, photoperiod treatment did not affect parenchymal DNA or lipid content. Our results suggest that mammary growth is not influenced by manipulation of day length during the prepubertal period.
Consistent with several other studies (Peters et al., 1980; Hansen et al., 1983; Schillo et al., 1983), LDPP accelerated the onset of puberty relative to that of SDPP heifers. Further, the onset of puberty may be associated with lean tissue accumulation. Therefore, heifers exposed to LDPP may have accumulated total lean tissue at puberty sooner than heifers exposed to SDPP. Heifers exposed to LDPP tended to be smaller in weight and height at pubertal age because they achieved puberty 21 d sooner than those exposed to SDPP. Importantly, earlier puberty did not impair ultimate skeletal growth as measured by BW, WH, HH, and HG, or mammary development. Because ultimate stature was not depressed in LDPP animals despite their earlier puberty, it appears that accelerating sexual maturity does not diminish ultimate growth under these conditions. Therefore, in the present study, we confirmed that LDPP promoted body growth in prepubertal heifers. In addition, a greater supply of RUP appeared to have a beneficial effect on BW, WH, and HH growth prepubertally. However, after the attainment of puberty, animals previously under SDPP during the treatment period grew faster than animals initially exposed to LDPP. Of interest, LDPP and SDPP heifers were moved into a natural photoperiod condition upon the onset of puberty at a time of year that coincided with a spring–summer photoperiod pattern. The transfer of heifers exposed to SDPP during the experiment into extended lighting conditions after puberty may have activated a compensatory mechanism that promoted growth in these animals relative to heifers exposed to LDPP, which, in contrast, actually saw a relative decrease in daylength.
Although the present study generally supports the concept that extended lighting is beneficial to heifer growth, the observation that LDPP may have limited effect under cold ambient temperatures cannot be discounted. The housing conditions of these heifers, however, were somewhat atypical for the industry, particularly with regard to stocking density and bedding. The sand stalls and concrete floors may have adversely affected the heifers that began treatment in December more so than those that began treatment under less extreme temperatures in October and late January, and the observation of growth rate depression supports that explanation. Under industry standard stocking density and bedding approaches that would limit heat loss, particularly in colder climates, the growth depression overall would be less. It is also important to note that LDPP did not negatively influence growth relative to SDPP, but rather, the expected improvement was not observed.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Heifers exposed to LDPP have greater growth and an earlier onset of puberty compared with heifers exposed to SDPP. Moreover, relative to SDPP, LDPP combined with a diet supplemented with RUP promotes growth at a higher rate, as measured by WH and HH gain. Photoperiod in combination with elevated dietary RUP provides a feasible management tool to the modern dairy industry to accelerate growth and puberty without limiting body size at breeding.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
Received for publication May 26, 2005. Accepted for publication July 25, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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), n = 8, SEM = 7.8; G2 (•), n = 12, SEM = 6.3; and G3 (
), n = 12, SEM = 6.3. Groups 1 and 3 have similar slopes compared with G2, where function for G1 = 16.2 x X + 95.4, G2 = 11.5 x X + 95.8 and G3 = 16.0 x X + 90.5. There was significant interaction between group and time (P = 0.001).
