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Influence of body condition at calving and postpartum nutrition on endocrine function and reproductive performance of primiparous beef cows^1,2

N. H. Ciccioli^*, R. P. Wettemann^*,3, L. J. Spicer^*, C. A. Lents^*, F. J. White^* and D. H. Keisler

^* Department of Animal Science, Oklahoma Agricultural Experiment Station, Stillwater 74078 and and Department of Animal Science, University of Missouri, Columbia 65211

	Abstract

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The influences of body condition score (BCS) at calving and postpartum nutrition on endocrine and ovarian functions, and reproductive performance, were determined by randomly allocating thin (mean BCS = 4.4 ± 0.1) or moderate condition (mean BCS = 5.1 ± 0.1) Angus x Hereford primiparous cows to receive one of two nutritional treatments after calving. Cows were fed to gain either 0.45 kg/d (M, n = 17) or 0.90 kg/d (H, n = 17) for the first 71 ± 3 d postpartum. All cows were then fed the M diet until 21 d after the first estrus. A replication (yr 2; M, n = 25; H, n = 23) was also used to evaluate reproductive characteristics. Concentrations of IGF-I, leptin, insulin, glucose, NEFA, and thyroxine were quantified in plasma samples collected weekly during treatment and during 7 wk before the first estrus. Estrous behavior was detected by radiotelemetry, and luteal activity was determined based on concentrations of progesterone in plasma. All cows were bred by AI between 14 and 20 h after onset of estrus, and pregnancy was assessed at 35 to 55 d after AI by ultrasonography. Cows that calved with a BCS of 4 or 5 had similar endocrine function and reproductive performance at the first estrus. During treatment, H cows gained BW and increased BCS (P < 0.01), and had greater (P < 0.05) concentrations of IGF-I, leptin, insulin, glucose, and thyroxine in plasma than M cows. However, during the 7 wk before the first estrus, plasma concentrations of IGF-I, leptin, insulin, glucose, NEFA, and thyroxine were not affected by time. Cows previously on the H treatment had a shorter (P < 0.01) interval to first postpartum estrus and ovulation, and a larger dominant follicle (P < 0.01) at first estrus, than M cows, but duration of estrus and the number of mounts received were not influenced by nutrient intake. Pregnancy rate at the first estrus was greater (P < 0.03) for H (76%, n = 38) than for M (58%, n = 33) cows. Increased nutrient intake after calving stimulated secretion of anabolic hormones, promoted fat deposition, shortened the postpartum interval to estrus, and increased pregnancy rate at the first estrus. Concentrations of IGF-I and leptin in plasma were constant during 7 wk before the first estrus, indicating that acute changes in these hormones are not associated with the resumption of ovarian function in primiparous beef cows.

Key Words: Beef Cows • Fertility • Insulin-Like Growth Factor • Leptin • Nutrition • Postpartum

	Introduction

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Optimal reproductive performance in beef cows is often limited by prolonged postpartum anestrous intervals. Heifers bred to calve at 2 yr of age resume ovarian function 20 to 40 d later than mature cows (Wiltbank, 1970). The stress of calving and the combined effects of growth and first lactation impose nutritional requirements that are often not fulfilled when cows graze low-quality pastures. Thus, inadequate nutrient intake before (Bellows et al., 1982) or after calving (Grimard et al., 1995) has greater detrimental effects on postpartum reproduction in primiparous than in mature cows.

Suckling (Williams, 1990; Stagg et al., 1998) and nutrition (Selk et al., 1988; Randel, 1990) are major regulators of the duration of the postpartum anestrous interval. Restricted nutrient intake prepartum results in thin cows at calving, a prolonged postpartum anestrous interval, and fewer cows in estrus during the breeding season (Dunn and Kaltenbach, 1980; Richards et al., 1986; Spitzer et al., 1995). Greater postpartum nutrient intake can enhance the secretion of LH and follicular growth (Perry et al., 1991a; Grimard et al., 1995), and effects of nutrition on reproduction may be more pronounced in thin cows than in cows with adequate BCS (Dunn and Kaltenbach, 1980; Richards et al., 1986; Spitzer et al., 1995).

Metabolites and metabolic hormones could mediate the effects of nutrient intake on reproductive function (Keisler and Lucy, 1996; Wettemann and Bossis, 2000). In beef cows, the roles of plasma concentrations of IGF-I (Stagg et al., 1998; Wettemann et al., 2003) and leptin (Spicer, 2001; Williams et al., 2002), in regulation of resumption of ovulation are not established. Therefore, this study was designed to determine the effects of BCS at calving and postpartum nutrient intake on endocrine and ovarian functions, and reproductive performance at the first postpartum estrus of primiparous suckled beef cows.

	Materials and Methods

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Animals, Diets, and Treatments
All experimental procedures described were approved by the Oklahoma State University Animal Care and Use Committee. During the last third of gestation, Angus x Hereford primiparous cows were maintained on dormant native range pasture and supplemented with either 0.9 or 1.8 kg/d of a 38% CP (on a DM basis) soybean meal-based supplement to result in loss or maintenance of BW, respectively, so that they would calve with a body condition score (BCS: 1 = emaciated; 9 = obese; Wagner et al., 1988) of 4 or 5. At calving (February and March), in each of 2 yr, cows were stratified by BCS and calving date and randomly assigned to one of two nutritional treatments for the first 71 ± 3 (range 45 to 96) d postpartum. Late-calving cows were on nutritional treatments for at least 45 d before the end of the feeding period (May 19). The end of treatment coincided with the time that adequate high-quality forage was available. Cows (yr 1, n = 34; yr 2, n = 48) were fed to gain 0.45 kg/d (moderate = M) or 0.90 kg/d (high = H). During treatments, cows had prairie hay (4% CP, on a DM basis) ad libitum. Moderate cows were supplemented with 2 kg/d of 38% CP (on a DM basis) range cubes, whereas H cows had free access to a high-energy feed (1.61 Mcal NE_m/kg DM, 0.90 Mcal NE_g/kg DM, and 11.1% CP). The ration was composed, on a DM basis, of rolled corn (39.7%), ground alfalfa pellets (35.5%), cottonseed hulls (22%), cane molasses (2.5%), and salt (0.3%). Cows on H consumed an average of 16 kg of supplement/animal daily. After the end of nutritional treatments, all cows were maintained in the same pasture and fed the M diet until detected in estrus. Body weight and BCS were determined every 30 d, after cows were denied access to feed and water for 16 h, from 90 d before to 150 after parturition. The last BCS recorded before calving was assigned as BCS at calving. The first body weight recorded after calving was used to determine BW changes during treatments. Calves were weighed within 48 h of birth and at 30-d intervals until weaning. Calves remained with cows and were denied access to feed and water for 16 h before weighing.

Blood Sampling and Hormone and Metabolite Assays
Blood samples were obtained three (Monday, Wednesday, and Friday) or two (Tuesday and Thursday) times a week during yr 1 and yr 2, respectively. Samples were collected from 30 d after calving to 3 wk after the first estrus or until 23 wk after calving for anestrous cows. Concentrations of insulin, IGF-I, leptin, thyroxine, glucose, and NEFA were determined in samples collected weekly from 3 wk before the end of the nutritional treatment to the week of first estrus in yr 1. Two cows were eliminated from endocrine analyses because they initiated estrous cycles early in the postpartum period and blood samples were not obtained for 3 wk before estrus. Missing samples resulted in the elimination of two cows from endocrine analyses. This resulted in 15 cows for each postpartum nutritional treatment. Endocrine data were evaluated only in yr 1 because 15 cows per treatment was considered an adequate number of animals to test the effect of treatment. Cows had access to feed before sampling. Caudal vein blood (10 mL) was collected into evacuated tubes containing EDTA. Tubes were immediately placed on ice, centrifuged (2,500 x g for 15 min) at 4°C within 3 h after collection, and plasma was recovered and stored at -20°C until hormones and metabolites were quantified.

Concentrations of insulin in plasma were quantified with a solid-phase RIA for human insulin (Coat-A-Count Insulin kit, Diagnostic Products Corp., Los Angeles, CA; Bossis et al., 1999) using bovine pancreatic insulin for standards (Sigma Chemical Co., St. Louis, MO). Intra- and interassay CV (n = 7 assays) were 12 and 17%, respectively. Concentrations of IGF-I in plasma were quantified by RIA (Echternkamp et al., 1990) after acid-ethanol extraction (16 h at 4°C). Intra- and interassay CV (n = 3 assays) were 11 and 14%, respectively. Concentrations of leptin in plasma were determined by RIA specific for ovine leptin and validated for use in bovine serum (Delavaud et al., 2000). Intraassay CV was 8.5%. Concentrations of thyroxine in plasma were quantified with a solid-phase RIA for human thyroxine (Coat-A-Count Total T4 kit, Diagnostic Products Corp.). Sensitivity of the assay was 10 ng/mL of plasma, and the addition of 16 ng of thyroxine to 1 mL of plasma resulted in 95% recovery (n = 4). When 5, 10, 15, 20, and 25 µL of bovine plasma were assayed, concentrations of thyroxine were parallel to those of the standard curve. The intraassay CV was 15%. Concentrations of glucose in plasma were determined with an enzymatic colorimetric procedure (no. 510, Sigma Chemical Co.) and intra- and interassay CV (n = 7 assays) were 3 and 7%, respectively. Concentrations of NEFA in plasma were quantified with a modified (McCutcheon and Bauman, 1986) colorimetric procedure (Wako-NEFA C, Wako Chemicals USA Inc., Dallas, TX). Inter- and intraassay CV (n = 7 assays) were 7 and 5%, respectively. Plasma concentrations of progesterone were quantified with a solid-phase RIA (Coat-A-Count Progesterone kit, Diagnostic Products Corp.; Vizcarra et al., 1997). Intra- and interassay CV were 3 and 6%, respectively. Plasma concentrations of estradiol 17-ß were determined by RIA (Estradiol MAIA, Polymedco Inc., New York, NY) with modifications (Vizcarra et al., 1997). The intraassay CV was 11% for the single assay in which all samples were quantified. Estradiol 17-ß concentrations were quantified in plasma samples collected at 18 or 30 h before the onset of the first estrus followed by ovulation.

Estrous Behavior, Ovarian Function, and Reproductive Performance
The number of mounts received by cows was continuously monitored using a radiotelemetric pressure-sensitive device (HeatWatch, DDx Inc., Denver, CO) attached to the rump of cows at 30 d postpartum. Date, time, and duration of each mount received were recorded and used to calculate the total number of mounts received and duration of estrus for each cow. Onset of estrus was defined as the first of two mounts received within a 4-h period. The end of estrus was defined as the last mount received with a mount 4 h before and with no mount during the next 12 h. Concentrations of progesterone were used to determine the incidence of luteal activity before and after the first postpartum estrus during yr 1. Luteal phases were classified as normal if at least five consecutive plasma samples (at least 10 consecutive days) had at least 0.5 ng/mL of progesterone; otherwise, they were classified as short luteal phases. Size of the dominant follicle (DF) was measured (yr 1 and 2), between 4 to 14 h after onset of estrus, by transrectal ultrasonography (Aloka 500-V ultrasound equipment with a 7.5-MHz probe; Corometrics Medical Systems, Wallingford, CT). Ovarian images were recorded on videotape. Size of the DF was the mean of the length and width of the largest follicle. Duration of postpartum anestrus and absence of luteal activity (postpartum interval [PPI], yr 1 and 2) was the number of days from calving to first estrus with a subsequent luteal phase. Ovulation at the first estrus was confirmed by at least two consecutive plasma samples with concentrations of progesterone greater than 0.5 ng/mL. Cows that were anovulatory and anestrous (H = 2 vs. M = 9) at 23 wk postpartum were assigned a PPI of 168 d (1 wk after the end of blood sampling) and were included in statistical analysis for PPI. Cows that were in estrus before 23 wk postpartum were artificially inseminated between 14 and 20 h after onset of estrus by the same technician using semen from one bull each year. Pregnancy status was determined at 35 to 55 d after AI by ultrasonography and confirmed by calving date.

Statistical Analyses
Changes in BCS and BW, ADG, number of mounts received, duration of estrus, PPI, and maximum diameter of DF were analyzed as a randomized complete block (year) design with a 2 x 2 treatment structure using a mixed model (PROC MIXED; Littell et al., 1996) with SAS (SAS Inst. Inc., Cary, NC). The model included year as a random effect, BCS at calving (4 or 5), postpartum nutrition (H or M), and the first-order interaction as fixed effects, with days on nutritional treatment as a covariate. All effects were tested with the pooled residual error term. Calf weights at birth, at the end of treatment, and at weaning were analyzed adding calf sex and all interactions to the previous model. Weaning weights were adjusted to 205 d of age before analysis. Concentrations of estradiol during proestrus were analyzed as a split-plot design with nutritional treatment, time of sampling (18 or 30 h) before first estrus, and its interaction as fixed effects. Cow within nutritional treatment was the error term to test the treatment effect. The residual mean square was the error term to test the effects of time of sampling and interaction.

Concentrations of metabolites and metabolic hormones across time (before and after treatment, and before the first estrus) were analyzed using generalized least squares and a mixed model for a randomized complete block (assay) design with repeated measures over the same experimental unit using the MIXED procedure of SAS. All samples for two or three cows per treatment were included in an assay, and samples were randomly distributed within assay. The statistical model included assay as a random effect and BCS at calving, postpartum nutrition, week postpartum, and all first- and second-order interactions as fixed effects. Cow within BCS x nutritional treatment was the error term to test treatment effects (BCS, nutritional treatment, and BCS x nutritional treatment), whereas the pooled residual was the error term to test week effect and all interactions with week. Degrees of freedom for the pooled error term were calculated using Kenward-Roger’s approximation (Littell et al., 1996). A first-order autoregressive function with lag equal to 1 was used to model the covariance structure for the repeated measures. If a significant treatment x week postpartum interaction was detected, simple effects of treatment were compared using the SLICE option of the LSMEANS statement of SAS. Pearson partial correlation coefficients were calculated to describe linear relationships among response variables using PROC CORR of SAS. Logistic regression analysis was used to compare the incidence of short luteal phases and pregnancy rates. The model included year, BCS at calving, nutritional treatment, and all first-order interactions as predictors, and number of cows pregnant divided by those inseminated at first estrus as the dependent variable. The model was fit using the GENMOD procedure of SAS with logit as a link function and assuming a binomial distribution for the error term.

	Results

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Body Condition Score, Body Weights, and Calf Performance
The two amounts of prepartum protein supplementation resulted in cows with BCS of 4.4 ± 0.1 or 5.1 ± 0.1 at calving (P < 0.01). Body condition score was similar (P = 0.60) for cows on M or H diets at the beginning of postpartum nutritional treatments, and greater nutrient intake postpartum increased (P < 0.01) BCS at the end of feeding, independently (P = 0.64) of BCS at calving (Table 1). Cows on the M diet lost an average of 0.26 in BCS, whereas cows on the H diet gained 0.45 in BCS. Average body weight before calving did not differ (P = 0.80) among cows assigned to different postpartum nutritional treatments. Body weight changes after calving were similar to changes in BCS (Table 1). Body condition score at calving did not influence (P = 0.97) change in BW during treatment, and H cows had a greater ADG during treatment than M cows (1.14 ± 0.10 vs. 0.35 ± 0.10 kg/d, respectively, P < 0.01). Calf birth weight (33.3 ± 1.6 kg) was not influenced by BCS at calving (Table 2). However, calves that suckled H cows were heavier (P < 0.01) at the end of nutritional treatment (71 ± 3 d) than M calves. Average daily gain was 0.25 kg/d greater (P < 0.01) for calves that suckled H cows compared with those that suckled M cows. Calf weight at the end of nutritional treatment was positively related to change in BW of the cow (r = 0.56; P < 0.01). Calves weaned from H cows were 10 kg heavier (P < 0.05) than those weaned from M cows.

View larger version (33K): [in this window] [in a new window]   Figure 1. Least squares mean concentrations of IGF-I (A), leptin (B), insulin (C), glucose (D), NEFA (E), and thyroxine (F) in plasma during 3 wk before and 3 wk after the end of nutritional treatment (High, n = 15; Moderate, n = 15) of primiparous lactating beef cows. *Treatment effect (P < 0.05). **Treatment effect (P < 0.01). Standard errors averaged over weeks were 3.4, 0.65, 0.14, 2.0, 30.0, and 2.5 for IGF-I, leptin, insulin, glucose, NEFA, and thyroxine concentrations, respectively.

During the 3 wk before and 3 wk after treatment, there was a treatment x week effect (P < 0.01) on plasma concentrations of NEFA (Figure 1E). Concentrations of NEFA were similar for H and M cows during treatment. However, concentrations of NEFA in plasma were greater (P < 0.01) for H than for M cows during the first 3 wk after the end of nutritional treatment (510 ± 30 vs. 264 ± 30 µEq/mL).

There was a treatment x week effect (P < 0.01) on concentrations of thyroxine in plasma during the last 3 wk before and 3 wk after nutritional treatment (Figure 1F). During treatment and the first week after treatment, plasma concentrations of thyroxine were greater (P < 0.01) for H than for M cows (41.0 ± 2.3 vs. 25.0 ± 2.4 ng/mL, respectively). However, concentrations of thyroxine were not different (P > 0.10) for H and M cows during 2 and 3 wk after treatment.

Partial correlation coefficients, adjusted for cow, for concentrations of IGF-I, leptin, insulin, glucose, NEFA, and thyroxine during the last 3 wk of nutritional treatment are shown in Table 3. Concentrations of IGF-I were positively correlated with concentrations of leptin (P < 0.05), insulin (P < 0.01), and glucose (P < 0.01). Concentrations of leptin were also positively correlated with concentrations of insulin (P < 0.01) and glucose (P < 0.05). Concentrations of NEFA were not correlated with any of the hormones quantified or with glucose. Concentrations of thyroxine were positively (P < 0.01) correlated with concentrations of IGF-I, leptin, insulin, and glucose.

View larger version (18K): [in this window] [in a new window]   Figure 2. Least squares mean concentrations of IGF-I (A), leptin (B), and insulin (C) in plasma during 7 wk before the first postpartum estrus of primiparous lactating beef cows previously fed a High (n = 15) or a Moderate (n = 15) energy diet after calving. Lines with open or solid circles represent cows that calved with a body condition score of 4 or 5, respectively. Standard errors averaged over weeks were 4.4, 0.6, and 0.2 for IGF-I, leptin, and insulin concentrations, respectively.

Concentrations of both insulin (Figure 2C) and glucose (Figure 3A) during 7 wk before the first estrus were not affected (P > 0.25) by treatment x BCS at calving x week, treatment x week, BCS at calving x week, or treatment x BCS at calving. Mean concentrations of insulin in plasma during 7 wk before first estrus were greater (P < 0.05) for cows with a BCS 5 at calving (1.08 ± 0.05 ng/mL) than for those with a BCS 4 (0.97 ± 0.05 ng/mL). Postpartum nutrition did not affect (P = 0.45) concentrations of insulin before estrus (1.01 ± 0.05 ng/mL and 1.04 ± 0.05 ng/mL, for M and H cows, respectively). Concentrations of glucose in plasma did not differ (P = 0.30) between cows that calved with a BCS 4 or 5 (61.5 ± 1.5 mg/dL and 62.5 ± 1.4 mg/dL, respectively). Cows that were previously on H nutrition (63.4 ± 1.4 mg/dL) had greater (P < 0.01) concentrations of glucose than those on M nutrition (60.6 ± 1.4 mg/dL). Although there was a tendency (P < 0.08) for week postpartum to influence concentrations of insulin and glucose before the first estrus, the changes with week postpartum were inconsistent (Figures 2C and 3A, respectively).

View larger version (19K): [in this window] [in a new window]   Figure 3. Least squares mean concentrations of glucose (A), NEFA (B), and thyroxine (C) in plasma during 7 wk before the first postpartum estrus of primiparous lactating beef cows previously fed a High (n = 15) or a Moderate (n = 15) energy diet after calving. Lines with open or solid circles represent cows that calved with a body condition score of 4 or 5, respectively. Standard errors averaged over weeks were 3, 30, and 4 for glucose, NEFA, and thyroxine concentrations, respectively.

Concentrations of thyroxine in plasma before first estrus were not affected (P > 0.25) by treatment x BCS at calving x week, week, and the first-order interactions of main effects with week (Figure 3C). There was a treatment x BCS at calving effect (P < 0.05) on concentrations of thyroxine before first estrus. When cows calved with a BCS of 4, plasma concentrations of thyroxine did not differ (P = 0.23) during 7 wk before first estrus for cows that were on H (29.3 ± 2.1 ng/mL) or M (26.2 ± 2.0 ng/mL) nutrition after calving. When cows calved with a BCS of 5, plasma concentrations of thyroxine during 7 wk before first estrus were greater (P < 0.01) in H cows (37.6 ± 2.0 ng/mL) than in M cows (26.8 ± 1.9 ng/mL).

Estrus, Ovarian Function, and Reproductive Performance
Estrus, ovarian function, and reproductive performance were not affected (P > 0.20) by BCS at calving or BCS at calving x postpartum nutrition treatment; thus, results are summarized for the effect of postpartum nutrition (Table 4). The incidence of short luteal phases before first postpartum estrus was not influenced (P > 0.25) by postpartum nutrition. Ninety percent of the cows (26/29) had a transient increase of progesterone in plasma ( 0.5 ng/mL for less than 10 d; maximum concentration = 1.60 ± 0.20 ng/mL) within 2 to 4 d before the first estrus. No cow had a normal luteal phase before the first postpartum estrus. After first estrus, all cows had normal luteal phases (progesterone 0.5 ng/mL for at least 10 consecutive days).

The interval from calving to first estrus with subsequent normal luteal phases (PPI) was shorter (P < 0.01) for H than for M cows (Table 4). Only 24% of M cows had ovulated and initiated a normal luteal phase before 80 d postpartum compared with 41% of H cows (², P = 0.13). When anovulatory cows (M = 9 and H = 2) were assigned an ovulation date of 1 wk after the last blood sampling date, the length of the postpartum anestrous interval was 34 d longer for M cows compared with H cows. Pregnancy rate after AI at the first estrus was greater (P < 0.03) for H than for M cows (Table 4).

	Discussion

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Increased nutrient intake for approximately 70 d after parturition increased BCS and BW of primiparous suckled beef cows that calved with a BCS of 4 or 5. In contrast, cows that had a moderate nutrient intake after parturition lost body energy reserves and weighed less than high cows at the end of nutritional treatment. High-energy diets fed after calving (Perry et al., 1991a; Stagg et al., 1995) or before puberty (Yelich et al., 1995) increase fat deposition in mature cows and growing heifers. Primiparous beef cows fed a high-energy diet postpartum partitioned a greater proportion of net energy (consumed) to grow maternal tissue than cows on moderate-energy diets (Lalman et al., 2000).

Calf birth weights were not influenced by BCS at calving. Nutrient intake of first-calf cows during gestation may (Corah et al., 1975; Bellows and Short, 1978; Spitzer et al., 1995) or may not (Whittier et al., 1988b; Goehring et al., 1989; Wiley et al., 1991) influence birth weight of calves. Calf birth weights of primiparous cows with a BCS of 5 were significantly heavier (1.5 kg) than those from cows with a BCS of 4 (Spitzer et al., 1995). In the present experiment, calves from cows with a BCS of 5 were only 0.5 kg heavier at birth than those from cows with a BCS of 4. Environmental and genetic factors affect birth weight of calves (Holland and Odde, 1992) and may influence the effect of nutrient intake on birth weight. In addition, nutrient intake during gestation must be drastically restricted to reduce calf birth weight because thin cows have enhanced placental growth, which may diminish some of the negative effects of reduced nutrient intake on fetal growth (Rasby et al., 1990).

Preweaning and adjusted 205-d weaning weights of calves were not affected by BCS at calving, which is consistent with previous results with primiparous beef cows (Whittier et al., 1988b; DeRouen et al., 1994; Spitzer et al., 1995). Minimal differences in BCS at calving may not have a significant effect on milk production and growth rate of calves; however, calves reared by thin cows (BCS of 3) were lighter at 105 d postpartum (Houghton et al., 1990) and at weaning (Corah et al., 1975) than those reared by adequately fed cows (BCS of 5).

Postpartum nutrient intake affected calf performance. Increased nutrient intake during lactation increased calf weight at the end of feeding and at 205 d of age. Restriction of the postpartum energy intake of cows reduced calf weight at 70 d of age (Perry et al., 1991a) and at weaning weight (Richards et al., 1986; Spitzer et al., 1995). Increased energy intake during lactation increases milk production (Perry et al., 1991a; Marston et al., 1995; Lalman et al., 2000). In the present study, calves suckling cows on the high-energy diet were heavier at 70 d of age and at weaning. Increased energy intake probably increased milk production as well as body energy reserves in H cows. Milk yield and weaning weights are positively correlated in beef cattle (Totusek et al., 1973; Marston et al., 1992). Alternatively, calves suckling H cows may have consumed some of the ration fed to cows, which could increase daily gain independently of the greater milk production of cows.

Nutrient intake after calving influenced concentrations of IGF-I in plasma of primiparous lactating beef cows. This is consistent with previous studies in which concentrations of IGF-I in plasma were directly related to nutrient intake in heifers (Armstrong et al., 1993; Yelich et al., 1996; Armstrong et al., 2001), primiparous (Lalman et al., 2000), and mature (Richards et al., 1991) beef cows. Reduced nutrient intake uncouples the GH–IGF-I axis (Thissen et al., 1994). Undernutrition increases GH secretion in cattle (Armstrong et al., 1993; Bossis et al., 1999), whereas serum concentrations of IGF-I and hepatic IGF-I mRNA are decreased (VandeHaar et al., 1995), probably due to an insulin-dependent down-regulation of the GH receptor (Thissen et al., 1994; Kobayashi et al., 1999; Butler et al., 2003). In the present study, M cows lost BCS during treatment, which reflects an inadequate nutritional status and, probably, an uncoupling of the GH–IGF-I axis. In contrast, H cows gained BCS and weight and had greater concentrations of IGF-I in plasma. The latter occurred simultaneously with increased concentrations of insulin in plasma that may have enhanced the hepatic sensitivity to GH in H cows. This nutritionally induced increase in concentrations of IGF-I in plasma could have reduced, directly and/or indirectly, the length of postpartum anestrus.

The results of the current study are the first to demonstrate that H nutrient intake after calving increased concentrations of leptin in plasma of lactating beef cows. A positive association between nutrient intake and concentrations of leptin in plasma has been reported in sheep (Delavaud et al., 2000; Ehrhardt et al., 2000) and cattle (Ehrhardt et al., 2000; Delavaud et al., 2002). The current study revealed an acute decrease in concentrations of leptin in H cows within the first week (4.0 ± 0.1 d) after nutritional treatment. Similarly, concentrations of leptin in plasma and in cerebrospinal fluid of sheep were influenced acutely and increased by 5 d after a change from a low- to a high-energy diet (Blache et al., 2000). These changes in leptin concentrations were probably not linked to changes in BCS or BW because they occurred in a few days and indicate that nutrient intake may affect concentrations of leptin in plasma independently of BCS or BW. Concentrations of leptin in bovine plasma may depend on amount of adipose tissue in the long term (Delavaud et al., 2002) but are influenced by changes in nutrient intake in the short term (Tsuchiya et al., 1998; Amstalden et al., 2000, 2002).

In the present study, concentrations of leptin, insulin, IGF-I, and glucose were positively correlated during the last 3 wk of nutritional treatments. Concentrations of leptin in plasma are directly related to concentrations of insulin in dairy cows (Block et al., 2001; Delavaud et al., 2002) and beef heifers (Amstalden et al., 2000, 2002). Insulin secretion in fasted heifers increased within 3 h of infusion of leptin into the lateral cerebral ventricle (Amstalden et al., 2002), and insulin increases the expression and secretion of leptin from bovine (Houseknecht et al., 2000) and rat (Barr et al., 1997) adipose tissue in vitro. The results of the current study provide evidence of a positive relationship between nutrient intake and secretion of leptin in postpartum beef cows. However, the specific roles of insulin, glucose, and/or NEFA in modulating leptin secretion have not been determined.

Greater nutrient intake after calving (H cows) increased concentrations of thyroxine in plasma, similar to the response in nonlactating cows (Delavaud et al., 2002). Concentrations of thyroxine and leptin in plasma were positively correlated during nutritional treatment. In agreement with this response, leptin administration increased pro-TSH gene expression (Legradi et al., 1997) and concentrations of thyroxine (Ahima et al., 1996) in fasted rodents. Because basal metabolic rate and energy expenditure are directly regulated by thyroxine, the results of the current study indicate that concentrations of leptin in plasma may be associated with rate of metabolism, increased secretion of anabolic hormones, and tissue accretion.

Concentrations of IGF-I, leptin, insulin, glucose, NEFA, and thyroxine did not change during the 7 wk before first estrus in cows that were previously on H or M intake. In agreement with these results, concentrations of IGF-I in plasma did not change with time postpartum in Bos taurus cows (Spicer et al., 2002) and concentrations of insulin and glucose were not predictive of the first luteal activity in primiparous postpartum beef cows (Vizcarra et al., 1998). However, plasma concentrations of IGF-I increased linearly during 75 d before first postpartum ovulation in beef cows suckled ad libitum or once daily (Stagg et al., 1998), and concentrations of IGF-I in plasma were greater at 2 and 10 wk after parturition in cows that resumed cycling by 20 wk after parturition compared with cows that were anestrous at 20 wk (Roberts et al., 1997). Systemic concentrations of IGF-I in cows are directly influenced by nutrient intake (Bossis et al., 2000), BCS (Bishop et al., 1994), and energy balance (Spicer et al., 1990). In the present study, H and M cows differed in only 0.75 unit of a BCS at the end of feeding and were on the same diet during the 7 wk before the first estrus. These conditions may explain why concentrations of IGF-I did not change before first estrus.

Increased concentrations of NEFA in plasma are indicative of lipid mobilization in cows during negative energy balance (Richards et al., 1989b; Staples et al., 1990; Bossis et al., 1999). In the current study, plasma concentrations of NEFA were greater for H cows than for M cows during 7 wk before first estrus. In agreement with Vizcarra et al. (1998) using the same experimental paradigm, increased NEFA in H cows could be related to a greater nutrient mobilization and energy available for milk production and reinitiation of estrous cycles. Plasma NEFA concentrations were negatively correlated with LH pulse frequency at 30 d postpartum in beef cows (Grimard et al., 1995), but NEFA are not the only metabolic signal that changes when energy balance is negative. Moreover, plasma NEFA concentrations are probably not a direct signal for the resumption of ovarian function because 1) lipid infusion that increased NEFA concentrations by twofold did not affect pulsatile secretion of LH in ovariectomized lambs (Estienne et al., 1990), 2) concentrations of NEFA in plasma did not predict the resumption of ovulation in postpartum primiparous cows (Vizcarra et al., 1998), and 3) concentrations of NEFA in plasma increased with time during realimentation of nutritionally induced anovulatory cows before resumption of ovulation (Bossis et al., 2000).

Cows that calved with a BCS of 5 and received the H diet for 71 d postpartum had greater concentrations of thyroxine in plasma before first estrus than cows with a BCS of 5 on the M diet or cows with a BCS of 4, regardless of diet. This response probably reflects a greater metabolic rate associated with increased milk production and greater feed intake (Richards et al., 1995). However, plasma thyroxine did not change during 7 wk before the first estrus. These and previous results (Stewart et al., 1993; De Moraes et al., 1998) support the concept that concentrations of thyroxine in plasma may not be limiting ovarian function in beef cows.

The first ovulation that occurs after parturition usually is not preceded by estrous behavior and is followed by a corpus luteum that is functional for a shorter time than normal (Murphy et al., 1990; Perry et al., 1991b; Werth et al., 1996). The current experiment is the first to determine that BCS at calving and postpartum nutrient intake do not influence the incidence of short luteal activity before the first estrus of primiparous beef cows. In agreement with other studies (Corah et al., 1974; Odde et al., 1980), duration of the estrous cycle after the first postpartum estrus usually is normal and not influenced by BCS at calving and postpartum weight gain (Looper, 1999; Lents et al., 2000).

Neither BCS at calving nor postpartum nutrition influenced estrous behavior at the first postpartum estrus. Duration of estrus and number of mounts received per estrus were comparable to other reports for mature lactating beef cows (Hurnik and King, 1987; Lents et al., 2000). In contrast, nonlactating beef cows were in estrus for 16 h and received 47 mounts (White et al., 2002). These results indicate that duration and intensity of estrous behavior may differ between cyclic cows and postpartum cows at the first estrus. Previous research (Spitzer et al., 1995) demonstrated that BCS at calving (range 4 to 6) influences the duration from parturition to estrus. The lack of effect of BCS at calving on reproductive performance in the current study could be related to the minimal but significant differences in BCS of the thin and moderate condition cows at calving, or that all cows had less than optimal BCS at calving for adequate performance. Spitzer et al. (1995) found greater pregnancy rates for cows with a BCS of 6 at parturition compared with cows with a BCS of 4 or 5, and recommended feeding to achieve a greater BCS at calving for primiparous than for mature cows.

Increased nutrient intake after parturition shortened the interval from calving to first estrus and ovulation in primiparous cows. Greater nutrient intake postpartum can have a positive effect (Wright et al., 1992; Stagg et al., 1995; Vizcarra et al., 1998) or no effect (Wright et al., 1987; Whittier et al., 1988a; Stagg et al., 1998) on duration of the postpartum anovulatory interval. Lack of consistency among studies may involve the amount of energy intake, duration of the feeding period, BCS at calving, age of cows, and so on. However, thin cows or primiparous cows at calving usually respond to increased postpartum nutrient intake with enhanced reproductive performance (Richards et al., 1986; Spitzer et al., 1995; Lalman et al., 2000), although reproductive performance may be less than adequate. Increased nutrient intake also induced fat deposition in H cows, which may be a prerequisite to reestablish ovarian function in postpartum cows. Increased BCS is required for the resumption of estrous cycles in nutritionally induced anestrous cows (Richards et al., 1989a) and heifers (Bossis et al., 2000), and body energy reserves influence the interval to ovulation after early weaning of beef cows (Bishop et al., 1994). The amount of fat reserves could be the reason why H cows resumed ovarian function 20 d earlier than M cows.

Increased energy intake after parturition enhanced pregnancy rate at the first postpartum estrus. Results of other experiments indicated that postpartum energy intake may influence pregnancy rate at the first postpartum estrus (Wiltbank et al., 1964; Richards et al., 1986); however, the effect of energy intake on pregnancy rate was not significant. Reduced nutrient intake during estrous cycles did not affect fertilization rate (Spitzer et al., 1978) but reduced conception rate (Hill et al., 1970).

Increased energy intake after calving stimulated secretion of anabolic hormones and increased the fat deposition and milk production of primiparous cows that calved with a thin or moderate body condition. However, resumption of ovarian function was limited during the first 3 mo after calving. Cows on a high-energy diet for 70 d postpartum not only resumed ovarian activity earlier after calving, but also had a greater pregnancy rate from AI at the first estrus. Although concentrations of IGF-I, leptin, insulin, glucose, and thyroxine in plasma were greater in H than M cows during treatment, only concentrations of IGF-I and thyroxine during 7 wk before the first estrus were greater for H than for M cows, and only in cows with a BCS of 5. Because none of the hormones studied, glucose, or NEFA were altered during the 7 wk before the first postpartum estrus, they may not be limiting factors or they could have an influence earlier in the postpartum period. Differences in endocrine function or metabolic signals during the first 3 mo after calving could influence ovarian activity and fertility at the first postpartum estrus.

	Implications

Top Abstract Introduction Materials and Methods Results Discussion Implications Literature Cited

 
Feeding a high-energy supplement after calving can increase reproductive performance of primiparous cows that calve with thin or moderate body condition. Cows that maintain or lose body condition during lactation have a prolonged interval from calving to estrus, are less fertile, and produce lighter calves at weaning. Estrus detection aids should be used in postpartum cows due to the duration of the estrous period and fewer mounts received. Concentrations of IGF-I, leptin, insulin, glucose, NEFA, or thyroxine in blood may not individually signal the onset of postpartum ovarian function, but they may act in concert with other factors to indicate the adequacy of nutrients. Additional research is necessary to elucidate the mechanism(s) that control postpartum ovarian activity and estrous behavior to maximize reproductive efficiency in beef cattle.

	Footnotes

 
¹ Approved for publication by the director, Oklahoma Agric. Exp. Stn. This research was supported under project H-2331 and a Fulbright–Universidad Nacional del Sur (Argentina) fellowship to N. H. Ciccioli.

² The authors acknowledge R. Jones, M. Anderson, and L. Mackey for technical assistance. Human insulin-like growth factor I antiserum was obtained from A. F. Parlow, National Hormone and Pituitary Program, Harbor-UCLA Medical Center.

³ Correspondence—phone: 405-744-6077; fax: 405-744-7390; E-mail: rpw{at}okstate.edu.

Received for publication April 11, 2003. Accepted for publication August 11, 2003.

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