Effect of Feed Intake During Late Develonment on Pubertal Onset and Resultini Body Comphsition in Crossbred Gilts1J2 E. A. Newtons and D. C. Mahan4

ABSTRACT8 A total of 105 nonboar-exposed, F2 ([Landrace x Yorkshire1 x Duroc) gilts were used in two replicates of a randomized complete block experiment to evaluate the effect of dietary feed intake on pubertal onset and subsequent body composition. Feed intakes were established at 50% of ad libitum (AL-501, 75% of ad libitum (AL-751, or at a d libitum (AL-100) levels from 4.5 to 9 mo of age. A corn-soybean meal diet fed to all gilts was formulated to meet or exceed nutrient requirements except for energy. Puberty was measured by two methods: 1) monitored once daily by back pressure applied by the herdsman or 21 from elevated plasma progesterone concentrations. Body composition was evaluated by the deuterium oxide method after plasma progesterone concentrations were elevated. Daily feed intake for the experimental period averaged 1.6, 2.3,and 3.2 kg, and the BW of gilts at 8 mo of age were 111, 131, and 154 kg for the AL-50, AL-75, and AL-100 groups,

respectively. Body weight, backfat thickness, and body fat content increased linearly (P < .011 as feed intake increased, but age at puberty was not severely influenced. A minimum body fat content or percentage did not seem to initiate pubertal onset. There was a trend for a lower percentage of the AL-50 gilts to ovulate (P = ,081 than those fed the AL-75 and AL-100 intakes. An inverse relationship resulted between the percentage of gilts that ovulated to the percentage that showed behavioral estrus. The percentage of gilts that ovulated was highest and the percentage that showed estrual behavior was lowest as feed intake increased. These data suggest that age had a greater effect on pubertal onset than did BW or body fat content, but energy restriction at approximately 50% of ad libitum levels tended to reduce the onset of ovulation. As feed intakes increased, behavioral estrus was more difficult to detect.

Key Words: Pigs, Energy, Protein, Puberty, Ovulation

J. Anim. Sci. 1992. 70:3774-3780

Introduction The effect of dietary nutrients provided during the late developmental period on pubertal onset in young gilts has been studied extensively (Hughes, 1982; den Hartog, 1984; den Hartog and Noordew-

'Salaries and research support provided by State and Federal funds appropriated to The Ohio Agric. Res. and Dev. Center, The Ohio State Univ., Manuscript no. 208-81. 2Appreciation is expressed to R.Sabine, S. Amos, G. Smith, and R.Schrock for data collection, to M. Day and M. Wright for laboratory assistance, to J. Holman for statistical analysis, and to J. Knauerhase for typing the manuscript. %Yesent address: Univ. of Nebraska, Lincoln 08583. 'To whom correspondence should be addressed: 2121 Fyffe Road. Received October 16, 19~91. Accepted July 24, 1892.

ier, 1984; Kirkwood and Aherne, 1985; Kirkwood et al., 19871. It has been proposed that a minimum body fat content may be essential for the onset of puberty in developing gilts (Kirkwood and Aherne, 1985; Beltranena et al., 1991). Although body fat reportedly has a positive correlation with estrous cyclicity IFrisch, 19881, evidence is lacking that supports a direct causal effect of body fat content on ovulatory regulation (Bronson and Manning, 1991). Bull presence has also been shown to interact with the differing nutritional status of peripubertal beef heifers in stimulating pubertal onset (Roberson et al., 19911, demonstrating the important interface of the male to the female in stimulating reproductive function. On commercial swine farms and in many reproductive experiments, boars are frequently used to detect behavioral estrus in gilts.

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Department of Animal Science, The Ohio State University and The Ohio Agricultural Research and Development Center, Columbus 43210-1095

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FEED INTAKE AND PUBERTAL ONSET IN GILTS

Materials and Methods Treatments and Management. Animals were selected at approximately 20 kg BW, placed in a completely enclosed facility in groups of eight per pen, and allowed ad libitum access to a vitamin and mineral fortified corn-soybean meal (C-SBM) diet until the experiment was initiated. At no time during the pretrial or experimental period were gilts exposed to boars. A total of 105 crossbred ([Landrace x Yorkshire1 x Duroc) gilts were allotted to outcome groups on the basis of ancestry and weight a t 4.5 ~t:.3 mo of age to one of three treatment groups in a randomized complete block experiment in two replicates. Replicate 1 was conducted from September 1989 to January 1990 using a total of 60 gilts, whereas Replicate 2 (February to June 1990) contained 45 gilts. Gilts were housed in an openfronted building in 5.2-m x 21-m concrete-floored pens, each equipped with one bowl waterer during the experimental period. The three treatments evaluated dietary feed intake provided to each group (50% of ad libitum [AL-501, 75% of ad libitum LAL-751, and ad libitum [AL-lOOI)from 4.5 to 9 mo of age. A C-SBM mixture was formulated to meet or exceed NRC (1988) requirements for all nutrients except energy when provided at 50% of ad libitum. Consequently, energy intakes provided 50 or 75% of that provided from ad libitum feeding for the AL-50 and AL-75 groups, respectively, whereas other nutrients were formulated to meet or exceed NRC (1988) requirements (Table I). Gilts in the AL-100 group had unlimited access to the diet from a self-feeder, whereas the r e s tricted groups (AL-50 and AL-75) were fed their daily feed allotment in two equal portions at 0900 and 1500 in individual feeding stalls located outside of but adjacent to their pen. Feed intake was adjusted at 2-wk intervals in the AL-50 and AL-75 groups based on measured feed intake of

Table 1. Experimental diet compositiona Ingredient

YO

Ground yellow corn Soybean meal (44% CP) Dicalcium phosphate Limestone Se premix" Trace-mineral saltC Vitamin mi# Antibacterial agente

80.53 35.25

2.27 .90

.15 .SO .30

.10 AL-7Sf

- -

AL-50f DaiIy intake, avg Feed, kg Protein, g Lysine, g ME, kcal

1.6 331 18.7 5,120

2.3

475 26.9

7,360

AL-lOOf 3.2

861 37.4 10,240

&Calculatedanalysis: CP, 20.88%,lysine, 1.17%;Ca, 1.01%;P, 3 2 % ; ME, 3,200 kcalkg.

bSe premix contributed .3 ppm of Se in a limestone carrier. CTrace-mineral salt mixture contribution per kilogram of diet: 8 mg of Cu, 120 mg of Fe, .2 mg of I, 15 mg of Mn, 120 mg of Zn, and 4.22 g of NaC1. dVitamin mix contribution per kilogram of diet: 8,660IU of vitamin A, 367 IU of vitamin Ds,37 IU of vitamin E, .84 mg of vitamin K, (menadione), 6.7 mg of riboflavin, 25 m g of d-pan tothenic acid, 20 mg of niacin, .5 mg of folacin, .33 mg of d-biotin, 33.4 w of vitamin Biz, and 2.1 g of choline. eAntibacterial agent contributed 22 g of tylosin per kilogram of diet. fAL-50, AL-75, and AL-100represent 50, 75, and 100% of ad libitum intake, respectively.

the AL-100 intake group during the previous period. Backfat thickness (Renco Lean-Meater, Minneapolis, MN) was measured a t 2-wk intervals with measurements collected approximately 5 cm from the midline at the first and last rib, last lumbar vertebrae (average values are reported). Criteria for Attainment of Puberty. Pubertal onset was identified using two methods, defined as luteal or estrual phases. The luteal phase or the period of ovulation was defined as occurring when the concentration of plasma progesterone was > 1 ng/mL, whereas concentrations < .5 ng/mL reflected prepuberty conditions. The estral condition was identified by signs of behavioral estrus, monitored once daily between 0900 to 1100 beginning at 4.5 mo of age by the same herdsman. Gilts were considered in estrus when they responded to back pressure applied by the herdsman. B2ood Collection and Analyses. At the initiation of the trial (4.5 mo of age) and at weekly intervals thereafter, blood samples were collected via jugular venipuncture. Blood was centrifuged (1,500 x g at 5OC) and plasma was stored at -2OOC until it was analyzed. The plasma samples that were collected during the week that progesterone was elevated (luteal phase) and during the previous week (peripubertal phase) were analyzed for concentrations of estradiol-17P (Ez).Individual blood collections were terminated at pubertal onset or

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Boar maturity (Kirkwood and Hughes, 1981) and the direct contact of boars with peripubertal gilts (Brooks and Cole, 1970; Caton et al., 1986b) can influence estrual behavior in female swine. This suggests that under experimental conditions boar exposure may confound specific treatment effects when evaluating pubertal onset in gilts. The objective of this experiment was to investigate the effect of various dietary feed intakes (with all nutrients except energy at or above recommended levels) initiated during late development on the subsequent pubertal onset in nonboar-exposed, crossbred gilts and to determine the resulting body composition of these gilts at puberty.

3776

NEWTON AND MAHAN

replicate that demonstrated luteal activity were excluded from this measurement, but the next eight gilts in Replicate 1 and five in Replicate 2 were used to determine body composition. Feed but not water was withheld for 18 h before D 2 O infusion (.22 g/kg BW) into the vena cava. Whole blood samples were collected before and at 1, 2, 4, and 6 h postinfusion, chilled on ice, and frozen at -20' C. Blood D2O concentration was determined by vacuum sublimation of whole blood followed by infrared analysis of the D20:H20 mixture (Byers, 1979). Prediction equations for nongravid swine developed by Shields et al. (1984) were used to calculate empty body component weights. Statistical Analysis. Data were analyzed using the GLM procedure of SAS (1985)and are reported as least squares means. Pens were considered the experimental unit. Orthogonal regression polynomials were used to define treatment contrasts. Percentage data were analyzed using chi-square analysis (Steel and Torrie, 19801.

Results Growth Performance. Body weights averaged 111, (P < .01) for the AL-

134, and 155 kg by 8 mo of age

50, AL-75, and AL-lo0 treatment groups, respectively. Weights increased linearly (P < .01) within each treatment group over time reflecting the growth pattern of the three groups from 4.5 to 9 mo of age (Figure 1). Backfat thickness increased linearly (P < .01) by age as feed intake increased (data not presented). Daily feed intake during the experimental

Table 2. Effect of dietary feed intake on age and weight at pubertal onset in crossbred gilts ~~~~~

~

~

Feed intakea

Item

AL-75

35 27d 20 74

35 31e 21 67

35 328 16 50

-

224.8 223.0

219.5 223.8

218.7 227.5

3.0 6.0

107.6 107.8

122.8 123.6

139.0 140.6

2.1** 3.6**

17.4 17.2

20.8 21.3

26.4 26.9

.72**

No. of gilts

Total Luteal" EstrualC EstrualAuteal, % Age, d Luted Estrual Body wt, kg Luteal Estrual Backfat, mm Luteal Estrual

--

AL-100

SE

AL-50

.eo**

*AL-50, AL-75, and AL-100represent 60, 75, and 100% of ad libitum intake, respectively. "Luteal inltial luted phase determined by plasma progesterone ( > 1 ng/mL) concentration. CEstrual first positive response to back pressure applied by the herdsman. d*eMeanswithin a row that do not have a common superscript differ (P .08). **Linearresponse (P < .001).

-

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no later than 9 mo of age. All gilts were retained and fed with their respective treatment group until replicate data collection was completed. Plasma concentrations of progesterone were analyzed using a direct double-antibody RIA technique (Cambridge Medical Diagnostics, Billerica, MA). Analysis for parallelism was performed with dilutions of porcine plasma that resulted in a log/logit curve with a slope of -.70 compared with a slope of -38 for the standard hormone curve. The addition of 10, 25, 40, and 50 pL of porcine plasma resulted in 1.94, 1.98, 1.86, and 1.90 ng/mL of progesterone, respectively. The intra- and interassay CV were 2.1 and 10.7%, respectively. Estradiol concentrations were determined by the method of Nephew et al. (19891, but charcoalstripped porcine plasma rather than bovine plasma served as the assay standard. Parallelism was performed by comparing the slope of the standard curve with the curve generated by adding a known amount of plasma to the standards (-1.12 vs -1.16, respectively). Recovery of a n internal standard of E2 averaged 85.6%. The addition of 25, 50,75, and 100 pL of porcine plasma resulted in 2.06, 2.01, 2.05, and 2.03 pg/mL of E 2 respectively. Intra- and interassay CV were 3.8 and 8.1YO, respectively. Body Composition. A total of 39 gilts (13 per treatment) were used to estimate body composition within 1 wk after they exhibited elevated plasma progesterone levels, using the deuterium oxide (D20) dilution procedure of Byers (1979). In an attempt to prevent bias of the population sample, the first two gilts within each pen per

-.-

FEED INTAKE AND PUBERTAL ONSET IN GILTS -e-

0) Y

-

c

140

-

120

-

AL-50

- * O - -AL-75

AL-IO0

JZ

m

z

>. 0

m

’

60 120

I 150

210

180

240

270

Age, d

Figure 1. Effect of dietary feed intake 50% [AL-50], 75% [AL-751, and 100% [AL-1001 ad libitum intake from 4.5 to 9 mo of age on body weight growth curves of

plasma E2 concentration, whereas after ovulation occurred, the AL-100 intake group had the highest (P < .051 E2 concentration (Table 3). Body Composition. The weight of gilts and their body compositions differed a t the onset of puberty for the different feed intake groups (Table 4). Weight of all body chemical components increased linearly (P < . O O l l as feed intake increased. Body fat content ranged from 31 to 47 kg as feed intake increased, an approximate 50% difference between dietary regimens at puberty. The weight of other body components ke., water, protein, and ash) also increased linearly (P < .0011 as feed intake increased, but not to the same magnitude as did body fat. Expressed as a percentage of total empty BW, fat content increased linearly (P < .0011 from 31 to 38%, whereas the percentages of water, protein, and ash decreased linearly (P < .001) as feed intake increased.

crossbred gilts.

Discussion period averaged 1.8, 2.3,and 3.2 kg reflecting 50, 72, and 100% of energy intake for the AL-50, AL-75, and AL-100 treatment groups, respectively. There was a small linear rise in feed intake from the 4.5- to 9-mo period for the AL-100 group and an approximate parallel rise in the AL-50 and AL-75 groups. The AL-50 group consumed approximately 2 1% less dietary protein than NRC (1988) requirements, but their daily amino acids and other nutrient requirements were met or exceeded by the diet provided (Table 11. Pubertal Onset. At the onset of puberty both BW (P < .001) and backfat thickness (P < .001) were higher as dietary feed intake increased (Table 2). This indicates that feed intake did not have a significant effect on age at puberty. The AL-50 intake gilts tended to be somewhat older (i.e., 6 dl when luteal function was identified than those fed the AL-75 or AL-100 plane of nutrition. This implies that feed intake may be a factor in pubertal onset when feed was restricted to 50% of ad libitum but was of lesser importance than gilt chronological age (Table 21. There was a trend (P = .o81 for a lower percentage of gilts in the AL-50 feed intake group to initiate luteal function than those in the AL-75 or AL-100 feed intake groups. The percentage of gilts that showed behavioral estrus to those with elevated serum progesterone concentration was highest with the AL-50 group and declined (P < -10)as dietary feed intake increased (Table 2). This suggests that a n inverse relationship exists between the percentage of gilts that ovulated and estral behavior as feed intake increased. During the week before ovulation (i.e., peripubertal period), feed intake had no effect on

Age seemed to be the principal factor that affected pubertal expression in sexually developing gilts, but feed restriction may be a secondary factor. The restricted group fed a t 50% of ad libitum tended to have a slight delay in pubertal onset. Although average age at pubertal onset for all gilts in our study was somewhat older than average, the animals in this study did not have the influence of boar contact. Nutrient intake, other than energy, exceeded current NRC (1988) requirements but was not constant among treatments. This partially confounds the interpretation among treatment groups. The most severely restricted group (AL-50) received 50% of the energy intake of the AL-100 group, but 2 100% of other nutrients, whereas the AL-100 group received 2 200% of all nutrients except energy. Excess protein, vitamins, and minerals have not been shown to have a negative effect on reproductive function. Because the development of other body tissues Le., muscle, bone) was not restricted a t higher nutrient intakes and available nutrients were ample for metabolic purposes, our conclusion is the level of energy restriction, not the excess of other nutrients, precipitated these treatment responses. The results imply that a 50% restriction of energy intake may be close to the critical level at which pubertal onset by age might be influenced, whereas the gilts fed 75 to 100% of a d libitum would be relatively uniform in terms of pubertal onset. This response is consistent with the results from other studies (den Hartog and Noordewier, 1984; Aherne and Kirkwood, 19851in which restriction of dietary protein and energy delayed puber-

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-0

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NEWTON AND MAHAN Table 3. Effect of dietary feed intake on concentration of estradiol-17P in plasma of pubertal gilts Feed intake* ~~~~~~

AL-50

AL-75

AL-100

SE

Gilts, no. Estradiol-l7P, pg/mL Peripubert alb LutealC

27

31

32

-

4.60 1.49

5.66 1.19

4.15 2.4 1

1.10 .37*

&AL-50,AL-75, and AL.100 represent 50, 75, and 100% of a d libitum intake, respectively. bPeripubertal = sample collected 1 wk before detection of ovarian function (luteal sample). %utea1 = sample containing > 1 ng/mL of progesterone. ,051. *Quadratic response (P

evident on age a t puberty. Growth rate and body fatness have been shown to be highly correlated with the onset of ovarian function (Beltranena et al., 1991). A minimum body fat content permissive to pubertal onset, though not necessarily a trigger for ovulation, has been proposed by den Hartog and Noordewier (19841, Frisch (19881, and Beltranena et al. (1991). Our results indicate that body fat content is not the major initiator of ovarian function in sexually developing gilts. Matamoros et al. (19911,however, suggested that certain metabolic hormones such as insulin and glucagon may influence ovulation and pubertal onset. In humans, excessive body fat has been associated with elevated insulin levels (Yalow et al., 1965) and concomitant insulin resistance (Friedman and Moon, 19851, both of which can interfere with ovarian function. A lower percentage of gilts in the AL-100intake group in our study were detected in behavioral

tal onset. Gunther (19741 reported that the reproductive system has a lower priority for dietary energy and protein than does body growth or maintenance. Therefore, severe nutrient restriction, particularly energy, imposed on the animal during the growth phase may delay pubertal onset in addition to reducing total somatic growth. Researchers who have examined moderate energy restriction (60to 80% of a d libitum) and the resulting effects on age at puberty have reported variable responses (Brooks and Cole, 1974; Hughes, 1982; Aherne and Kirkwood, 1985; Beltranena et al., 1991). One factor influencing pubertal onset seems to be the amount of body tissue development attained before the imposition of the dietary restriction. Etienne et al. (19831 demonstrated that young gilts fed at 77% of ad libitum from 28 to 60 kg BW had a delayed pubertal onset, whereas when energy restriction was not imposed until 60 kg BW, no effect was

Table 4. Effect of dietary feed intake on empty body composition of crossbred gilts at puberty Feed intakea ~~

Item

~

AL-50

Observations, no. Weight, kgb Body component, kgC

AL-75

13 100.5

Fat Water Protein

Ash

13 116.0

~

AL-100 13 139.6

SE

2.4**

3 1.44 48.10 17.39 3.46

39.21 52.94 18.83 3.72

46.88 57.40 20.18 3.97

1.41** 1.11** .22** .04**

31.32 47.91 17.32 3.45

34.18 46.16 16.42 3.24

36.40 44.76 15.74 3.10

.65** .66** .14** .03**

Body component, TO

Fat Water

Protein Ash ~~~~

~

&AL-50,AL-75, and AL-100 represent 50, 75, and 100% of a d libitum intake, respectively. bEmpty (ingesta-free) body weight. CEmpty body chemical component weight. dEmpty body chemical component as a percentage of total body component weight. ,001). **Linear response (P

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Item

FEED INTAKE AND PUBERTAL ONSET IN GILTS

Implications The greatest influence on pubertal onset was age of the gilt, but feed restriction, if excessive, may be a secondary factor. Feed intake influenced

the proportion of gilts that ovulated or were detected in standing estrus. Gilts with higher energy intakes were observed in estrus by the herdsman proportionally less often even though they ovulated. These results suggest that age was the determining factor in achieving puberty, but moderate energy restriction during late development may be effective in recognizing estrous cyclicity in replacement gilts. Ad libitum intakes do not seem to hinder ovulation but may result in an increased incidence of silent estrus.

Literature Cited Aherne, F. X., and R. N. Kirkwood. 1985. Nutrition and sow prolificacy. J. Reprod. Fertil. [Suppl.) 33:189. Apter, D.. and V.Reijo. 1990. Serum sex hormone-binding globulin and sex steroids in relation to pubertal and postpubertal development of the menstrual cycle. Prog. Reprod. Biol. Med. 14:58. Beltranena, E., F. X. Aherne, G. R.Foxcroft, and R.N. Kirkwood. 1991. Effects of pre- and postpubertal feeding on production traits at first and second estrus in gilts. J. Anim. Sci. 89:886. Bronson, F. H., and J. M. Manning. 1991. The energetic regulation of ovulation A realistic role for body fat. Biol. Reprod. 44:945.

Brooks, P. H., and D.J.A. Cole. 1970. The effect of the presence of a boar on the attainment of puberty in gilts. J. Reprod. Fertil. 23:435. Brooks, P. H., and D.J.A. Cole. 1974. The effect of nutrition during the growing period and the oestrous cycle on the reproductive performance of the pig. Livest. Prod. Sci. 1:7. Byers, F. M. 1979. Extraction and measurement of deuterium oxide at tracer levels in biological fluids. Anal. Biochem. 98: 208.

Caton, J. S.,G. W. Jesse, B. N. Day, and M. R.Ellersieck. 1986a. The effect of confinement on days to puberty in gilts. J. h i m . sci. 62:1203. Caton, J. S.,G. W. Jesse, B. N. Day, and M. R.Ellersieck. 1986b. The effect of duration of boar exposure on the frequency of gilts reaching first estrus. J. Anim. Sci. 62:1210. den Hartog, L. A. 1984. The effect of energy intake during rearing on reproductive traits in gilts. Neth. J. Agric. Sci. 32:281.

den Hartog, L. A,, and G. J. Noordewier. 1984. The effect of energy intake on age at puberty in gilts. Neth. J. Agric. Sci. 32:263.

Elsaesser, F., K. Stickney, and G. Foxcroft. 1982. A comparison of metabolic clearance rates of oestradiol-17P in immature and peripubertal female pigs and possible implications for the onset of puberty. Acta Endocrinol. 100:806. Etienne, M., S. Camous, and A. Cuvillier. 1983. Effect of feed restrictions during growth on puberty and reproductive performance in gilts. Reprod. Nutr. Develop. 23:309. Friedman, C. I., and H. K. Moon. 1985. Obesity and its effects on reproductive function. Clin. Obstet. Gynecol. 28:645. Frisch, H. L. 1988. Fatness and fertility. Scient. Am. 258(31:88 Gunther, K.D. 1974.Zur Frage der beeinflussung von Fortpflanzungsprozessen beim weiblichen Rind durch Grunfutter von intensiv gedungten Roduktionsflachen. Tieraerztl. Umsch. 29:411. Hughes, P. E. 1982. Factors affecting the natural attainment of puberty in the gilt. In: D.J.A. Cole and G. R. Foxcroft (Ed.) Control of Pig Reproduction. pp 117-138. Butterworths, London. Hurtgen, J. P., A. D. Leman, and B. Crabo. 1980. Seasonal

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estrus by the herdsman than gilts in the other treatment groups, whereas the proportion of gilts that ovulated, as measured by progesterone analysis, was lower when gilts were restricted in their feed intake (Table 2). This suggests a greater difficulty in estrous detection when gilts are fed higher planes of nutrition. Concomitantly, plasma E2 concentrations during the luteal phase were greater (P < .05) in the AL-100 than in the AL-50 and AL-75 intake groups (Table 4). This implies that higher dietary energy intakes and(or1 higher body fat contents may influence the circulating pool of free estrogen. It has been suggested that the fat cell possibly serves in a capacity to store, metabolize, and convert circulating steroids (Lustig et al., 1989) as well as effect a change in sex hormone-binding globulins (Apter and Reijo, 1990). Elsaesser et al. (1982) reported that pubertal development in gilts is associated with a decrease in metabolic clearance rate of estrogen. Consequently, the lower nutritional regimen and lower body fat content of developing gilts may affect E2 metabolic clearance rates, which may subsequently influence the estrogen feedback on the ovulatory LH surge and subsequent estrual behavior. The higher E2 concentration in the AL-100 gilts during the luteal phase implies different responses in the hypothalamo-adenohypophysealovarian feedback axis, which allowed ovulation but decreased the magnitude of estrous expression in these gilts. Obviously nutrient restriction can dramatically reduce the synthesis of reproductive hormones, but this level of restriction is probably below that used in our study. Lack of boar exposure likely resulted in increased age of gilts at puberty in our study. Other factors including genetics (Hutchens et al., 19811, social environment (Caton et al., 1980a1, and season (Hurtgen et al., 1980; Szarek et al., 19811 can influence age at puberty. By eliminating the effects of the intact male from our study, we hoped to evaluate the specific effects of dietary intake on reproductive response. Roberson et al. (1991) reported that peripubertal beef heifers, when fed to achieve high rather than moderate growth rates, exhibited a greater response to bull exposure. Perhaps the reduction in age at puberty when gilts were fed to achieve high growth rates in some studies (Kirkwood and Aherne, 1985; Beltranena et al., 1991) can be attributed jointly to the boar as well as to the nutritional effects being evaluated.

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NEWTON AND MAHAN trauterine treatment with catecholestradiol. Domest. Anim. Endocrinol. 6:383. NRC. 1988. Nutrient Requirements of Swine (9th Ed.). National Academy Press, Washington, DC. Roberson, M. S., M. W. Wolfe, T. T. Stumpf, L. A. Werth, A. S. Cupp, N. Kojima, P. L. Wolfe, R. J. Kittok, and J. E. Kinder. 1991. Influence of growth rate and exposure to bulls on age at puberty in beef heifers. J. Anim. Sci. 89:2092. SAS. 1985. SAS User's Guide: Statistics. SAS Inst. Inc., Cary, NC. Shields, R. G., Jr., D. C. Mahan, and F. M. Byers. 1984. In vivo body composition estimation in nongravid and reproducing fist-litter sows with deuterium oxide. J. h i m . Sci. 59:1239. Steel, R.G.D., and J. H. Torrie. 1980. Principles and Procedures of Statistics: A Biometrical Approach (2nd Ed.). McGrawHill Book Co., New York. Szarek, V. E., D. G. Levis, and J. H. Britt. 1881. Characteristics of summer infertility in sows. J. Anim. Sci. 53(Suppl. 11:3 (Abstr.). Yalow, R. S., S. M. Glick, J. Roth, and S. A. Berson. 1965. Plasma insulin and growth hormone levels in obesity and diabetes. Ann. N.Y. Acad. Sci. 131:357.

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influence on estrous activity in sows and gilts. J. Am. Vet. Med. Assoc. 178:119. Hutchens, L. K., R. L. Hintz. and R. K. Johnson. 1981. Genetic and phenotypic relationships between pubertal and growth characteristics of gilts. J. Anim. Sci. 53446. Kirkwood, R. N., and F. X. Aherne. 1985. Energy intake, body composition and reproductive performance of the gilt. J. Anim. Sci. 60:1518. Kirkwood, R. N., D. C. Cumming, and F. X. Aherne. 1987. Nutrition and puberty in the female. Proc. Nutr. SOC.48:177. Kirkwood, R. N., and P. E. Hughes. 1981. A note on the influence of boar age on its ability to advance puberty in the gilt. Anim. Prod. 32:211. Lustig, R. H., H. L. Bradlow, and J. Fishman. 1989. Estrogen metabolism in disorders of nutrition and dietary composition. In: K. M. F'irke, W. Wuttke, and U. Schweiger (Ed.)The Menstrual Cycle and Its Disorders. pp 119-132, SringerVerlag, Heidelberg. Matamoros, I. A., N. M. Cox, and A. B. Moore. 1891. Effects of exogenous insulin and body condition on metabolic hormones and gonadotropin-induced follicular development in prepuberal gilts. J. Anim. Sci. 89:2081. Nephew, K. P., S. P. Ford, M. L. Day, and W. F. Pope. 1989. Extension of short cycles in postpartum beef cows by in-