J. Anim. Sci. 2005. 83:2653-2662
© 2005 American Society of Animal Science
Incidence of puberty in beef heifers fed high- or low-starch diets for different periods before breeding1
N. H. Ciccioli2,
S. L. Charles-Edwards3,
C. Floyd4,
R. P. Wettemann5,
H. T. Purvis,
K. S. Lusby6,
G. W. Horn and
D. L. Lalman
Department of Animal Science, Oklahoma Agricultural Experiment Station, Stillwater 74078
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Abstract
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Spring-born Hereford x Angus heifers (n = 206) were used to determine effects of energy supplementation programs and amount of starch in the diet on incidence of puberty. In Exp. 1, heifers (205 ± 5 kg; n = 68) grazing dormant native pasture were fed 0.9 kg/d (as-fed basis) of a 42% CP supplement from November until February 14. Heifers were stratified by weaning weight and allotted randomly to treatment before breeding (May to July). Treatments were 1) 0.9 kg (as-fed basis) of a 42% CP supplement/d and pasture (control); 2) a high-starch (HS) diet (73% corn; 53% starch) fed in a drylot for 60 d (HS-60); 3) a HS diet fed in drylot for 30 d (HS-30); or 4) a low-starch (LS) diet (49% corn; 37% starch) self-fed on pasture for 30 d (LS-30). The HS-60 and HS-30 heifers were limited-fed to gain 0.9 kg/d, and the LS-30 heifers had ad libitum access to the diet. High-starch-60 and LS-30 heifers were heavier (P < 0.05) than control and HS-30 heifers at the beginning of the breeding season. Thirty-one, 25, and 26% more HS-60 heifers were pubertal (P < 0.05) on May 1 compared with LS-30, HS-30, and control heifers, respectively. At puberty, HS-60 heifers were 24 and 22 d younger (P < 0.05) than LS-30 and control heifers, and 31 kg lighter (P < 0.01) than LS-30 heifers. In Exp. 2, heifers grazed dormant pasture and were fed 0.9 kg (as-fed basis) of a 42% CP supplement/d from weaning in October to late February; then heifers were assigned randomly to treatments for 60 d before the breeding season. In two years, control heifers (n = 46) grazed pasture and received 0.9 kg of SBM supplement/d; LS (n = 46) heifers were self-fed a distiller’s grain and soybean hull-based diet in drylot; and HS heifers (n = 46) were limited-fed a corn-based diet in drylot. During treatment, HS and LS heifers had greater weight gains than control heifers. Pubertal BW (313 ± 6 kg) was not influenced by treatment, but HS and LS heifers were younger (P < 0.03) than control heifers at puberty. During a 60-d breeding period, the incidence of puberty was greater (P < 0.05) for HS and LS heifers than for control heifers and was greater (P < 0.05) in HS than in LS heifers in Year 1. Feeding a LS or a HS diet for 30 d before breeding may be inadequate to stimulate puberty in beef heifers, but feeding a diet with a greater amount of starch for 60 d before breeding may increase the incidence of puberty during breeding of heifers that have inadequate yearling weight.
Key Words: Beef Cattle • Diet Composition • Nutrition • Puberty • Starch
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Introduction
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Heifers that initiate estrous cycles and conceive early during a constrained breeding season will have greater lifetime production than heifers conceiving later (Lesmeister et al., 1973
). Age at puberty is influenced by growth rate (Schillo et al., 1992; Yelich et al., 1995), and energy intake is positively related to growth rate and inversely related to age at onset of puberty (Wiltbank et al., 1969; Arije and Wiltbank, 1971; McShane et al., 1989). Changes in rate of gain before puberty may not influence reproductive performance (Lynch et al., 1997). Consumption of a high-concentrate (HC) diet for 60 d before breeding decreased age at puberty (Marston et al., 1995). Spring-born heifers grazing dormant native grass require large amounts of supplemental energy to attain puberty as yearlings. Feeding heifers a high-protein supplement during winter and then a HC diet before breeding may decrease feed costs. Diets or feed additives that increase propionate:acetate in the rumen may decrease age at puberty (Moseley et al., 1977, 1982; McCartor et al., 1979). Ruminal propionate concentrations are directly related to intake of rapidly fermentable carbohydrates (Krause et al., 2003); however, use of low-starch (LS) ingredients will allow a high-energy diet to be self-fed with roughage and thereby decrease labor input.
Our hypothesis was that a greater quantity of dietary starch would decrease age at puberty. The objectives of the first experiment were to determine whether limit-feeding a HC diet for 30 or 60 d or self-feeding a diet with less starch will decrease the age at puberty. The objective of the second experiment was to determine the percentage of pubertal heifers during the breeding season, and pregnancy rate, when beef heifers had high starch (HS) or LS intake for 60 d before the breeding season.
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Materials and Methods
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All experimental procedures described were approved by the Oklahoma State University Animal Care and Use Committee.
Experiment 1
Hereford and Angus x Hereford heifers (n = 68) born between February 15 and May 2 (394 ± 8 d of age on April 24) were stratified by weaning weight (7-mo-old; 16 h after removal from feed and water) and allotted randomly to four treatments. Treated heifers either grazed dormant native tallgrass pasture and were fed 0.9 kg of a 42% CP soybean meal-based pelleted supplement/d (control; 19.0 mm o.d. pellet; Table 1), were fed a HS diet in a drylot pen (25 m x 30 m) for 60 d (HS-60) or 30 d (HS-30) just before the breeding season, or grazed native tallgrass pasture and were self-fed a LS diet for 30 d just before the breeding season (LS-30).
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Table 1. Ingredient and nutrient composition (% of DM) of high- and low-starch diets and supplement fed to heifers (Exp. 1)a,b,c
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All heifers grazed the same dormant native tallgrass pasture and received 0.9 kg (as-fed basis) of a 42% CP of soybean meal-based supplement/d (Table 1
) from weaning (October 25) to the initiation of treatments. The native dormant tallgrass pasture was adequate in quantity and quality. Predominant forage species were little bluestem (Andropogon scoparius) and big blue-stem (Andropogon gerardii). Beginning on January 3, all heifers were on dormant native tallgrass pasture and had ad libitum access to native tallgrass hay composed of the same species as the pasture (6% CP, DM basis). Crude protein content of hay was determined by combustion (LECONS 2000; Leco Corporation, St. Joseph, MI) in accordance with AOAC (1990). The HS-60 and HS-30 heifers were maintained in drylot pens during treatment and group-fed daily at 0800. The HS-60 heifers began adaptation to a corn-based HS diet (Table 1) on February 14 (329 ± 8 d of age). Initially, HS-60 heifers were fed 1.36 kg of the HS diet/d and 4.5 kg of grass hay/d. Hay was fed after the HS diet was consumed. Over a 10-d period, the quantity of HS diet was increased by 0.8 kg every other day, and the quantity of hay was decreased by 0.8 kg (as-fed basis). When heifers were adapted to the HS diet, they were fed 0.45 kg of hay (as-fed basis) each day. After adaptation, BW was monitored every 28 d, and the daily ration was adjusted to achieve a gain of 0.9 kg/d. High-starch-30 heifers began the 10-d adaptation to the HS diet on March 14 and were limited-fed for 30 d to gain 0.9 kg/d. During the adaptation period, one HS-60 and one HS-30 heifer were removed from the experiment because of ruminal acidosis; another HS-30 heifer was removed because of an injury unrelated to the study. Low-starch-30 heifers had ad libitum access to a LS diet (Table 1) and dormant native grass pasture for 30 d, beginning when the HS-30 heifers ended the adaptation period (March 23).
Body weights were taken every 14 d from February 14 until April 11 after restriction of feed and water intake for 16 h. A full BW was taken at 0800 on April 25 after all heifers were placed in a drylot and fed native hay for 4 d to minimize differences in body fill. Weight (feed and water restricted for 16 h) at the beginning of breeding (April 28) was determined by averaging weights taken on two consecutive days at the end of 4 fcd on native hay, and heifers were weighed every 14 d until the end of the breeding season (July 1). Between February 14 and July 1, blood was obtained every 7 d via tail venipuncture into 10-mL tubes containing EDTA. After collection, blood samples were placed on ice and centrifuged (2,600 x g for 30 min) within 4 h. Plasma was aspirated and stored at –20°C until progesterone was quantified (Bishop and Wettemann, 1993). Puberty was defined as the first of two consecutive sampling dates with a concentration of progesterone in plasma >1 ng/mL. Weight at puberty was the weight taken on that day or the linear interpolation of the every-other-week weights before and after puberty. Two HS-30, two HS-60, and three LS-30 heifers did not become pubertal by the end of the breeding season and were eliminated from the analyses of age and weight at puberty. Body condition scores (Wagner et al., 1988) were estimated by four independent evaluators on April 27. On April 28, all heifers were placed with two bulls in a single pasture for 64 d. Pregnancy was diagnosed by rectal palpation 90 d after the end of the breeding season.
Data were analyzed as a completely randomized design using the GLM procedure (SAS Inst., Inc., Cary, NC). Heifers were stratified by weaning weight, with dietary treatment as the independent source of variation. If the treatment effect was significant (P < 0.05), treatment means were compared by t-tests using the PDIFF option of SAS.
Experiment 2
Angus x Hereford heifers (Year 1, n = 69; Year 2, n = 69), born between February 5 and May 9 and weaned on October 7, were used in this experiment. Shrunk weights (feed and water restricted for 18 h) were determined at weaning. From October to February 20, heifers grazed dormant native tallgrass pasture, had access to grass hay, and were fed 0.9 kg of a soybean meal-based 42% CP supplement/d (as-fed basis). The pasture and hay were the same as those described for Exp. 1.
In each year of the study, shrunk BW were recorded on February 20 (328 ± 7 d of age), and heifers were stratified by weight and allotted randomly to treatments. Treatments were dormant native grass pasture with 0.9 kg (as-fed basis) of a 42% CP supplement per heifer/d (control, n = 46); HS diet (n = 46); or LS diet (n = 46). High-starch and LS diets were formulated to result in similar caloric intakes, provided CP in excess of NRC (1996) requirements, and were fed in a drylot. Feed ingredients and nutrient concentrations are presented in Tables 2 and 3, respectively. Heifers received 0.5 kg of grass hay/d (6% CP) during acclimation and treatment. Heifers received 3.6 kg (as-fed basis) of their respective diets/d for 3 d, 4 kg/d for 4 d, then the LS group had ad libitum access to feed and HS heifers were fed 80% of estimated ad libitum intake each day. High-starch and LS diets were fed for 60 d after the acclimation period. Average daily consumption of the diets (on a DM basis) was 6.34 and 6.69 kg, respectively, for HS and LS heifers. Estimated daily consumption of starch (on a DM basis) was 3.36 and 2.44 kg, respectively, for HS and LS heifers. Shrunk BW were recorded after 28 d, and daily intake of HS heifers was adjusted to achieve similar ADG for LS and HS heifers. During May 4 to 9, all heifers were maintained in a drylot with free access to water and hay. Shrunk BW were recorded on May 9 and July 22 (Year 1) and on May 8 and August 5 (Year 2).
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Table 2. Ingredient composition (% of DM) of high- and low-starch diets and supplement fed to heifers (Exp. 2)a,b,c
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Blood samples were collected every 7 d, from March 16 to August 31, and progesterone was quantified as described for Exp. 1. Puberty was defined as the first of two consecutive sampling dates with a concentration of progesterone in plasma >1 ng/mL. Weight at puberty was determined by interpolation of BW before and after puberty.
Commencing on May 9, heifers were exposed to two 3-yr-old fertile Angus bulls in the same native grass pasture until August 24 (Year 1) or August 4 (Year 2). Heifers were not fed supplement during the early breeding season. Beginning on July 1, all heifers were fed 0.45 kg of a 42% CP supplement/d (as-fed basis) to meet nutrient requirements. Pregnancy rate was determined by rectal palpation 75 d after the end of the breeding season.
Body weight, ADG, and age at puberty were analyzed by ANOVA using a statistical model containing the main effects of year, treatment, and the year x treatment interaction (GLM procedure of SAS). The main effects and the interaction were tested using the residual mean square as the error term. If the year x treatment interaction was significant (P < 0.05), separate analyses were performed for each year of the experiment. To determine the effects of presence and amount of starch in the diet on response variables, treatment means were compared by orthogonal contrasts (control vs. HS and LS; LS vs. HS).
Survival analyses (LIFETEST procedure of SAS) were used to evaluate the effect of treatment on the onset of puberty during the first 8 wk of the breeding season. The analyses regressed the proportion of prepubertal heifers on week of the breeding season. Differences between treatment survival curves were tested separately for each year using the Wilcoxon test (SAS). When the treatment effect was significant, contrasts were used to compare control vs. HS and LS, and HS vs. LS were compared by deleting control from the data set.
Differences in pregnancy rates (number of pregnant heifers divided by the number of heifers exposed to bulls x 100) were tested using a logistic regression model (GENMOD procedure of SAS) that included the effects of treatment, year, and treatment x year. When the treatment effect was significant, pregnancy rates were compared using 
2 tests (PROC FREQ, SAS) for control vs. HS and LS and for HS vs. LS.
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Results
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Experiment 1
Initial BW on October 25 were similar for heifers on all treatments (Table 4). Heifers on the four treatments had similar ADG from October 25 to February 14, at which time the HS-60 heifers began adaptation to the HC diet. From February 14 to May 1 (beginning of the breeding season), HS-60 and LS-30 heifers gained more than control and HS-30 heifers. Thus, HS-60 and LS-30 heifers weighed more on May 1 than control and HS-30 heifers (P < 0.05), and had greater (P < 0.05) BCS than control heifers at the beginning of the breeding season. The HS-30 heifers had a greater (P < 0.05) BCS than control heifers, but were thinner (P < 0.05) than HS-60 heifers. During the breeding season, control heifers had greater (P < 0.03) ADG than HS-60 and LS-30 heifers, and HS-60 heifers gained 0.14 kg/d less (P < 0.05) than LS-30 heifers.
Limit-feeding the HC diet to HS-60 heifers decreased (P < 0.01) age and weight at puberty by 24 d and 31 kg, respectively, compared with LS-30 heifers (Table 5). On May 1, 31, 25, and 26% more of the HS-60 heifers were pubertal (P < 0.05) than LS-30, HS-30, and control heifers, respectively; however, treatment did not influence the percentage of pubertal heifers on May 23 or June 27.
The HS-60 heifers were pubertal 22 d earlier than control heifers (P < 0.05), but they were not significantly (P = 0.33) lighter at puberty (Table 5). The control, HS-30, and HS-60 heifers were 19, 33, and 31 kg lighter at puberty than LS-30 heifers. Age at puberty did not differ (P = 0.39) for control, HS-30, and LS-30 heifers. Dietary treatments did not affect (P = 0.82) pregnancy rates (Table 5).
Experiment 2
Average daily nutrient and DMI are presented in Table 3. Heifers were consistently heavier (P < 0.01) in Year 2 than in Year 1 (Table 6). At the end of the winter feeding period (November through February), before the start of nutritional treatments, BW was similar for heifers assigned to HS, LS, or control treatments within year (Year 1, P = 0.90; Year 2, P = 0.72). Average daily gains for HS (0.89 kg/d) and LS (0.84 kg/d) heifers for both years were similar during treatment, whereas control heifers gained less weight (0.22 and 0.51 kg/d in Year 1 and 2, respectively). At approximately 75 d after the end of treatment (July 22 or August 5), HS and LS heifers maintained the increased weight achieved during treatment in Year 1 and weighed 18% (P < 0.001) more than control heifers, but this was not the case not in Year 2.
Age of heifers at the beginning of treatment did not differ between years (363 ± 3 d; P = 0.80). There was not a treatment x year effect for age at puberty (P = 0.93), but heifers reached puberty 32 d younger in Year 2 (P < 0.001; 480 ± 7 vs. 448 ± 4 d for Year 1 vs. 2, respectively; Table 7). Heifers fed high-energy diets were younger (P < 0.03) at puberty than control heifers in Year 2 but not in Year 1 (P = 0.53). Treatment did not influence (P = 0.30) weight at puberty in either year (HS = 311 ± 4 kg; LS = 319 ± 5 kg; control = 309 ± 8 kg).
Survival analyses modeled the proportion of prepubertal heifers during the first 8 wk of the breeding season. Because the proportion of pubertal heifers and BW at the beginning of the breeding season were influenced (P < 0.05) by year, the effect of treatment was tested within year. In both years, treatment affected (Year 1, P < 0.001; Year 2, P < 0.03) the survival curves for prepubertal heifers during a 60-d breeding season. In Year 1 (Figure 1A), all heifers were prepubertal at the beginning of the breeding season. The proportion of prepubertal heifers was relatively similar for HS, LS, and control heifers during the first 4 wk of breeding, whereas the percentage of prepubertal heifers on the HS treatment decreased during 4 to 8 wk of breeding compared with LS and control heifers. The survival curves for control vs. HS and LS and for HS vs. LS in Year 1 were different (P < 0.002 and P < 0.05, respectively). In Year 2 (Figure 1B), the proportions of prepubertal heifers at the beginning (HS and LS = 67%; control = 100%; P < 0.01) and during the first 3 wk of the breeding period were less for HS and LS than for control heifers. The survival curves for HS and LS heifers differed from control (P < 0.02) heifers, but survival curves for HS and LS heifers were similar (P = 0.38). The proportion of prepubertal heifers decreased more rapidly for control heifers than for HS and LS heifers during the 4th wk of the breeding period, and survival curves were not influenced by treatment during wk 4 to 8.
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Figure 1. Proportion of prepubertal heifers each week during a 60-d breeding period. Survival curves represent heifers fed a complete high-starch (HS) diet, a complete low-starch (LS) diet, or dormant native grass pasture with 0.9 kg (as-fed basis) of a 42% CP supplement per heifer/d (control) for 60 d before the onset of breeding in Year 1 (A) and Year 2 (B).
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There was a treatment x year effect on pregnancy rate (P < 0.05; Table 8). In Year 1, pregnancy rates did not differ (P = 0.36) between HS or LS heifers, but they were less (P < 0.001) for control heifers. Treatment did not influence (P = 0.46) pregnancy rate in Year 2.
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Table 8. The effect of high- or low-starch diets on pregnancy rates of heifers during a 107-d (Year 1) or 87-d (Year 2) breeding season (Exp. 2)
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Discussion
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Experiment 1
Nutrition is a major determinant of the onset of puberty in beef heifers (Schillo et al., 1992). In this experiment, we attempted to increase the number of pubertal heifers during the breeding season using alternative feeding strategies that differed mainly in dietary energy content and feeding regimen. Treatments, as expected, influenced ADG before, during, and after the breeding period, and this increased the number of pubertal heifers at the beginning of the breeding season.
A HS diet fed for 60 d or a self-fed LS diet fed for 30 d before breeding were equally effective to increase BW and BCS of heifers at the beginning of the breeding season compared with feeding 0.9 kg of a 42% CP supplement/d to heifers grazing dormant native grass pasture. Feeding a diet containing more starch for 30 d before the onset of breeding promoted a moderate and inadequate increase in ADG because HS-30 heifers did not differ in BW from control heifers at the beginning of breeding. In agreement with previous results (Marston et al., 1995), control heifers compensated for the minimal winter ADG and gained more weight during the breeding season than heifers fed a HS diet for 60-d before breeding.
Low-starch-30, control, and HS-30 heifers were similar in age at puberty and older than HS-60 heifers. Onset of puberty was probably limited by weight for control and HS-30 heifers. As a result of the greater ADG from February 14 to May 1, LS-30 heifers had a greater BCS than control heifers and also were heavier than control and HS-30 heifers at the beginning of the breeding season (Table 4); however, age at puberty and percentage of LS-30 heifers that were pubertal at the start of the breeding season were similar to control and HS-30 heifers. These results do not agree with results from other researchers, who found that an increase in energy intake resulted in younger and heavier heifers at puberty (Wiltbank et al., 1969; Patterson et al., 1991; Marston et al., 1995; Yelich et al., 1995). Nonetheless, in those experiments, the increase in growth rates occurred over longer periods of time than in the present study, suggesting that a 30-d self-feeding period was insufficient to induce puberty. Low-starch-30 heifers had similar BCS and BW at the beginning of the breeding season as HS-60 heifers. Although a 30-d self-fed period of the LS diet was adequate to deposit the same amount of weight and body fat reserves as a 60-d feeding of a HS diet with limited intake, it did not enhance the onset of puberty. The diets differed in the source of energy and duration of feeding, which could affect the physiological processes that cause puberty.
Experiment 2
Heifers in Year 2 were heavier at the end of the winter feeding period because pretreatment feeding was managed more efficiently than in Year 1. Frequent monitoring of BW during winter was used to adjust the amount of hay fed to heifers. Increasing energy intake of prepubertal heifers grazing dormant native grass can increase pregnancy rates without affecting weight or body condition at puberty (Marston et al., 1995).
Body weight at the beginning of breeding (May) and ADG reflected the amounts of energy provided by treatments. Because HS and LS diets were isocaloric and because feed intake was adjusted to achieve similar ADG, heifers had similar weights at the beginning of breeding, and HS and LS heifers consistently weighed more than control heifers. This difference was greater in Year 1 than Year 2 because control heifers gained 0.29 kg/d less during Year 1. Forage quality/availability and/or environmental conditions are extremely variable between years and might have contributed to differences in animal performance. These factors might have caused the effect of treatment on BW at 75 d of the breeding season in Year 1 but not in Year 2.
Heifers attained puberty at a younger age in Year 2, but pubertal weights were similar in both years. High-starch and LS heifers were younger at puberty than control heifers in Year 2 but not in Year 1. This year effect is probably caused by a majority of control heifers that were prepubertal at the end of the experiment in Year 1. The difference between years in BW at the end of the winter feeding period was maintained throughout the breeding season (May to August); thus, at the same BW, heifers in Year 2 were younger than those in Year 1. Despite this difference in age, the onset of puberty occurred at similar BW in both years. This finding agrees with the concept that, if age is not limiting (Yelich et al., 1995), puberty occurs at a critical BW (Kennedy and Mitra, 1963).
Supplementation of growing heifers for a 60-d period before breeding can increase the number of pubertal heifers during a constrained breeding season; however, the effectiveness of this nutritional strategy may depend on the weight of heifers at the end of the first winter (yearling weight). In the present study, the beneficial effects of feeding a HS diet were more evident in Year 1, when heifers weighed 211 kg in late February, which reflects a minimal winter growth rate, and control heifers weighed 53 kg less than HS heifers at the end of treatment (beginning of the breeding period). The difference in growth rate persisted throughout the breeding period, when all heifers were on the same pasture, and resulted in 60% of the HS heifers that had attained puberty by the end of the first 8 wk of the breeding period, in contrast to <10% of the control heifers. It is widely accepted that puberty is delayed in undernourished heifers (Wiltbank et al., 1969; Day et al., 1986; Yelich et al., 1995). Conversely, in the second year of study, heifers were considerably heavier (245 kg) at the start of treatment than in Year 1 (211 kg), and control heifers were only 23 kg lighter than HS heifers at the end of treatment. Thus, all heifers gained more weight during winter in Year 2 than in Year 1. Consequently, 33% of the HS heifers, but none of the control heifers, were pubertal at the beginning of breeding. The number of pubertal heifers was equivalent for control and HS heifers during the second half of the breeding season, and >90% of heifers on all treatments had initiated estrous cycles by the end of breeding. This finding implies that diets with greater amounts of starch may induce puberty in heifers that have inadequate BW at the start of the breeding season.
Feeding diets with greater amounts of starch to lightweight prepubertal heifers before the start of breeding may have additional benefits for a cow-calf operation. Fertility of heifers increases with the number of estrous cycles preceding breeding (Byerley et al., 1987). If more HS heifers are cycling at the start of breeding, as occurred in Year 2 of the present experiment, HS heifers also should become pregnant earlier in the breeding season. Heifers that become pregnant early during the first breeding season produce heavier calves at weaning and more kilograms of calf in their lifetime (Lesmeister et al., 1973). The major effect of diet on pregnancy rate observed in Year 1 was due to an increased number of pubertal heifers during the breeding season. This result, however, does not preclude a possible beneficial effect of HS diet on fertility at the first estrus following an anestrous condition (Ciccioli et al., 2003).
The present experiment was designed to test whether the amount of starch fed during the prepubertal period may hasten the onset of puberty. Survival curves from Year 1 support this hypothesis because more HS than LS prepubertal heifers attained puberty during the breeding season. An explanation for this positive response to HS supplementation does not involve differential growth rates and/or nutrient intake because HS and LS groups had 1) similar BW at the beginning of feeding; 2) similar ADG during feeding and, therefore, similar BW at the end of feeding; and 3) similar feed intake of isonitrogenous-isocaloric diets. Growth rate during breeding also was similar for HS and LS heifers; however, amount of starch in the diet may alter hormone/metabolite production to hasten the onset of puberty. A linear increase in the amount of dietary fermentable carbohydrate, corn starch, shifted ruminal fermentation and caused a linear increase in propionate concentration in ruminal fluid (Krause et al., 2003). Dietary effects on production of VFA were not determined in the present study, but the amount of dietary starch might have altered acetate:propionate in the rumen (Krause et al., 2003). A decrease in age and weight at puberty of heifers occurred concomitantly with a change in ruminal fermentation, which decreased acetate:propionate (Moseley et al., 1977, 1982; McCartor et al., 1979). Heifers consuming diets that caused production of greater propionate were younger and lighter at puberty. Abomasal infusion of propionate into prepubertal heifers increased blood glucose concentrations and enhanced the secretion of LH after a GnRH challenge (Rutter et al., 1983).
Increased propionate in heifers on a HS diet could increase concentrations of insulin and glucose in plasma (Oba and Allen, 2003). Greater release of splanchnic glucose increases body energy retention in beef heifers (Reynolds et al., 1991), enhancing metabolic efficiency and, perhaps, reproductive function. Nutrient composition of the diet may decrease age at puberty (Lalman et al., 1993), and heifers fed diets that produce more propionate had greater concentrations of insulin during the peripubertal period, were fatter (Yelich et al., 1995), and were younger at puberty (McCartor et al., 1979; Marston et al., 1995; Yelich et al., 1995). The specific mechanisms that link nutrient intake with timing of puberty are not completely understood; however, there is increasing evidence that indicates that concentrations of insulin may have an important role in the onset of puberty. Increased concentrations of insulin in blood may act directly or indirectly through glucose uptake by the central nervous and/or ovarian systems to modulate reproductive activity. Insulin, in the presence of glucose, increased GnRH output in vitro from hypothalamic tissue of rats (Arias et al., 1992). Propionate-induced increases in plasma concentrations of insulin did not affect LH secretion in ovariectomized beef heifers (DiCostanzo et al., 1999), which indicates a minor role of peripheral concentrations of insulin on brain centers that control LH secretion in cattle. A direct action of insulin on the ovary cannot be ruled out. Insulin, in presence of gonadotropins, directly stimulates steroidogenesis of cultured bovine thecal (Stewart et al., 1995) and granulosal cells (Spicer et al., 2002), and, in presence of constant normal concentrations of glucose, increases estradiol secretion by the dominant follicle of anovulatory cows (Butler et al., 2004). Supporting this hypothesis, increased ruminal propionate, and therefore insulin, enhanced the ovarian response to gonadotropins (Bushmich et al., 1980), and feeding a diet that increased plasma concentrations of insulin during the anovulatory period advanced the first postpartum ovulation (Gong et al., 2002).
We conclude that 1) self-feeding a LS diet or limited-feeding a HS diet for a 30-d period preceding the breeding season is insufficient to increase the number of pubertal heifers at the onset of breeding, 2) feeding a diet with HS content for 60 d before breeding may decrease age at puberty when heifers have an inadequate yearling weight and may increase the number of pubertal heifers at the beginning of the breeding season, and 3) increased amounts of starch in the diet may induce puberty without affecting BW.
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Implications
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Feeding a high-concentrate diet for less than 30 d may be inadequate to stimulate puberty of spring-born beef heifers; however, feeding high- or low-starch diets for 60 d will increase the number of pubertal heifers during the first breeding season if they do not gain sufficient weight after weaning. Isocaloric-isonitrogenous diets that contain greater amounts of starch may hasten pubertal development compared with diets containing less starch; however, the effect may be influenced by the degree of development or body weight before the first breeding season.
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Footnotes
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1 Approved for publication by the director, Oklahoma Agric. Exp. Stn. This research was supported under Project H-2331. 
2 Deceased (2005) after preparation of the manuscript.
3 Current address: 1900 Napa Valley Rd., Edmond, OK 73013.
4 Current address: First National Bank, P.O. Box 719, Johnson, KS 67855.
6 Current address: Dept. of Anim. Sci., Univ. of Arkansas, Fayetteville 72701.
5 Correspondence: 114 Animal Science (phone: 405-744-7390; fax: 405-744-7390; e-mail: Bob.Wettemann{at}okstate.edu).
Received for publication August 20, 2004.
Accepted for publication July 6, 2005.
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Abstract
Introduction
