Supplemental Feed Protein Concentration and Weaning Age Affects Replacement Beef Heifer Performance

The Professional Animal Scientist 21 (2005):278–285

Supplemental Feed Protein Concentration and Weaning Age Affects Replacement Beef Heifer Performance W. J. SEXTEN, PAS, D. B. FAULKNER1, PAS, and J. M. DAHLQUIST Department of Animal Sciences, University of Illinois, Urbana 61801

Abstract Sixty-four Simmental heifer calves were utilized in a 2 × 2 factorial treatment arrangement designed to determine the effects of weaning age and supplemental feed CP concentration on performance, reproduction, and milk production of heifer calves reared as replacements. Weaning occurred at an average of 89 ± 1.6 d of age (early weaning; EW) or 232 ± 1.8 d of age (normal weaning; NW); supplemental diets were either 12 or 17% CP. A weaning age × supplemental feed interaction (P<0.01) was observed for calf performance. Providing EW heifers additional CP improved (P=0.03) ADG; however, additional CP decreased (P=0.003) ADG in NW heifers. Normal-weaned heifers provided a 12% CP supplement had greater (P=0.001) ADG than did EW heifers; however, weaning age did not influence (P=0.20) ADG of heifers consuming a 17% CP supplement. Supplemental DMI was greater (P<0.001) for EW heifers. Early-weaned heifer BW was lesser (P≤0.08) through breeding. Despite lesser BW, more (P=0.05) EW heifers (81.3%) were pubertal at 8 mo of age than NW

1

To whom correspondence should be addressed: [email protected]

heifers (59.4%). By 83 d postpartum, no BW differences were observed (P=0.56) relative to weaning age. Diet did not influence puberty onset or postweaning BW (P≥0.30). Neither dam weaning age nor previous supplemental CP concentration influenced (P>0.15) first or second lactation milk production and calf performance. The influence of supplemental feed CP concentration on heifer performance is dependent on weaning age; however, reproduction and milk production were not affected. Early weaning reduced heifer BW until breeding and increased percentage of pubertal replacements at an early age without influencing milk production. (Key Words: Beef Cattle, Supplemental Feed, Protein, Weaning Age, Milk Production.)

Introduction Creep feeding and early weaning (EW) are management strategies used to improve performance of heifer calves during early development (Richardson et al., 1978; Hixon et al., 1982). Several problems have been associated with creep feeding and EW management of replacement heifers. In one study, EW increased heifer development costs by 12.8% compared with normal wean-

ing (NW) management because of higher feed and labor requirements (Story et al., 2000). Providing supplemental feed to EW and nursing calves reduces future milk production (Holloway and Totusek, 1973b; Martin et al., 1981). Heifers are most susceptible to the impact of feeding level during the prepubertal mammary growth phase, which starts as early as 3 mo of age (Sinha and Tucker, 1969; Sejrsen et al., 1982). Nutritional management strategies using minimal feed and labor inputs to promote prebreeding growth without negatively influencing future productivity may offer solutions to the problems of creep feeding and EW. Supplementing weaned and nursing calves grazing high quality forage with undegraded intake protein increased calf ADG (Lardy et al., 2001). Increasing creep feed CP concentration from 14 to 18% improved preweaning growth of beef heifers grazing mixed cool season pastures while increasing early first lactation milk production (Sexten et al., 2004). Increasing dietary protein to support greater ADG of high energy diets fed in confinement during the prepubertal mammary growth period has shown the potential to improve dairy heifer growth without sacrificing mammary development

Supplemental Feed and Weaning Age Affects Beef Heifers

TABLE 1. Supplemental feed ingredients and diet composition (DM basis). Item Ingredient, % Cracked corn Dried distillers grains with solubles Soybean meal, 48% CP Cane molasses Ground limestone Composition DM, % OM, % NDF, % ADF, % CP, % Undegradable intake protein, % CP TDN, %b NEm, Mcal/kgb NEg, Mcal/kgb a

12% CPa

17% CP

88.0 0.0 5.0 5.0 2.0

62.0 26.0 5.0 5.0 2.0

83.3 90.1 10.5 3.9 11.8 55.4 80 2.07 1.41

84.2 87.4 16.3 6.5 17.0 54.3 78 2.00 1.36

12% CP = 12% CP diet; 17% CP = 17% CP diet. Calculated values from wet chemistry ADF (%).

b

(Radcliff et al., 1997; Van Amburgh et al., 1998). VandeHaar (1997) reviewed 12 published studies using dairy heifers managed to gain in excess of 0.9 kg/d and indicated that high dietary protein may improve mammary development. The objectives of this experiment were to evaluate the effects of weaning age and supplemental feed CP concentration on performance, reproduction, and milk production of heifer calves reared as replacements.

Materials and Methods Sixty-four Simmental heifer calves were utilized in a 2 × 2 factorial arrangement to determine the effects of weaning age and supplemental feed protein concentration on replacement heifer calf growth and milk production. Thirty-two heifers were weaned at 89 ± 1.6 d of age (EW), and 32 heifers were weaned at 232 ± 1.8 d of age (NW). Heifers were assigned to one of two experimental diets containing different CP concentrations, as presented in Table 1. Eight pasture groups were uti-

lized, resulting in two replications per treatment combination. All experimental procedures followed those approved by the University of Illinois Laboratory Animal Care Advisory Committee. All heifers were given ad libitum access to the 12% CP diet starting on May 2 (EW). Heifers were given access to the 12% CP supplemental feed prior to pasture turnout to encourage feed consumption by EW heifers. Supplemental dietary treatments of 12 and 17% CP began 57 d later on June 28 and concluded after an 84-d feeding period ending on September 20 (NW). During the 84d feeding period, cattle grazed mixed pastures of endophyte-infected tall fescue (Festuca arundinacea), orchardgrass (Dactylis glomerata), white clover (Trifolium repens), and alfalfa (Medicago sativa). Cattle groups were rotated through pastures to minimize forage quality differences. All heifer calves were given ad libitum access to a diet composed of one-half 12% CP supplemental feed and one-half 17% CP supplemental feed for 21 d after NW. Then, all heifers were limit-fed

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a 50% ground alfalfa hay, 45.8% ground corn, 2.3% soybean meal, and 1.9% mineral diet at 1.7% of BW from October 11 through breeding. Heifers were randomly allotted to dietary treatment by sire and d 1 BW. Calf performance data were determined from unshrunk BW and hip height recorded on June 28 and NW. Dams of nursing heifer calves gained 0.18 ± 0.09 kg/d during this period, resulting dam BW at NW was 568.3 ± 11.8 kg and hip height that was 136.4 ± 0.77 cm. Dams ranged in age from 2 to 6 yr old with an average parity of 2.5 ± 0.2. Milk production and composition estimates were taken at dietary treatment initiation and NW to estimate milk protein intake of suckling calves.Twenty-four hour milk production estimates were determined using a 12-h weigh-suckle-weigh technique (Beal et al., 1990). Six hours following the weigh-suckle-weigh, a subsample of 6 cows were milked using a commercial portable milk machine (Porta Milker, The Coburn Company, Inc., Whitewater, WI). Cows were administered 20 USP units of oxytocin (Phoenix Scientific, St. Joseph, MO) intravenously within 2 min of milking to initiate milk letdown. Milk was sampled and sent to Dairy Lab Services (Dubuque, IA) for compositional analysis. At dietary treatment initiation, milk production estimates were 7.05 ± 0.29 kg/d with a milk protein content of 2.92 ± 0.04%, at NW milk production estimates were 6.13 ± 0.62 kg/d with a milk protein content of 3.36 ± 0.07%. Based on the composition and production estimates, NW heifers were receiving an additional 206 g of CP/d while suckling dams. Weekly blood sampling was initiated August 10, via jugular venipuncture, to determine percentage of heifers pubertal at 8, 10, and 12 mo of age. Serum was separated by centrifugation at 1000 × g for 20 min within 6 h of collection. Serum was stored at −20°C until assayed for progesterone concentration. Progesterone con-

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centrations were determined by validated enzyme immunoassay (Kesler et al., 1990). Heifers were considered pubertal when progesterone concentration exceeded 1.5 ng/mL on 1 sample d. Heifers were estrus synchronized using melengestrol acetate and prostaglandin F2α. Melengestrol acetate (0.5 mg per heifer; MGA威; Pfizer, New York, NY) was fed daily for 14 d starting February 28, 19 d after the completion of MGA feeding heifers were administered a luteolytic dose of prostaglandin F2α (25 mg per heifer; Lutalyse威; Pfizer). Heifers were artificially inseminated on April 4 at an average age of 427.1 ± 1.2 d. All cattle were monitored for estrus activity after the first AI; upon estrus detection, heifers were artificially inseminated a second time. Heifers were turned out onto mixed breeding pastures on April 29 after two potential AI opportunities. One mature Angus bull was also turned out at this time and remained with the heifers for a 29-d natural breeding season. Cattle remained on mixed pastures and were rectally palpated for pregnancy 42 d after bull removal. Heifers were supplemented for 66 d prior to initiation of the calving season. Cattle were limit fed 11.7 kg/d per heifer of a 50% ground alfalfa hay, 50% corn diet until calving. Birth BW and calving ease data were utilized for calves that died prior to weaning. Heifers that gave birth to twins or failed to raise their own calves were excluded from the performance data set but were included in reproductive analysis. Heifers were maintained in a dry lot after calving. Supplementation included 4.54 kg/d per heifer of alfalfa hay and 5.7 kg/d per heifer of dry corn gluten feed or dried distillers grains with solubles. Heifer milk production estimates were collected for first and second lactations. First lactation milk production estimates were determined when heifers averaged 63.4 ± 1.9 d postpartum; heifer and calf perfor-

Sexten et al.

mance data were collected when calves were an average age of 83.4 ± 1.9 d. Second lactation milk production and calf performance data were determined when heifers were 62.4 ± 3.7 d postpartum. All milk production estimates were conducted using a 12-h weigh-suckle-weigh. Treatment effects were considered significant at an α level of 0.05. The full model included weaning age, supplemental feed protein concentration, and the weaning age × supplemental feed protein interaction. Prebreeding performance data were analyzed using the MIXED procedure of SAS威 (Littell et al., 1996; SAS Inst., Inc., Cary, NC), and pen was the experimental unit. Postbreeding performance analysis also utilized the MIXED procedure of SAS威 (Littell et al., 1996); however, animal served as the experimental unit. Reproductive data were analyzed using the GENMOD procedure of SAS威 with animal as the experimental unit. The reproductive data followed a binomial distribution with zero representing failure and one equaling reproductive success.

Results and Discussion The mixed pastures of endophyteinfected tall fescue, orchardgrass, white clover, and alfalfa were maintained in a vegetative state. Therefore, available degradable protein was not considered limiting; however, protein intake from pasture was not determined. Supplemental undegradable intake protein (UIP) by heifer calves was determined using diet analysis from Table 1 and supplemental DMI from Table 2: [dietary CP (%) × undegraded CP (%) × supplemental DMI]. Early weaned heifers fed the 12% CP diet consumed 356 g/d of supplemental UIP, EW heifers fed the 17% CP diet consumed 514 g/d of supplemental UIP, NW heifers fed the 12% CP diet consumed 232 g/d of supplemental UIP, and NW heifers fed the 17% CP diet consumed 298 g/d of supplemental UIP. Milk consumption during the

supplemental feeding period provided an additional 206 g/d CP to NW calves based on milk production estimates and milk composition analysis conducted at the initiation and termination of the supplemental feeding period. Because of the closure of the esophageal groove during suckling, milk protein bypasses ruminal fermentation (Ruckebusch, 1988); therefore, all of the milk protein should be available to the suckling calf as UIP. An interaction was observed between weaning age and supplemental feed protein concentration for BW at NW (P=0.002) and ADG (P= 0.003) during the supplemental feeding period (Table 2). Energy limits growth in suckling calves consuming fescue-based diets (Cremin et al., 1991; Faulkner et al., 1994) as well as native range (Loy et al., 2002). However, Lardy et al. (2001) suggested high quality forage was limiting in metabolizable protein for both weaned and suckling calves, and milk represented an important source of metabolizable protein to young calves. Normal weaning improved (P≤0.001) ADG during the supplemental feeding period in heifers fed the 12% CP diet, suggesting that UIP might have been limiting growth, because NW heifers fed the 12% CP diet received an additional 82 g/d of UIP because of their milk consumption compared with the consumption by EW heifers fed the 12% CP diet. Weaning age did not influence (P≥0.20) performance in 17% CP diet heifers; however, the calculated UIP intake difference was negligible between EW (514 g/d of UIP) and NW (504 g/d of UIP). Improved (P≤0.03) supplemental feeding period performance of EW heifers fed a 17% CP diet compared with a 12% CP diet also supports UIP as the limiting nutrient, as the 17% CP diet provided an additional 158 g/d of UIP. Lusby and Wetteman (1986) indicated that protein creep was best utilized when forage quality or dam milk production limited calf BW gain. A response in calf

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Supplemental Feed and Weaning Age Affects Beef Heifers

TABLE 2. Influence of weaning age and supplemental feed protein concentration on performance (least squares means) of replacement heifer calves during the summer grazing perioda. Treatmentb EW Item Initial Pen, n Animal, n BW, kg Hip height, cm Final BW, kg Hip height, cm ADG, kg/d Height gain, cm Supplemental DMI, kg/d

NW

P

12% CP

17% CP

12% CP

17% CP

SEM

Wean

Diet

Wean × diet

2 16 180.9 105.0

2 16 181.9 104.0

2 16 183.3 105.1

2 16 182.5 106.8

— — 0.5 0.48

— — 0.05 0.04

— — 0.79 0.54

— — 0.17 0.05

280.1 112.7 1.18 7.7 5.45

287.2 112.1 1.26 8.1 5.57

298.2 114.1 1.37 9.0 3.55

285.1 114.8 1.22 8.0 3.23

1.5 0.9 0.02 1.2 0.18

0.005 0.09 0.01 0.66 < 0.001

0.10 0.96 0.10 0.83 0.61

0.002 0.49 0.003 0.62 0.28

c

a

Summer grazing period began June 28 and ended September 20. EW = early wean, NW = normal wean, 12% CP = 12% CP diet, and 17% CP = 17% CP diet. c Largest standard error of least squares means. b

performance to UIP may be expected at high energy intake, as demand for UIP increases with high BW growth rates (NRC, 1996). Jensen et al. (1999) suggested that high energy intake was necessary to justify feeding high protein supplements. Supplemental DMI was high for all calves in this experiment, as NW calves were consuming 1.4% of BW and EW heifers were consuming 2.4% of BW. The 17% CP diet depressed (P≤0.003) supplemental feeding period performance in NW cattle. Rationale for the depression in performance is not clear. Previously, increasing protein supply using creep feed has either not affected (Cremin et al., 1991) or increased (Lardy et al., 2001; Sexten et al., 2004) calf performance during the summer grazing season. Degradable intake protein was considered adequate for all treatments because of pasture intake, and no interaction (P=0.28) was observed for supplemental DMI. A numerically greater supplemental DMI (by 0.32 kg/d) by NW heifers fed the 12% CP diet (Table 2) may partially explain the decreased performance

in NW heifers fed the 17% CP diet. Additionally, the interaction of weaning age and diet reduced replication of experimental units to two for ADG during the supplemental feeding period. Hip height gain was not influenced by weaning age (P= 0.66) or supplemental feed CP (P= 0.83). Percentage of heifers pubertal by 8 mo of age was greater (P=0.05) for EW heifer calves than for NW heifer calves (Table 3). Day et al. (2001) reported precocious puberty (245 ± 8.5 d of age) in >50% of EW heifers fed high concentrate diets. Precocious puberty was induced in 7 of 9 heifers early weaned (73 d of age) and fed a high energy diet; none of the 9 EW heifers receiving the low energy diet exhibited precocious puberty. Managing heifers to attain 65% of mature weight prior to breeding is the most practical method to ensure that a high percentage of heifers are pubertal prior to the breeding season (Patterson et al., 1992). Age at puberty was negatively correlated to BW gain from birth to puberty (Arije and Wiltbank, 1971). Increasing preweaning (Buskirk et al., 1996) or

postweaning (Buskirk et al., 1995) BW gain has resulted in earlier puberty. In this study, a greater percentage of EW heifers were pubertal at 8 mo of age, yet ADG for EW heifers was less than or equal to the ADG of NW cattle. These data suggest high energy diets are involved in the attainment of puberty, although the role may not relate specifically to increased ADG. Moseley et al. (1982) reported age at puberty was reduced independent of ADG or BW of heifers fed monensin. The authors hypothesized a relationship among puberty, energy metabolism, endocrine system responsiveness, and rumen fermentation patterns favoring propionate. Although rumen fermentation patterns were not quantified in this study, EW heifers would be expected to have less acetate:propionate based on greater (P<0.001) supplemental DMI (Table 2). Despite the apparent differences in fermentation patterns caused by weaning age, physiological effects attributable to dam presence or absence must also be considered. By 10 mo of age, weaning age did not affect (P=0.30)

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TABLE 3. Influence of weaning age and supplemental feed protein concentration on reproductive performance (actual means) of replacement heifer calves. Treatmenta EW Item n Percentage pubertal byb 8 mo of agec 10 mo of age 12 mo of age nd Pregnancy rate, %e

NW

P

12% CP

17% CP

12% CP

17% CP

Wean

Diet

Wean × diet

16

16

16

16

—

—

—

87.5 93.8 100.0 15 86.7

75.0 100.0 100.0 15 93.3

56.3 93.8 100.0 16 75.0

62.5 93.8 100.0 15 73.3

0.05 0.30 — — 0.09

0.62 0.30 — — 0.65

0.35 0.30 — — 0.57

a

EW = early wean, NW = normal wean, 12% CP = 12% CP diet, and 17% CP = 17% CP diet. Heifers were considered pubertal when serum progesterone concentration exceeded 1.5 ng/mL. c Heifers classified as 8, 10, or 12 mo of age at puberty were ≤240, 300, or 365 d of age, respectively. d Three heifers were removed from study prior to breeding because of positive neospora serum tests. e Pregnancy rate to two AI opportunities and a 29-d natural service period; determined 42 d after bull removal. b

percentage of pubertal heifers. All heifers were weaned 10 d prior to attaining 8 mo of age; therefore, the potential physiological effects of the dam on puberty onset were removed after 8 mo of age. The interaction of puberty onset, high-energy diets, rumen fermentation pattern, and physiological dam effects merits further investigation. Supplemental feed protein concentration did not influence (P>0.15) puberty onset. Increasing feed protein decreased age at puberty in lightweight Zebu heifers, although increased protein also improved growth rate, which was cited as the reasoning for earlier puberty (Oyedipe et al., 1982). When prepubertal BW gain does not differ, increasing prepubertal dietary protein did not influence age at puberty (Fajersson et al., 1991). In this study, the 17% CP diet enhanced growth of EW heifers and depressed growth of NW cattle, yet no weaning age × diet interaction (P=0.35) was observed for percentage of heifers pubertal by 8 mo of age. Pregnancy rates tended (P=0.09) to be greater in EW heifers (Table 3). Supplemental feed protein concentration did not influence (P=

0.65) pregnancy rates. Previous work (Holloway and Totusek 1973b; Richardson et al., 1978) reported no differences in pregnancy rates because of weaning age in beef heifers early weaned at 120 or 140 d of age or normal weaned at 210 or 240 d of age. Sexten et al. (2004) previously reported no effect of creep feed protein level on pregnancy rates. Reproductive responses should be viewed with caution, as the number of available experimental units does not allow for robust statistical analysis. Yearling (P=0.08) and prebreeding (P=0.06) BW tended lesser because of EW management (Table 4). Body weights at specified time periods are commonly used to determine whether replacement heifers are growing at an adequate rate. Considerable flexibility exists in the timing of BW gain from weaning to breeding, early, constant, and delayed BW gain from weaning to breeding has not influenced reproductive performance of replacement heifers as long as heifers reach adequate breeding weight (Clanton et al., 1983; Lynch et al., 1997). Varied experimental conditions associated with EW, such as weaning age and man-

agement after EW and NW, have yielded diverse results regarding EW heifer performance. Early weaning at 120 to 150 d of age has depressed BW gains by EW heifers from EW to NW followed by increased ADG from normal weaning until 1 yr of age (Richardson et al., 1978; Story et al., 2000). Holloway and Totusek (1973a) reported no differences between EW (140 d of age) and NW (240 d of age) heifers for BW or BCS from 2 until 4.5 yr of age. Despite lesser prebreeding BW, no differences (P=0.56) in heifer BW at an average postpartum interval of 83.4 ± 1.9 d were observed because of weaning age. Breeding BW differences caused by preweaning (Buskirk et al., 1996) and postweaning (Buskirk et al., 1995) treatments are no longer observed as heifers mature. Supplemental feed protein concentration did not influence heifer BW at a year of age (P=0.79), prebreeding (P=0.65), or 83 d postpartum (P=0.70). Prepubertal dietary protein intake has not influenced calving BW of Holstein (Dobos et al., 2000) or beef heifers (Sexten et al., 2004). Neither weaning age (P=0.58) nor supplemental feed CP concentra-

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Supplemental Feed and Weaning Age Affects Beef Heifers

TABLE 4. Influence of weaning age and supplemental feed protein concentration on replacement heifer calf BW (least squares means) from a year of age until first calf early weaning. Treatmentb EW Item n Yearling BW, kg Prebreeding BW, kg Cow at calf early weaning, 83.4 ± 1.9 d postpartum n BW, kg BCSc

NW

P

12% CP

17% CP

12% CP

17% CP

SEM

Wean

Diet

Wean × diet

15 321.3 393.7

15 329.2 400.2

16 346.5 424.3

15 334.1 408.5

— 8.5 10.2

— 0.08 0.06

— 0.79 0.65

— 0.23 0.28

11 530.8 6.2

10 518.5 6.3

9 532.2 6.3

9 533.5 6.0

— 14.6 0.12

— 0.56 0.41

— 0.70 0.29

— 0.63 0.22

c

a

EW = early wean, NW = normal wean, 12% CP = 12% CP diet, and 17% CP = 17% CP diet. Largest standard error of least squares means. c Body condition score (1- to 9-point scale). b

tion (P=0.53) influenced first lactation milk production (Table 5). One of the objectives of this study was to determine whether increasing protein intake of replacement heifers could increase milk production. In a review of 12 experiments, VandeHaar (1997) indicated that high dietary protein may improve mammary development of dairy heifers managed for BW gain in excess of 0.9 kg/d. Increasing heifer calf pro-

tein intake by increasing supplemental feed protein concentration or permitting calves to suckle dams during the summer grazing period did not improve milk production under the conditions of this study. Similar results have been reported in intensively managed dairy heifers. A high-protein, low-energy diet resulting in BW gains of 0.95 kg/d was not able to improve milk production in dairy heifers compared

with a low protein, high energy corn silage diet (Waldo et al., 1998). Whitlock et al. (2002) concluded that dietary protein does not have an effect on mammary development of rapidly grown dairy heifers provided the diet contains adequate protein for growth. Dobos et al. (2000) and Sexten et al. (2004) reported improved heifer growth because of greater prepubertal protein levels. However, total first lactation

TABLE 5. Influence of weaning age and supplemental feed protein concentration on first lactation maternal performance (least squares means) of replacement heifer calves. Treatmentb EW

NW

P

Item

12% CP

17% CP

12% CP

17% CP

SEM

Wean

Diet

Wean × diet

Dam milk production, kg/dc Calf, n Calving eased Birth BW, kg 83-d BW, kg ADG, kg/d

8.89 11 1.27 36.2 111.6 0.90

9.29 10 1.42 39.9 114.0 0.88

8.30 8 1.33 36.7 106.8 0.86

8.95 9 1.36 39.3 121.0 0.96

0.89 — 0.21 1.8 6.5 0.07

0.58 — 0.99 0.99 0.84 0.72

0.53 — 0.68 0.08 0.16 0.50

0.88 — 0.76 0.76 0.32 0.29

a

c

EW = early wean, NW = normal wean, 12% CP = 12% CP diet, and 17% CP = 17% CP diet. Largest standard error of least squares means. c Heifers averaged 63.4 ± 1.9 d postpartum. d 1 = no difficulty, 2 = pulled by hand, 3 = mechanical assistance, and 4 = Caesarean section. b

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TABLE 6. Influence of weaning age and supplemental feed protein concentration on second lactation performance (least squares means). Treatmentb EW Item Cowc n BW, kg BCSd Milk production, kg/d Calf Birth BW, kg 62 d BW, kg ADG, kg/d

NW

P

12% CP

17% CP

12% CP

17% CP

SEM

Wean

Diet

Wean × diet

10 556.2 5.1 10.2

9 534.1 5.0 9.6

5 555.5 5.0 10.8

8 565.0 4.9 8.8

— 27.3 0.4 1.9

— 0.47 0.82 0.92

— 0.76 0.82 0.38

— 0.45 0.96 0.64

42.0 115.9 1.06

42.3 113.1 1.04

42.6 106.1 0.96

39.8 116.5 1.08

0.61 0.44 0.58

0.47 0.36 0.38

0.40 0.13 0.23

c

2.4 5.5 0.07

At weigh-suckle-weigh, 62.4 ± 3.7 d postpartum. EW = early wean, NW = normal wean, 12% CP = 12% CP diet, and 17% CP = 17% CP diet. b Largest standard error of least squares means. c Body condition score (1- to 9-point scale). a a

milk production was not influenced in either experiment. Silva et al. (2002) suggested high energy intake during the prepubertal growth period may result in increased BW gain and reduced milk production by a group of cattle. However, rapid BW gain was not considered the cause of depressed milk production per se, as body fatness at breeding was a better indicator of impaired mammary development in Holstein heifers. Neither breeding fat thickness nor BCS were measured in this experiment. Lammers and Heinrichs (2000) reported that increasing the ratio of protein to energy in prepubertal Holstein heifer diets decreased BCS and increased mammary development. Neither weaning age (P=0.99) nor supplemental feed CP concentration (P=0.68) influenced calving ease (Table 5). Heifers fed the 17% CP diet tended (P=0.08) to give birth to heavier BW calves. Body weight and ADG at 83 d of age were not influenced (P>0.15) by dam weaning age or supplemental feed CP concentration. Calf performance differences were not expected, as weaning BW and calf

ADG are highly correlated to milk production (Totusek et al., 1973). Weaning age and supplemental feed CP concentration did not influence (P>0.15) the performance of heifers remaining in the herd for a second lactation (Table 6). Additionally, performance of the second calf was not influenced (P>0.15) by weaning age or supplemental feed CP concentration of the dam.

Implications Providing supplemental feed in a creep feeder to EW replacement heifers can be utilized as a management practice to reduce labor required for EW heifer development. Increasing the supplemental feed protein concentration improved performance of EW d heifers and depressed performance of NW heifers while grazing pasture. Increased protein intake caused by normal weaning management improved performance during the summer grazing period of heifers consuming a 12% CP diet. Weaning age did not influence heifer performance during the summer grazing period when heif-

ers were fed a 17% CP diet. Early weaning management increased the percentage of heifers pubertal by 8 mo of age despite lighter BW. Weaning age and supplemental feed protein concentration did not influence long-term heifer performance or milk production.

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Supplemental Feed and Weaning Age Affects Beef Heifers

after weaning on the development of replacement beef heifers. J. Anim. Sci. 56:280. Cremin, J. D., Jr., D. B. Faulkner, N. R. Merchen, G. C. Fahey, Jr., R. L. Fernando, and C. L. Willms. 1991. Digestion criteria in nursing beef calves supplemented with limited levels of protein and energy. J. Anim. Sci. 69:1322. Day, M. L., J. E. Huston, and D. E. Grum. 2001. Early weaning, puberty and cow reproduction. J. Anim. Sci. 79(Suppl. 2):44. (Abs.)

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