Effect of constant and of changing photoperiods on age at first egg and related traits in pullets

1. The effects of constant photoperiods and of single (5 h) changes in photoperiod applied at 12 or 17 weeks of age upon age at first egg (AFE) were studied using ISA Brown and Shaver 288 pullets.

2. Birds reared from 2 d of age until after maturity on constant 10 h photoperiods matured 8 d earlier than birds reared on constant 8 h and 5 d earlier than the average for 13 or 18 h photoperiods.

3. A single increment in photoperiod from 8 to 13 h advanced AFE by 23 d (compared to 8 h constant day controls) when applied at 84 d, but by only 6 d when given at 119 d. An increase in photoperiod from 13 to 18 h advanced AFE by only 4 d, averaged across breeds and age at increase. A reduction in photoperiod from 13 to 8 h delayed AFE by 22 d when given at 84 d and by 16 d at 119 d. A similar 5 h reduction in photoperiod, but from 18 to 13 h, retarded maturity by 11 d in ISA Brown pullets, but only when given at 84 d, and delayed AFE in Shaver 288 by 12 d, but only when given at 119 d. This interaction may be partly explained by the different physiological stages reached by the two breeds when the photoperiod was changed.

4. Under constant daylengths cumulative food intake before first egg was positively correlated with photoperiod, but the early AFE for birds on 10 h photoperiods resulted in this group having the lowest cumulative food intake to first egg.

5. A 5 h increase in photoperiod at 84 d significantly reduced the food consumed to first egg, but had no effect when given at 119 d. A 5 h decrease in photoperiod generally increased the food consumed to first egg, but the effect was only significant when the daylength was reduced from 13 to 8 h at 119 d. Food intake to first egg in birds subjected to a change in photoperiod was highly correlated with AFE.

6. The data confirm that sexual development in growing pullets responds more to changes in photoperiod than to the absolute daylength, that changes made at different daylengths are not equivalent and that sensitivity changes with age.     

Effect of exogenous oestradiol and lighting regime on age at first egg in domestic pullets

1. Groups of ISA Brown pullets were transferred from 8- to 16-h photoperiods at 34, 44 or 54 d. In each group, 12 birds were injected on alternate days over a 12-d period starting 6 d before the change in photoperiod with beta-oestradiol-3-benzoate (1 mg/kg body weight) or with arachis oil vehicle (controls). Short-day controls were similarly injected from 28 to 40 d. Long-day (16 h) controls were also included in the trial but were not injected. Age at first egg (AFE) was recorded and plasma luteinising hormone (LH) concentrations were measured around the time of oestradiol treatment. 2. Mean AFE for birds photostimulated at 34 d was not significantly different from short-day controls. Birds photostimulated at 44 and 54 d matured at similar ages but 3 weeks earlier than short-day controls (P<0.05). 3. There was a tendency for oestradiol to advance AFE for birds photostimulated at 34 d (P=0.15) but to delay AFE following photostimulation at 44 d (P=0.23). Oestradiol significantly delayed AFE for the birds photostimulated at 54 d (P=0.01). 4. Plasma LH levels during 6 d of oestradiol injection but before transfer from 8- to 16-h photoperiods tended to fall between 28 and 34 d, were relatively constant between 38 and 44 d, but declined significantly between 48 and 54 d. Following photostimulation at 34 d, increases in plasma LH levels for oestradiol-injected birds were significantly greater than for controls. Oestradiol treatment had no significant effect on changes in plasma LH concentrations after photostimulation at 44 or 54 d. 5. This trial confirms previous work showing that pullets are unresponsive to photostimulation before 6 weeks of age. It also demonstrates that raising circulating oestrogen levels by injecting 0.5 mg/kg oestradiol benzoate on alternate days enhances the LH response to photostimulation at 34-d, but only very slightly sensitises a 34-d old bird to an increase in photoperiod which, 10 d later, is capable of advancing AFE in control birds by 24 d. Increased circulating oestrogen might be a factor which allows pullets to advance AFE in response to an increase in daylength.     

Effect of one or two pre-pubertal long days on age at first egg in domestic pullets

1. Two trials were conducted, using 288 brown-egg hybrid pullets in each, to determine the effect on age at first egg (AFE) of exposure to one or two 'long days' during the rearing period. In the first trial, birds were given a single 'long day' of 10, 12 or 14 h at 75, 89 or 103 d of age, with controls maintained on 8-h photoperiods. All treatment groups were transferred to cages at 110 d, and half the birds from each treatment combination given a 6-h increment in photoperiod at 116 d, with the remainder held on 8-h photoperiods. In the other trial, birds were given one or two long days of 14 or 16 h at 96, 107, 117 or 128 d of age; controls were again held on 8-h photoperiods. All groups were moved to cages at 130 d but maintained on 8-h photoperiods. 2. AFE was not significantly affected by one or two pre-pubertal long days, irrespective of when the long day was given or its length. AFE was advanced by 2 to 3 weeks following a transfer from 8 to 14 h at 116 d, independently of whether or not the birds had received a previous long day exposure. 3. It appears that a full-fed egg-type hybrid requires more than two cycles of long days to initiate rapid gonadal development, and that exposure to a single long day during the rearing period will have minimal effect on the timing of sexual maturation and no influence on the response to a subsequent permanent transfer to long days.     

Effect of constant and of changing photoperiod on plasma LH and FSH concentrations and age at first egg in layer strains of domestic pullets

1. ISA Brown pullets were transferred from 8 to 14 h or from 14 to 8 h photoperiods at 35 or 56 d of age. Controls were maintained on constant 8 or 14 h photoperiods from day 1. 2. Blood samples were obtained immediately before each daylength change and subsequently at 7 d intervals until 1st egg in the treated groups and at 70 d of age and then at 14 d intervals until 1st egg in the constant photoperiod controls. Plasma luteinising hormone (LH) and follicle stimulating hormone (FSH) concentrations were determined using homologous radioimmunoassays. 3. Prior to 16 weeks, LH was consistently higher in birds on constant 14 h photoperiods than in those on constant 8 h, but was down-regulated as birds approached maturity so that LH concentrations in the 2 groups were similar during the final 10 d before the first egg was laid. FSH concentrations rose steadily with age but with a tendency for concentrations to be higher in the 8 h than in the 14 h treatment. Birds on constant 8 h daylengths matured 18.3 d later than those on constant 14 h photoperiods. 4. A 6 h increment in photoperiod given at 35 d or 56 d, resulted in an increase in LH within 7 d in both cases. FSH concentration did not respond to an increase in photoperiod at 35 d but rose following the same increase at 56 d. This was associated with a 3-week advance in sexual maturity, whilst age at 1st egg in birds photostimulated at 35 d was similar to the age with a constant 14 h photoperiod. 5. LH concentration fell when photoperiod was reduced from 14 to 8 h at either 35 or 56 d and remained below the constant 8 h controls for many weeks before rising to a concentration not significantly different from other groups in the final 10 d before 1st egg. FSH concentrations in birds exposed to a decreased daylength at 35 d, although more oscillatory, were similar to the constant 8 h photoperiod controls. In birds exposed to the same decrease at 56 d, FSH concentration initially tumbled but was similar in the 2 groups during the latter stages of rearing; neither differed significantly from the constant daylength controls during the 60 d before 1st egg. Sexual maturity in both groups given a reduction in photoperiod was delayed by about 2 weeks compared with constant 8 h controls. 6. Change in FSH concentration following an increase in daylength was a better predictor of age at 1st egg than change in LH. However, FSH concentrations after 14 weeks of age were rather similar in short day and long day controls and in the 2 groups given reductions in photoperiod at 35 d and 56 d, despite differences of nearly 5 weeks in mean age at 1st egg amongst these 4 treatments.     

Effect of age of release from light or food restriction on age at sexual maturity and egg production of laying pullets

1. Lohmann Brown pullets, in one trial, and Hyline Brown pullets in another, were reared from day 2 on short daylengths, and from week 8 in trial 1 (week 16 in trial 2) on food restriction. These restrictions were lifted at various times during the rearing period as a means of determining the relative importance of the day length and food restriction stimuli on the attainment of sexual maturity and subsequent laying performance. 2. A total of 2304 pullets were used in each trial. The birds were reared in light proof rooms, and subjected to 8L:16D until they were moved to a laying facility where a light stimulus of 16L:8D was applied. In trial 1 the six ages at which light stimulation was applied were 115, 122, 129, 136, 143 and 171 d. Within each light treatment, food restriction of pullets, which consisted of feeding 72 g of food/bird d, was lifted at six different ages, namely, 115, 129, 143, 157, 171 and 185 d. In trial 2 both the light stimulation and the lifting of food restriction occured at 111, 125, 139, 153, 167 and 181 d of age, producing 6x6=36 treatments in both trials. 3. The first trial was terminated when the pullets were 28 weeks old, soon after all the birds had commenced laying, because of an outbreak of Egg Drop Syndrome. However, because age at maturity was the variable of major interest, data from this experiment could be used in the analysis. The second trial ended when the birds reached 40 weeks of age. Variables measured were age at maturity, food intake and body weight gain subsequent to the lifting of restrictions and, in the second experiment, rate of lay, peak rate of lay and egg weight at various ages. 4. The mean age at sexual maturity was influenced by the date of release from light restriction (P<0.001) and from food restriction (P<0.001) in both trials. In addition, the interaction between the age at release from light and from food restriction was significant. Regression equations were produced for each trial to describe the relationships between the age at sexual maturity and the age at release from light restriction and food restriction. 5. There was an effect of both light restriction (P<0.001) and of food restriction (P<0.001) on the increase in food intake (g/bird d) in the week following release from food restriction in both experiments. These effects were not independent: the effect of the interaction of light and food restriction on this increase in food intake was also highly significant. The longer the birds were subjected to light restriction, the less dramatic the increase in food intake when food restriction was lifted. The more sustained the period of food restriction, the higher the increase in food intake in the week following release from the restriction. 6. Mean egg weight was 4 g heavier at 22 weeks of age in birds released from food restriction at 16 and 18 weeks, than from those released at 24 and 26 weeks of age. However, by 30 weeks of age, birds restricted for longer produced heavier eggs than their earlier-maturing counterparts. This effect continued to the end of the trial at 40 weeks of age, at which time there was a 2.3 g difference in egg weight between these treatments. 7. Both light and food restriction have an effect on the age of maturity in laying hens. The length of time between the release from light or from food restriction to the onset of laying depended on the age of the pullets when the release occurred. Egg weight at a given age was significantly affected by the age at release from food restriction, but not from light restriction.     

Effect of timing and size of photoperiod change on plasma FSH concentration and the correlation between FSH and age at first egg in pullets.

1. ISA Brown pullets were transferred at 6, 9, 12, 15, 18 or 20.3 weeks of age from an 8 h photoperiod to an 8, 10, 13 or 16 h photoperiod. Plasma follicle stimulating hormone (FSH) concentration was measured at transfer at 7 and 14 d afterwards, and age at first egg (AFE) was recorded. 2. Plasma FSH concentration in pullets reared on constant 8 h photoperiods generally increased with age but with a trough at 12 weeks. Plasma FSH increased during the first 14 d of photostimulation to a significantly higher concentration, compared with constant 8 h controls, when the photoperiod was increased to 13 or 16 h at 9, 12 or 15 weeks; but for the increase from 8 h to 10 h photoperiods FSH was only significantly higher than controls when the change was made at 12 weeks. 3. The change in plasma FSH concentration 14 d after photostimulation was significantly correlated with mean AFE (reported in Lewis et al., 1997) and appears to be a better predictor of gonadal development than concurrent changes in plasma LH concentration previously reported (Lewis et al., 1994).     

Changes in light intensity can influence age at sexual maturity in domestic pullets

1. Shaver White and ISA Brown pullets were reared to 140 d in groups of 8 in cages on a 10-h photoperiod of incandescent light and maintained at an illuminance of 3 or 25 lux, or transferred from 3 to 25 lux or from 25 to 3 lux at 63 or 112 d of age. 2. There was no significant difference in sexual maturity, measured as eggs per 100 bird.d at 139 and 140 d, for ISA Brown maintained on 3 or 25 lux, but Shaver White pullets exposed to constant 3 lux matured significantly later than those maintained on 25 lux. 3. In Shaver Whites, sexual maturity was significantly delayed by an increase from 3 to 25 lux at 63 and 112 d, and advanced by a decrease from 25 to 3 lux at 112 d. Sexual maturity of ISA Browns was not significantly affected by a change in illuminance at 63 or 112 d, though responses were in the same direction as for Shaver Whites. 4. In both breeds, total feed consumed to 112 d was higher for birds on 3 lux than 25 lux, but lower between 112 d and 140 d when birds on 25 lux underwent rapid sexual development. In both breeds, body weight at 63 d was higher for birds exposed to 3 lux than 25 lux, but body weight gain thereafter was similar for the two light intensities. 5. In both breeds, plasma luteinising hormone (LH) concentration at 63 and 112 d was lower in birds maintained on 3 lux than 25 lux. At 63 and 112 d, transfers from 25 to 3 lux depressed, whereas transfers from 3 to 25 lux at 63 d, but not at 112 d, increased plasma LH. 6. Advances or delays in sexual maturity induced by changes in illuminance were not correlated with differences in feed intake, body weight gain, or with changes in plasma LH. 7. One possible explanation for the inverse relationship between the direction of change in illuminance at 63 and 112 d in pullets exposed to a 10-h photoperiod and the age at which they became sexually mature is that changes in light intensity and/or spectral composition affect the entrainment of the circadian rhythm of photoinducibility, to effect a phase shift in the photoinducible phase and/or the responsiveness of phototransduction pathways.     

Light intensity can influence plasma FSH and age at sexual maturity in domestic pullets

1. Shaver White and ISA Brown pullets were reared to 140 d in cage groups of 8 on a 10-h photoperiod of incandescent light and maintained at an illuminance of 3 or 25 lux, or transferred from 3 to 25 lux or from 25 to 3 lux at 63 or 112 d of age. 2. Plasma follicle stimulating hormone (FSH) concentration at 63 and 112 d was higher in both breeds for pullets maintained at an illuminance of 25 lux compared with 3 lux. After 2-4 d, and relative to constant-illuminance controls, plasma FSH increased significantly for ISA Brown transferred from 3 to 25 lux at 63 d and for Shaver White transferred at 112 d. Irrespective of genotype, plasma FSH for pullets given a decrease in illuminance at 63 or 112 d showed a tendency for less change than did constant-illuminance controls. 3. There was no significant difference in sexual maturity for ISA Brown maintained on 3 or 25 lux, but Shaver White pullets exposed to constant 3 lux matured later than those maintained on 25 lux. Shaver White matured later following an increase from 3 to 25 lux at 63 and 112 d, and earlier subsequent to a decrease from 25 to 3 lux at 112 d. ISA Brown pullets were not significantly affected by a change in illuminance at 63 or 112 d, though their responses were in the same direction as Shaver White. 4. Changes in plasma FSH in the 2- to 4-d period following a change in illuminance at 63 or 112 d were not significantly correlated with sexual maturity.     

Effect of 5 hour increases in photoperiod and in feeding opportunity on age at first egg

1. ISA Brown pullets were given an 8‐h photoperiod and fed ad libitum to 63 d of age. At 63 d the photoperiod was either kept at 8 h or increased to 13 h, and the photostimulated birds were subjected to 1 of 3 feeding systems: ad libitum, 8 h daily access to food or a daily individual allocation of food equal to that given to the 8 h control group.

2. Mean age at first egg (AFE) of the groups given the photoperiod increase was on average 33 d earlier than that of the 8 h controls. Within the photostimulated groups, limiting daily feeding opportunity to 8 h delayed maturity by 4 d compared with ad libitum feeding. The mean AFE of the birds which were given allocated quantities of food was intermediate and not significantly different from either of the other groups.

3. Light was the principal factor which determined AFE, but moderate food restriction had a small modifying influence, consistent with earlier evidence.

4. The 3 groups given a 5‐h increase in photoperiod consumed similar quantities of food to first egg, which was laid around 15 weeks of age. The 8 h control group ate a similar amount of food to this age, but needed more than 40% extra food to reach their first egg at 20 weeks.     

Effect of age and body weight at photostimulation on the sexual maturation of broiler breeder pullets transferred from 8L:16D to 16L:8D

1. In two trials, broiler breeders were grown to a mean body weight of 2.0 to 2.2 kg at 20 weeks on 8L:16D and transferred to 16L:8D at various ages between 69 and 175 d of age at mean body weights varying between 1.0 and 2.7 kg. Other groups were grown to have a mean 20-week body weight 0.87, 1.15, 1.25 or 1.37 of the normal birds and photostimulated simultaneously with the normal weight birds. Controls were held on 8L:16D in each weight group. In a third trial, broiler breeders were transferred from 8L:16D to 16L:8D at 45 (ad libitum fed), 75 or 90 d (mild feed control), when the mean body weight of each group had reached 2.1 kg. Controls weighing 2.0 kg at 20 weeks were held on 8L:16D or 16L:8D. 2. In trial 1, photostimulating normal weight birds at 69, 76, 83 or 97 d delayed maturity and widened the spread of individual maturities, whilst transferring to 16L:8D at 111 or 125 d advanced maturity. Increasing the mean 20-week body weight to 2.8 kg advanced maturity in birds transferred to 16L:8D at 97, 111 or 125 d. Birds photostimulated at 69, 76 and 83 d matured at a similar age to controls held on 8L:16D. 3. In trial 2, transferring normal weight birds to 16L:8D at 91, 112, 133, 147, 161 or 175 d advanced maturity. Reducing the 20-week mean body weight to 1.91 kg delayed maturity in birds photostimulated at 91 or 112 d, but advanced it in birds photostimulated at 112, 133, 147, 161 or 175 d. Sexual maturity was similar for birds with a 20-week mean body weight of 2.54 or 2.74 kg, with advances for all groups photostimulated between 91 and 175 d. 4. In trial 3, birds maintained on 16L:8D matured later than birds held on 8L:16D, but maturity was advanced when birds were transferred to 16L:8D at 45, 75 or 90 d at a common body weight of 2.1 kg.     

Growth, intakes and metabolic responses of artificially reared lambs weaned at 14 d of age

In two experiments, a total of 46 Finn Cross and Dorset lambs were artificially group-reared. Thirty eight were weaned abruptly at 14 d of age from a commercial milk replacer (MR) to a dry starter diet (SD). Lambs were self-fed cool (10 to 15 C) MR reconstituted to 25% dry matter (DM). The ground (2-mm screen) SD averaged 26.2% crude protein and 7.4% fat (DM basis). For both experiments, birth and weaning weights averaged 3.8 and 8.0 kg, respectively. Experiment 1 tested a strategy for encouraging postweaning DM intake. Fifteen lambs received MR reconstituted to 33% DM from d 11 to 14, and 15 lambs received standard 25% DM MR. Between d 14 and 15, intakes of DM, gross energy, crude protein and water dropped 86, 89, 85 and 64%, respectively. Lambs doubled their birth weights during the milk-feeding period and consumed 1.41 kg SD between d 14 and 25. The MR reconstitution rate did not affect weaning weight, postweaning SD or water intake, or growth check (GC, P greater than .10). Postweaning GC averaged 12.2 d and was not influenced (P greater than .10) by birth weight, sex or weaning weight. Mortality and disease rates under these conditions were negligible. Experiment 2 was designed to differentiate between the dual effects at weaning of altering the type of diet and of reducing the level of nutrient intake. Eight lambs were weaned to SD at 14 d, and eight lambs were bottle-fed isocaloric levels of MR from d 12 to 30. At weaning, plasma glucose concentration declined 1.4 mM from 6.7 mM due to fasting and an additional 1.0 mM due to the change of the type of diet (P less than .01). Plasma acetate and urea N concentrations rose steadily after d 16 in the SD-weaned lambs, but not in the MR-fed lambs (P less than .01), suggesting that the SD-weaned lambs absorbed ruminal fermentation products. These results indicate that artificially reared lambs may be routinely weaned to a dry diet at 14 d of age. The major alterations in plasma metabolites that occur within 6 to 8 d after abrupt weaning may define the period when these lambs become functional ruminants.     

A model for the effect of constant photoperiods on the rate of sexual maturation in pullets

1. This paper reviews evidence from 15 experiments, reported over a span of 44 years, in which pullets were reared from hatching to sexual maturity on 2 or more constant photoperiods. 2. The evidence strongly indicates that earliest age at first egg (AFE) was observed when pullets were held on constant 10 h days (though earlier maturity is easily induced by increasing the photoperiod during rearing). The pair of equations which best describe the relationship between AFE (y,d) and photoperiod (x,h) are for x < 10 h, y = 175.8- 1.731x; for x > 10 h, y=155.5 + 0.301x. 3. This 2-straight-line model, hinged at 10 h, should be used in preference to curvilinear models published earlier, which wrongly predict that pullets reared on long days (14h to 17h) mature faster than birds reared on constant 10 h.     

Effect of size and timing of photoperiod increase on age at first egg and subsequent performance of two breeds of laying hen

1. ISA Brown and Shaver 238 pullets were changed from 8 h to 8, 10, 13 or 16 h photoperiods at 42, 63, 84, 105, 126 or 142 d of age.

2. Age at first egg (AFE) was curvilinearly affected by the size and timing of the change in photoperiod. AFE was advanced most by a photoperiod change from 8 to 13 h made at 63 or 84 d. ISA birds were generally more responsive than Shaver to the photoperiod changes.

3. Longer photoperiods significandy increased survivors' egg production, but decreased liveability to 504 d, so that eggs per hen housed were unaffected. Retarding AFE by 10 d reduced survivors' egg numbers by 7.0, but increased mean egg weight by 1.26 g. Egg output by Shaver birds was unaffected by AFE, but that of ISA was curvilinearly affected, with an apogee at an AFE of 135 d. In both breeds, egg weight and egg output were greater following an early or late, rather than a mid‐term photostimulation.

4. Photoperiod significandy increased mean daily food intake during lay by 1.26 g/h. A 10 d retardation in AFE resulted in a reduction in food intake of 1 g/d. Efficiency of food conversion deteriorated according to the square of the photoperiod, and changed curvilinearly according to age at photostimulation. Food conversion efficiency improved by 0.05 g/g for each 10 d delay in AFE.

5. Shell quality was unaffected by AFE, but deteriorated with increasing photoperiod and was curvilinearly affected by age at photostimulation with the smallest shell weights associated with photostimulation at 63 d. The incidence of double‐yolked (DY) egg production increased with photoperiod and decreased with delayed photostimulation. There was an exponential regression of DY eggs on AFE.

6. Body weight at first egg increased by 75 g/d delay in AFE, but body weight at 504 d of age was unaffected by AFE, photoperiod or age at photostimulation. Body weight gain during lay increased by 15 g/h increase in photoperiod, decreased by 6 g per 10 d delay in photostimulation and by 40 g per 10 d delay in AFE. Fat content at 504 d increased by about 10 g/kg and by 23 g/bird for each 10 d delay in AFE.

7. Mortality in lay increased by 0.8%/h increase in photoperiod, but was unaffected by either age at photostimulation or AFE.     

光照制度对蛋鸡开产日龄和生产性能的影响研究

对育成期罗曼商品褐壳蛋鸡提供10～12 h光照时间和10 Lx的光照强度,后备母鸡提前5～8 d开产;45周龄之前,产蛋量和产蛋数均高于8 h、10 Lx光照;64周龄时产蛋量、产蛋数各组间无差异,但平均蛋重下降;至72周龄时产蛋量、平均蛋重和饲料利用率均下降,并呈现明显差异(P＜0.05).     

The influence of pelleted feed on the response of growing pullets to photoperiods of less than ten hours.

1. An experiment was designed to test the hypothesis that delayed sexual maturity in pullets reared on very short (4 h) constant photoperiods might be partly attributable to limitation of food intake and that offering a pelleted feed might circumvent this effect. 2. The factors investigated were 2 strains (Amber Link and Hyline Brown), 3 photoperiods (4, 7 and 10 h) and 2 forms of food (mash throughout rearing or crumbs from 0 to 4 weeks followed by pellets). All 12 combinations of these factors were tested with 14 replications of 18 pullet chicks allocated to each combination. 3. Mean ages at first egg for 4, 7 and 10 h rearing photoperiods were 189, 184 and 162 d respectively. Pullets given the pelleted diet ate 2% less food to 20 weeks but were 6% heavier at that age. However, the pellet-fed birds were 6 d later in mean age at 50% lay. There was no interaction between form of food and photoperiod in the data for age at first egg. 4. It is concluded that constant short photoperiods during rearing cause delayed sexual maturity entirely due to the effect of light on gonadal development and that limitation of food intake is not a factor in this response.     

Effect of calflhood morbidity on age at first calving in New York Holstein herds

The effect of calhood morbidity on age at first calving was investigated in 948 heifer calves in 21 herds in the vicinity of Cornell University. Heifers were born from July 1983 to April 1985 and neither died nor were sold prior to 90 days of age. All calvings occurred on or before 31 May 1987.

The specific calfhood morbidity variables of interest were the first occurrences of the following clinical signs as diagnosed by farmers on check-off forms: scours/diarrhea within 14 days of birth and from 15 to 90 days of age; dull, listless, droopy ears, or off of feed within 90 days of birth; cough, runny nose or eyes, or trouble with breathing within 90 days of birth. The Kaplan-Meier product-limit life-table technique and Cox's proportional hazards model were used to evaluate the effect of calfhood morbidity on the age distribution of first calving. Season and year of birth were included (forced) into the Cox's proportional hazards model as possible confounding variables. The model was stratified on herd (allowing a different survival function for each herd) in order to account for herd effects.

The final Cox's proportional hazards model included season and year of birth, signs of respiratory illness within 90 days of birth and dullness within 90 days of birth. Heifers without respiratory illness as calves were twice as likely subsequently to calve and calved 6 months earlier when compared to those with respiratory illness as calves. Heifers with dullness as calves were 1.6 × more likely to calve and calved 2 months earlier when compared to calves without dullness as calves.