Effects of early versus late spring calving on beef cow-calf productivity

Transcription

1 Effects of early versus late spring calving on beef cow-calf productivity H. Pang 1, 2, M. Makarechian 1, L. A. Goonewardene 2, and R. T. Berg 1 1 Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2P5; 2 Alberta Agriculture, Food and Rural Development, Edmonton, Alberta, Canada T6H 5T6. Received 18 November 1997, accepted 13 May Pang, H., Makarechian, M., Goonewardene, L. A. and Berg, R. T Effects of early versus late calving on beef cow-calf productivity. Can. J. Anim. Sci. 78: Early (April) vs. late (May/June) calving was compared over 3 yr at the University of Alberta Ranch at Kinsella, Alberta, Canada. Three synthetic lines, two of which were predominantly beef breeds (BS1 and BS2) and one with approximately 60% dairy breeding (DS), were used and 995 records were analyzed. Cows from each line were randomly allotted to the early or late breeding schedules at the time of breeding. The same bulls bred cows in their own line, beginning in June and in August for two 6-wk periods. Calving season had no significant effect (P > 0.05) on pregnancy rate (%), calving assistance, calving and weaning rate (%), adjusted off-test weight or post-weaning ADG, suggesting that late calving can be a viable management option. Birth weights of all calves were significantly lower in the early group than in the late group (38.9 vs kg, P < 0.01). In the BS1 and BS2 lines, early calves had higher adjusted pre-weaning ADG: BS1 (1.18 vs kg d 1, P < 0.01), BS2 (1.12 vs kg d 1, P < 0.01) but the differences in the DS line were not significant (P > 0.05). The early calves were older at weaning and therefore were expected to have higher pre-weaning ADG. The fact that the late-born calves in the DS line equalled the early calves in pre-weaning ADG indicates that they were growing relatively faster. Late calving DS heifers had significantly higher weights (P < 0.01) at breeding in June, higher ADG to breeding, and cows had higher body condition scores at calving and weaning (P < 0.01), suggesting that DS cattle benefited more by late calving. The late-calving management system appears to be a viable option for cow-calf production in Alberta. Key words: Calving season, reproductive traits, weight and growth, beef cattle Pang, H., Makerechian, M., Goonewardene, L. A. et Berg, R. T Effets comparatifs de la précocité du vêlage de printemps sur la productivité des élevages de veau d embouche. Can. J. Anim. Sci. 78: Nous avons comparé pendant trois ans les effets du vêlage précoce (avril) et du vêlage tardif (mai-juin) au ranch de l Université de l Alberta à Kinsella en Alberta (Canada). Nous utilisions pour cette analyse 995 relevés de production concernant trois souches synthétiques, dont deux à dominance bouchère (BS1 et BS2) et une à 60 % d ascendance laitière (AL). Les vaches de chaque souche étaient assignées au hasard à un protocole de mise à la reproduction, précoce ou tardif. Les mêmes taureaux étaient utilisés sur les vaches de même souche, à partir de juin ou d août, chaque fois pour une durée de six semaines. La saison de vêlage n avait pas d effets significatifs (P < 0,05) sur le taux de gravidité (%), sur l assistance requise au vêlage, sur le taux de vêlage et de sevrage (%), ni sur le poids corrigé en fin de test ou sur le GMQ en post-sevrage, ce qui porte à considérer le vêlage tardif comme une option de gestion valable. Le poids à la mise bas de tous les veaux était significativement plus bas en vêlage précoce qu en vêlage tardif, soit 38,9 contre 41,2 kg (P < 0,01). Dans les souches BS1 et BS2, les veaux du régime de vêlage précoce affichaient un GMQ corrigé en présevrage meilleur, soit, pour BS1, 1,18 contre 1,11 kg j 1 et pour BS 2, 1,12 contre 1,05 kg j 1, valeurs toutes deux significatives à P < 0,01. La souche AL ne manifestait pas de différences significatives à cet égard. Les veaux du régime précoce étaient plus âgés au sevrage et devaient forcément avoir eu un GMQ plus élevé jusqu à ce stade. Le fait que les veaux nés en fin de printemps dans la souche AL avaient un GMQ en présevrage égal à celui des veaux nés en avril laisse supposer qu ils croissaient relativement plus vite. Les génisses du régime vêlage tardif étaient significativement plus lourdes (P < 0,01) à leur mise à la reproduction en juin, leur GMQ jusqu à cette étape était plus élevé et les vaches adultes affichaient un meilleur état corporel au vêlage et au sevrage (P < 0,1), autant de signes suggérant que la souche AL profitait davantage d une conduite en vêlage tardif. Ces observations viennent donc conforter la validité de cette option pour la production de veaux d embouche en Alberta. Mots clés: Saison de vêlage, caractères de reproduction, poids et croissance, bovins de boucherie 249 In recent years, calving season on farms in Alberta, has been gradually moved into early spring and late winter. Early calving is often preferred because it affords a more favourable labour distribution by avoiding a conflict with spring farming operations. Calves born early in the calving season weigh more at weaning because they are older than late-season calves (Sellers et al. 1970; Lesmeister et al. 1973; Keller and Brinks 1978; Deutscher et al. 1991). Cows calving early may have a greater chance of achieving higher pregnancy rates when the breeding season is longer (Burris and Priode 1958; Warnick et al. 1967). Body condition of cows at calving and energy intake (nutrition) are also known to influence subsequent reproductive performance (Selk et al. 1988). It is therefore necessary to consider a number of factors when deciding on an appropriate calving season system. In Alberta, early-calving cows may be exposed to extreme cold during and after calving, thereby increasing risks (Berg et al. 1991). Abbreviations: ADG, average daily gain; BCS, body condition score; BS1, Beef Synthetic 1; BS2, Beef Synthetic 2; CA, calving assistance; DS, Dairy Synthetic

2 250 CANADIAN JOURNAL OF ANIMAL SCIENCE There is limited information on the effects of the month of calving on cow productivity (Bellido et al. 1981; Bagley et al. 1987). Therefore, a 3-yr study was undertaken to compare effects of early (April) and late (May/June) calving under range conditions in east-central Alberta. The objectives of this study were to examine the effects of calving season on reproductive performance, calving pattern, calf growth rate (pre- and post-weaning), heifer growth rate, cow weight and body condition score (BCS) in three synthetic lines of range beef cattle. MATERIALS AND METHODS Animals Data generated from 995 cows and heifers, calving during the period from 1991 to 1993, at the University of Alberta Ranch at Kinsella, Alberta were used. They represented three synthetic lines: Beef Synthetic 1 (BS1), Beef Synthetic 2 (BS2) and Dairy Synthetic (DS). The breed composition of the synthetic lines and management practices were described in earlier reports (Berg et al.1990; Pang et al. 1995). Briefly, the BS1 was composed of 33% each of Angus and Charolais, 20% Galloway, with the remainder of other beef breeds. The BS2 was made up of approximately 60% Hereford and 40% other beef breeds. The DS was composed of 60% dairy breeds (Holstein, Brown Swiss and Simmental) and 40% beef breeds. In 1991 only two lines, BS1 and DS, were represented while in 1992 and 1993 all three lines were represented. All animals used for the study were managed according to the Canadian Council on Animal Care guidelines (1984). Management and Climate The Kinsella ranch is located in East Central Alberta and the annual precipitation is around 48 cm. The total precipitation for the months of March, April, May and June was 18.5, 45.0, 54.6 and 92.3 mm, respectively, in 1991; 20.0, 26.5, 81.4 and 19.3 mm, respectively in 1992; and 14.0, 54.0, 35.4 and 54.6 mm, respectively, in Winters in this part of Alberta persist to the end of March, and April snowstorms often occur. The minimum and maximum temperatures at Kinsella for the months of March, April, May and June were 11.0 and 1.4 C, 0.5 and 13.1 C, 4.2 and 17.3 C, and 8.8 and 20.2 C, respectively, in 1991; 3.4 and 6.9 C, 0.0 and 11.8 C, 3.9 and 15.1 C, and 9.3 and 21.8 C, respectively, in 1992; and 6.9 and 2.9 C, 1.5 and 9.6 C, 4.4 and 18.6 C, and 6.9 and 20.2 C, respectively, in The three lines and calving groups were maintained and managed similarly, and were subjected to the same selection program. Selection of sires within each line was based primarily on pre- and post-weaning gain. Heifers and cows failing to wean a calf in any year were culled. The breeding herds grazed a mixture of native rough fescue and improved pastures year round and were supplemented with an alfalfabrome hay and green feed (oats) during the winter. Cows were wintered on open range pastures which had poplar bluff shelters and which were either minimally grazed or not grazed in the previous summer. Winter supplementation usually began in December and continued through to March. Table 1. Calving dates by year and calving group Calving date Calving period Year Group n Mean Beg. End (d) 1991 Early z Late Early Late Early Late Overall Early Late z Calving date expressed as Julian dates, January 1=1, etc. Straw bedding was provided for cows and heifers in sheltered areas on the range. Cows calved on the open range and heifers calved out in separate easily accessible pastures. Calves were weaned in September or October and following a 28-d adjustment period, all males were placed on a standard feed test. Male calves were not castrated, and were on the performance test for approximately 140 d and fed a high-energy feedlot diet (Berg et al. 1990). After an adjustment period of 3 wk following weaning, heifer calves were fed 2.27 kg roughage (grass and alfalfa hay) and 2.27 kg grain and a mineral supplement daily until the following April when they were transferred to pasture. These heifer calves also consumed some of the straw supplied as bedding and gained 0.3 kg d 1 while in dry lot. Within each line, age group and year, cows were randomly allotted to one of the two calving season treatments. Breeding in the early group began on 21 June to commence calving on approximately 1 April Breeding in the late group, began on 4 August to commence calving on approximately 15 May (Table 1). The breeding season continued for approximately 6 wk. On the day following termination of the early breeding season, the same bulls were assigned to the late group of cows for a further 6 wk. In all cases, cows were exposed only to bulls of their own line. Pregnancy was diagnosed at 4 mo after breeding by rectal palpation. The calves in the two groups were weaned on the same day each year, 5 September, 8 September and 25 October in 1991, 1992 and 1993, respectively. Data collected and/or calculated included pregnancy rate, calving date and distribution, calving difficulty, calving rate, calf birth weight, weaning rate, weaning weight, calf pre-weaning ADG, off-test weight, post-weaning ADG, heifer weight at breeding, heifer ADG from birth to breeding, cow BCS at calving and weaning, and cow weight at weaning. Pregnancy rate, calving rate and distribution, and weaning rate were based on the number of cows exposed to breeding. Calving difficulty was scored from 0 to 5 where, 0 = no assistance, and the assisted scores were, 1 = light pull, 2 = hard pull, 3 = calf puller used, 4 = veterinary assistance and 5 = Caesarean section (Naazie et al. 1989). The Canadian five score system was used to evaluate the BCS.

3 PANG ET AL. EFFECT OF CALVING SEASON ON LOW CALF PRODUCTIVITY 251 Fig. 1. Calving distribution for all cattle from 1991 to Fig. 2. Cumulative calving frequency for all cattle, Generally, a cow with a BCS of 2 is considered to be thin, whereas a cow with a BCS of 4 is fat. A moderate condition score of 3 (5 in the United States) is considered close to ideal. Statistical Analyses Calving assistance for heifers and cows were analyzed separately by chi-square tests with females classified as unassisted (score of 0) or assisted. The data on pregnancy rate, calving rate, weaning rate and calving distribution (expressed as a percentage) were analyzed using a chisquare test and the CATMOD procedure of the SAS Institute, Inc. (1990). Other data were analyzed using the General Linear Model (GLM) procedure of the SAS Institute, Inc. (1990). Fixed effects of year, line, calving season group, sex of calf, age of dam, and interactions of year x season and breed x season were included in the model. Calf pre-weaning ADG, cow weight and BCS at weaning were adjusted for calf weaning age by the analysis of covariance. Calf post-weaning ADG and off-test weight were adjusted for calf age off-test. Heifers born in the treatment groups and subsequently used for breeding were evaluated for heifer weight at breeding and heifer ADG from birth to breeding, which were adjusted for heifer age at breeding (June). Calving dates were expressed as Julian dates: 1 January = 1, 10 April = 100 etc. RESULTS AND DISCUSSION Cows in the early and late groups received 1270 kg and 1261 kg of hay and 809 kg and 772 kg of green feed respectively, cow 1 yr 1. Although the amounts differed for the two calving season groups in different months, the total winter supplementary feed consumption for the two groups was similar. Reproductive Performance The 3-yr average calving date for the early group was day 105 (15 April in non-leap years), and for the late group was day 148 (28 May). The average calving period (time between the first and last calf) for the early group was longer than that of the late group, 53 vs. 47 d, respectively (Table 1). The tighter calving period can be considered an advantage for the late-calving group. Overall, average calving frequency and cumulative frequency at 5-d intervals during the calving season are presented in Figs. 1 and 2. The general distributions of calving patterns were similar in both early and late groups, but a lower percentage of calves was born in the first 21 d of the season in the early group than in the late group (51.5 vs. 61.8%, respectively). Morris (1984) studying calving distributions in Angus and Hereford cows in New Zealand found that early calving cows were delayed in achieving a successful subsequent conception. Pregnancy rates (%), calving rates (%) and weaning rates (%) by year and line did not differ significantly (P > 0.05) between the two groups 89.2 vs. 91.9, 82.2 vs. 83.2, and 78.3 vs. 78.3% for early and late groups, over the 3-yr, respectively (Table 2). There was a non-significant trend towards higher pregnancy, calving and weaning rates in the late over early-calving DS cows. Bagley et al. (1987) using Angus and Hereford x Angus cows in Louisiana, found similar conception rates in two calving season groups when data were averaged over a 5-yr period. Similar observations were reported by Lesmeister et al. (1973) who found no significant difference in percent calf survival from birth to weaning between early (87%) and late-calving (100%) treatments at one location, but percent calf survival was significantly higher for early calves at a second location. However, Deutscher et al. (1991) reported that in Nebraska herds, cows calving late, compared with those calving early showed a significantly (P < 0.01) higher percentage of calves surviving from birth to weaning. The CA in the herds for heifers and cows was very low, perhaps negating the possibilities of detecting season differences. The calving assistance for heifers was 15 and 10% for early and late groups, respectively (Table 3). The difference

4 252 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 2. Pregnancy, calving and weaning rate by year, line and calving season Year or Rate (%) z line Group n Preg. Calving Weaning 1991 Early Late Early Late Early Late BS1 Early Late BS2 Early Late DS Early Late Overall Early Late z Pregnancy, calving and weaning rates were based on the number of cows bred. The differences between the early and the late groups are not significant (P > 0.05). between early and late groups was not significant (P > 0.05), at 1 and 0.5%, respectively. Calf Weights and Growth Traits Least squares means and standard errors of means for birth weights, adjusted pre-and post-weaning ADG and adjusted off-test weights for the early and late groups by year, line and overall are shown in Table 4. Birth weights of calves, adjusted for sex were significantly lower (P < 0.01) for the early-calving group than for the late group, 38.9 vs kg, respectively. Similar results were reported by Lesmeister et al. (1973) but non-significant differences in birth weights among early- and late-calving groups were reported by Deutscher et al. (1991) and Bagley et al. (1987). The largest difference in birth weights between the early- and late-calving groups in our study occurred in the DS line (41.3 vs kg, respectively). Heavier birth weights in the late group may be a result of the more favourable nutrition from new growth pastures. The differences were greater for the DS cows which had significantly lower body condition scores for the early vs. the late groups as well as greater differences in calf birth weights than for the beef lines (Tables 4 and 5). Overall, calf pre-weaning ADG in the early group was significantly higher (P < 0.01) than in the late group, 1.17 vs kg d 1, respectively. This result is in agreement with the findings of Lesmeister et al. (1973) who reported that older calves show higher gains as they are probably able to utilize pasture to a greater advantage and consume larger quantities of milk produced by the dam at its peak milk production. However, in our study, the interaction between calving season and line was significant (P < 0.01). The difference in pre-weaning ADG between early and late groups was not significant (P > 0.05) for DS, but it was highly significant (P < 0.01) for BS1 and BS2. The similarity in preweaning ADG between the early- and late-weaned DS calves indicates that the late-group calves (being younger) Table 3. Calving assistance (CA) for heifers and cows Number Animal Group Normal CA z χ 2 Prob. Heifers Early Late 19 2 Total 36 5 Cows Early Late Total z The CA category included animals with calving difficulty scores between 1 and 5. were growing relatively faster than the calves in the earlyborn group. The cows in the DS line have been shown to have higher milk yields than the beef lines, resulting in higher calf preweaning average daily gains (Butson et al. 1980). The post-weaning ADG of male calves adjusted for age did not differ (P > 0.05) between the early and late groups (1.57 and 1.56 kg d 1, respectively), indicating that calving season did not influence the growth rate of calves following weaning. Post-weaning off-test weights of male calves adjusted for age were not different (P > 0.05) between the early and late groups (418.5 vs kg, respectively). The calf ages off-test in the early group were higher by design than those in the late group (333 vs. 291 d, respectively). The age adjusted post weaning ADG and off-test weights between early- and late-calving groups in all three lines were similar. Heifer Growth Performance The least squares means and standard errors of the means of the adjusted heifer weights at breeding and adjusted heifer ADG compared for calving season by year, line and overall, are presented in Table 5. The adjusted weights of heifers at breeding in June were not significantly different (P > 0.05) between the early and late groups, vs kg, respectively. The heifer weight at breeding and the heifer ADG to breeding were not different (P > 0.05), similar to the results reported by Rege and Famula (1993). The adjusted heifer ADG from birth to breeding were not significantly different (P > 0.05) between the early and late groups (0.64 vs kg d 1 ) between the two beef lines. However, for DS heifers, body weight at breeding and gain to breeding were significantly (P < 0.01) higher for the heifers in the late group (304.9 vs kg, 0.65 vs kg d 1 ), and the late DS heifers retained their earlier weight advantage. Cow Weight and BCS The least squares means and standard errors of the means for cow BCS at calving and at weaning, and cow weights at weaning are shown in Table 5. The overall cow BCS at calving for the early and late groups were not significantly different (P > 0.05, 2.50 vs. 2.56, respectively). Deutscher et al. (1991) reported similar cow condition scores at pre-calving (February and March), pre-breeding (May and June) and weaning (September and October). However, our data show that the difference in BCS between the early and late cows

5 PANG ET AL. EFFECT OF CALVING SEASON ON LOW CALF PRODUCTIVITY 253 Table 4. Least squares means (LSM) and standard errors (SEM) of calf weights and growth traits (kg) for calving season by year, line and overall. All calves Male calves Year Birth weight Adj. pre-wean. ADG z Adj. post-wean. ADG y Adj. off-test weight y or LSM LSM LSM LSM line Group n (kg) SEM Prob x (kg d 1 ) SEM Prob n (kg d 1 ) SEM Prob (kg) SEM Prob 1991 Early Late Early Late Early Late BS1 Early Late BS2 Early Late DS Early Late Overall Early Late z Adjusted by calf weaning age. y Adjusted by calf off-test age. x Probability of significance for the least-squares means (LSM) between the early and the late calving groups. in the DS line was significant (P < 0.01, 2.17 vs. 2.41, respectively). The body condition score at calving is a good indicator of the energy reserves of the cow following a winter on pasture. The study indicated that DS cows lost more condition during winter than the BS1 and BS2 cows. These results suggested that high milk production composites such as DS could be bred later to allow them to take advantage of the better quality feed in early summer. This management option may give the late-calving, high-milk production cows a slight advantage when they commence lactation. The adjusted cow BCS at weaning in the early group was significantly higher (P < 0.01) than those of the late group in all the lines. The overall BCS at weaning were 3.1 vs 2.8 for the early and the late groups, respectively. Furthermore, in all lines, the cows in the early calving group showed significantly (P < 0.01) higher BCS than the cows in the late group. The cow weights at weaning in the early group were somewhat higher than those of the late group. However, the difference was not significant (P > 0.05). Bellido et al. (1981) and Deutscher et al. (1991) also reported that earlybred cows were heavier than late-bred cows at weaning. As all calves were weaned on the same date, the late-calving group weaned their calves approximately 43 d earlier than those of the early group (Table 1). As such, the latecalving cows nursed their calves 43 d less than the early group of cows. On the basis of annual cow nutrient requirements, late calving accompanied by a shorter lactation period, is expected to be an advantage to the nutritional status of the cow. Inclusion of large dairy breeds in a composite has resulted in higher calf weaning weights and pre-weaning ADG (Berg et al. 1986). This is directly due to higher milk yields of the dam introduced through the genetics of the dairy types. In the present study, the post-weaning growth rates and off-test weights in the DS line were comparable with the BS1. However, the DS cows were slightly lower in condition than the BS1 and BS2 cattle at calving, but this difference in condition had no effect on reproduction. Our study shows that DS animals benefited more from late season calving than did the beef synthetics, as the differences between early and late calving was not significant in the DS for pre-weaning ADG, off-test weight and post-weaning ADG. The similarity in pre-weaning ADG between early- and late-born calves in the DS line could be mainly due to timing of calving and milk production. Among late calves, the need of the calf is better synchronised with the time at which pasture availability and quality is optimum. However, it does appear that although the late-born calves in the DS line were weaned earlier than the early calves, rate of gain was similar at weaning between the younger and older calves. Another advantage for late calving is that calving early in the colder months of the winter can be quite stressful to cows and calves. As the temperature drops, cattle need more feed to maintain body temperature and beef cows in average body condition are known to be cold stressed at temperatures below 20 C (Webster 1974). Calving in the late spring, when it is warmer, timing milk production with the period of greatest demand by the calf to coincide with pasture availability is a viable management option for the DS line.

6 254 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 5. Least squares means (LSM) and standard errors (SEM) of cow weight and BCS, and heifer weight at breeding and growth for calving season by year, line and overall Female calves Cows Year Adj. heifer wt in June z Adj. heifer ADG to June z Cow BCS at calving Adj. cow wt weaning y Adj. cow BCS wean. y or LSM LSM LSM LSM LSM line Group n (kg) SEM Prob x (kg d 1 ) SEM Prob n (score) SEM Prob (kg) SEM Prob (score) SEM Prob 1991 Early Late Early Late Early Late BS1 Early Late BS2 Early Late DS Early Late Overall Early Late z Adjusted by heifer age in June (at breeding). y Adjusted by calf weaning age. x Probability of significance for the least-squares means (LSM) between the early and the late calving groups. CONCLUSIONS Similar pregnancy, calving, weaning rates, post-weaning ADG and off-test weights in the early- and late-born beef calves, higher birth weights and a tighter calving period among late-born calves suggests that late calving is an alternate management strategy to the more traditional system of calving in late winter or very early spring. The pre-weaning average daily gains were generally higher for the early-born than for the late-born calves in the two beef synthetic lines, as the early calves were older. However, the dairy synthetic cows were more suited to the late-calving management practice as the pre-weaning gains among early- and late-born calves were similar. Late-season calving in the dairy synthetic line resulted in higher heifer weights at breeding and higher average daily gains to first breeding. Delaying breeding and calving, so that calving and peak lactation coincide with pasture availability, may be considered a practical and useful management option for cow-calf producers. ACKNOWLEDGMENTS The assistance of Gary Minchau and the staff of the Kinsella ranch are gratefully acknowledged. Bagley, C. P., Carpenter, J. C., Jr., Feazel, J. I., Hembry, F. G., Huffman, D. C. and Koonce, K. L Influence of calving season and stocking rate on beef cow-calf productivity. J. Anim. Sci. 64: Bellido, M. M., Wallace, J. D., Parker, E. E. and Finkner, M. D Influence of breed, calving season, supplementation and year on productivity of range cows. J. Anim. Sci. 52: Berg, R. T., Goonewardene, L. A. and Makarechian, M The University beef breeding project 1986 update. University of Alberta, Annual Feeders Day Rep. 65: 3 6. Berg, R. T., Makarechian, M. and Arthur, P. F The University of Alberta beef breeding project after 30 years A review. University of Alberta, Annual Feeders Day Rep. 69: Berg, R. T., Price, M. A. and Makarechian, M Calving season: should it be later? University of Alberta, Annual Feeders Day Rep. 70: 71. Burris, M. J. and Priode, B. M Effect of calving date on subsequent calving performance. J. Anim. Sci. 17: Butson, S., Berg, R.T. and Hardin, R.T Factors influencing weaning weight of range beef and dairy-beef calves. Can. J. Anim. Sci. 60: Canadian Council on Animal Care Guide to the care and use of experimental animals. Vol.2. CCAS, Ottawa, ON. Deutscher, G. H., Stotts, J. A. and Nielsen, M. K Effects of breeding season length and calving season on range beef cow productivity. J. Anim. Sci. 69: Keller, D. G. and Brinks, J. S Mating system by environment interactions for weaning weight in Hereford cattle. J. Anim. Sci. 46: Lesmeister, J. L., Burfening, P. J. and Blackwell, R. L Date of first calving in beef cows and subsequent calf production. J. Anim. Sci. 36: 1 6. Morris, C. A Calving dates and subsequent intercalving intervals in New Zealand beef herds. Anim. Prod. 39: Naazie, A., Makarechian, M. and Berg, R.T Factors influencing calving difficulty in beef heifers. J. Anim. Sci. 76:

7 PANG ET AL. EFFECT OF CALVING SEASON ON LOW CALF PRODUCTIVITY 255 Pang H., Makarechian, M., Price, M. A., Liu, M. F. and Berg, R. T Effect of calving season on pregnancy rate, calving pattern and calf survival of beef cattle in Alberta. In Proc. West. Sec. Am. Soc. Anim. Sci., Lethbridge, Canada, 6 8 July : Rege, E. O. and Famula, T. R Factors affecting calving date and its relationship with production traits of Hereford dams. Anim. Prod. 57: SAS Institute, Inc SAS/STAT user s guide: Statistics. Version 6, 4th ed. SAS Institute, Inc., Cary, NC. Selk, G. E., Wettermann, R. P., Lusby, K. S., Oltjen, J. W., Mobley, S. L., Rasby, R. J. and Garmendia, J. C Relationships among weight change, body condition and reproductive performance of range beef cows. J. Anim. Sci. 66: Sellers, H. I., Willham, R. L. and debaca, R. C Effect of certain factors on weaning weight of beef calves. J. Anim. Sci. 31: Warnick, A. C., Kirst, R. C., Burns, W. C. and Koger, M Factors influencing pregnancy in beef cows. J. Anim. Sci. 26: 231 (Abstr.). Webster, A. F. J Physiological effects of cold exposure. Pages in D. Robertshaw, ed. Int. Rev. of Physiol., Env. Physiol. I, V 7, Butterworths, London, UK.