Postpartum intervals will typically range from 35 to 70 days in reasonably managed beef cows. However, it is possible to have intervals as short as 10 to 20 days in well-nourished, nonsuckled cows and over 100 days in suckled, nutritionally stressed cows. This chapter aims to review how suckling affects postpartum interval and to discuss what management alternatives can be considered to decrease the inhibitory effects of suckling and lactation. In order for a weaning treatment to have an immediate effect on postpartum reproduction, it must occur before or early in the breeding season. The weaning options that will affect postpartum anestrus and have the most practical application in the long run have to do with the age of calf at normal weaning. Most cow-calf operations wean their calves somewhere around 6 or 7 months of age, but it would not be uncommon to delay weaning to 8 or 10 months.

Pinus ponderosa or western yellow pine (Pinaceae) is widely distributed in the US, but is most abundant in the western US and western Canada. It was suggested by L. R. James et al. that in addition to the possible toxins in Pinus ponderosa needles, several other factors may be associated with and/or predispose animals to abort following their consumption. These include: stage of gestation when agent(s) are ingested, environmental stresses, condition of the animal, and the general physiology of the animal. The feeding of Pinus ponderosa needles to late pregnant cattle results in an abortion in from 3 days to 3 weeks, with most occurring 5-15 days after the initiation of daily feeding. Although Pinus ponderosa needle abortion is a common name for the syndrome afflicting cattle, it is in some ways misleading, since abortion is normally taken to mean expulsion of a nonviable fetus.

Summary Weight (WT), hip height (HT) and heart girth (HG) were measured on 211 Hereford and 229 crossbred cows. Body condition scores were assigned based on palpation over the ribs and backbone (SP) and on visual observation (SV). Correlations among WT, HT and HG were all positive (P<.01) and greater than .54 within both Herefords and crossbreds. Palpated score and SV were both correlated positively (P<.01) with both WT and HG in Herefords and cross- breds. Palpated score and SV were correlated (P<.01) with HT (.33 and .40, respectively) in Herefords but not (P>.05) in crossbreds (-.01 and. 14). Palpated score was correlated .71 with SV in Herefords and .78 in crossbreds (both P<.01). No evidence of a nonlinear relationship between SP and SV was found. No differences were noted in relation to WT, HT and HG between the SP and SV methods of assigning condition scores. A method of assigning nutri- tive condition suggested by Brody (1945) was correlated (P<.01) .43 with SP and .49 with SV in the Herefords and .48 with SP and .62 with SV in the crossbreds. The WT:HT ratio was correlated (P<.01) to SP and SV, although the form of the relationship was not consistent. A parabola described the data better (P<.01) than a straight line for SP in both Herefords and crossbreds and for SV in crossbreds, but a straight line was the better (P<.01)descripti on

A 3-yr study was conducted to evaluate the effects of calving system, weaning age, and postweaning management on growth and reproduction in beef heifers. Heifer calves (n = 676) born in late winter (average birth date = February 7 +/- 9 d) or early spring (average birth date April 3 +/- 10 d) were weaned at 190 or 240 d of age, and heifers born in late spring (average birth date May 29 +/- 10 d) were weaned at 140 or 190 d of age. Heifers were managed to be first exposed to breeding at approximately 14 mo of age. After weaning, the calves were randomly assigned to treatments. Heifers on the constant gain treatment were fed a corn silage- and hay-based diet. Heifers on delayed gain treatments were placed on pasture but were fed grass hay or a supplement, or both, depending on the forage conditions. Three months before their respective breeding seasons, delayed gain heifers were moved to drylot and fed a corn silage- and barley-based diet (late winter or early spring) or moved to spring rangeland (late spring). The data were analyzed using mixed model procedures with calving system, weaning age, and postweaning management options creating 12 treatments. Average daily gain was 0.36 +/- 0.05 (SED) kg/d less (P < 0.001) for delayed gain heifers during the initial phase, whereas these heifers gained 0.44 +/- 0.03 kg/d more (P < 0.001) than constant gain heifers during the last 90 d before breeding. Body weights at the beginning of the breeding season did not differ (P = 0.97) between constant gain and delayed gain heifers but were affected by calving system and weaning age, reflecting some of the differences in initial BW. Prebreeding BW for heifers weaned at 190 d of age were 36 +/- 6.4 kg heavier (P < 0.001) for those born in late winter and early spring compared with late spring and were 388, 372, and 330 kg for heifers weaned in October at 240, 190, or 140 d of age (linear effect, P < 0.001). The proportion of heifers exhibiting luteal activity at the beginning of the breeding season was not affected (P = 0.57) by treatment. Approximately half of the heifers were randomly selected for breeding. Treatment had no effect (P = 0.64) on pregnancy rates. In conclusion, heifers from varied calving systems and weaning strategies can be raised to breeding using either constant or delayed gain strategies without affecting the percentage of heifers cycling at the beginning of the breeding season. These results suggest that producers have multiple options for management of heifer calves within differing calving systems.

The impact of varied calving and weaning times on post-weaning production of steer calves was evaluated in a 3yr study. Steers (approximately 12 steers per calving-weaning treatment per year) born in late winter or early spring were weaned at 190 or 240 d of age, and steers born in late spring were weaned at 140 or 190 d of age after grazing with their dams on native range. Steers were pen-fed a growing diet until approximately 375 kg BW. They were then moved to an individual feeding facility and fed a higher-energy diet. Steers were allotted to harvest dates based upon visual estimates of degree of fat cover. Mixed model procedures were used to evaluate fixed effects of treatment and random effects of year and year × treat1This research was conducted under a cooperative agreement between USDA-ARS and the Montana Agric. Exp. Sta. USDA-ARS is an equal opportunity and affirmative action employer and all agency services are available without discrimination. Mention of a proprietary product does not constitute a guarantee or warranty of the product by USDA, Montana Agric. Exp. Sta., or the authors and does not imply its approval to the exclusion of other products that may also be suitable. 2To whom correspondence should be addressed: elaine@larrl.ars.usda.gov 3Retired. ment. Predefined contrasts were used to delineate treatment effects. Initial steer BW was affected by calving system and age at weaning. No treatment differences in ADG during the growing phase or ADG, feed intake, or feed efficiency during the finishing phase occurred. Steers averaged 537 ± 4 kg at harvest and age at harvest did not differ among treatments. Late winter steers had greater hot carcass weights, marbling scores, and quality grades than early spring steers. Calving later in the calendar year without altering weaning times produced younger calves at weaning with lighter BW at the start of the feeding period, which required greater number of days from weaning to harvest and produced carcasses with less marbling and lesser quality grades.

In environments where cool-season forages are the major forage resource, shifting calving season from winter to spring may match up forage nutrient supply with animal nutrient demands and thereby reduce purchased feed cost. However, calf BW at weaning in the fall may be less. The objective of this experiment was to determine the effect of calving season and age at weaning on stocker and finishing phase performance. Cows were assigned to late winter (LW), early spring (ES), or late spring (LS) calving seasons. Steers in the LW and ES groups were weaned at 190 and 240 d of age. Steers in the LS group were weaned at 140 and 190 d of age. Delaying the calving season reduced (P < 0.01) the age and BW of the calf at entry into the stocker phase and finishing phases. For the steers grown and developed in Oklahoma, overall stocker gain was similar (P > 0.10) among the 6 treatments groups. Steers developed in Montana had greater stocker ADG than steers grown in Oklahoma, and stocker ADG decreased (P < 0.01) as weaning age decreased. Steers born in LS produced carcasses that had less ( P < 0.05) marbling, fat thickness, quality grade, and yield grade than steers born in LW or ES, but had greater (P < 0.10) ADG. Finishing steers on pasture with ad libitum access to high-energy feed produced leaner carcasses, but pasture-finished steers were not as efficient in converting feed DM to gain as steers finished in dry lot. In a vertically integrated enterprise, delaying the calving season resulted in more of the BW gain being achieved in the finishing phase.

Data from a 3-yr study in Montana were utilized to evaluate impacts of season of calving, weaning strategy, and retained ownership of steer calves on enterprise profitability. Calving seasons were late winter (LW), early spring (ES), or late spring (LS). Each season had 2 weaning times: 190 (LW190, ES190) or 240 (LW240, ES240) d for LW and ES, and 140 (LS140) or 190 (LS190) d for LS. Backgrounding options included shipping steers to Oklahoma (OK1), or backgrounding in Montana to a constant age (MT2) or weight (MT3). Steers from OK1 and MT2 were finished in Oklahoma in confinement or via self-feeders on pasture and harvested in Texas. Steers in MT3 were finished in Montana in confinement and harvested in Colorado. Performance of each system was modeled based on actual animal performance, market prices, and variable input costs. When calves were sold at weaning, gross margins per cow were greatest for LS190 (P < 0.05) and lowest for LW240. During backgrounding, costs of gain were similar among cow-calf systems, and gross margins per steer were greatest for LS140 (P < 0.05), but not different among backgrounding systems. During finishing, costs of gain were greatest for steers from MT2 due to transportation costs to Oklahoma (P < 0.05), and gross margin per steer favored MT3 (P < 0.05). Gross margin for a ranch with a fixed land base did not differ among systems if calves were sold at weaning, but was greatest for LS systems after backgrounding or finishing (P < 0.05).

The impact of varied calving and weaning times on post-weaning production of steer calves from the Northern Great Plains was evaluated in a 3-yr study. Steers (n = 215) born in one of three calving seasons (late winter (LW), early spring (ES), or late spring (LS)) were weaned at 4 (LS1), 6 (LW1, ES1, LS2), or 8 (LW2, ES2) mo of age after grazing with their dams on native range. Later weaned cow- calf pairs continued to graze native range until weaning. Steers were pen-fed a corn silage and alfalfa hay-based diet until the weaning group averaged 375 kg. They were then moved to an individual feeding facility and fed a higher energy diet. Steers were individually allotted to harvest dates based upon visual estimates of fat thickness. Data were analyzed as a completely random design with fat thickness as a covariate using mixed model procedures. Year and year by treatment were random effects. Non- orthogonal estimates were used to delineate treatment effects. Initial steer weights averaged 216 ± 12 kg but were affected by calving season and age at weaning, with LS2 steers weighing 24 ± 10 kg less (P < 0.05) than the average of the LW1 and ES1, LW and ES steers weaned at 6 mo averaging 26 ± 8 kg less (P < 0.01) than those weaned at 8 mo of age, and LS1 weighing 26 ± 11 kg less (P < 0.05) than LS2. There were no treatment differences in ADG during the growing or finishing phases. Total days to harvest averaged 311 ± 15 d and differed between LW and ES steers weaned at 6 versus 8 mo of age due to a 37 ± 12 d difference (P < 0.01) in time to reach harvest. Total days to harvest did not differ between LS steers weaned at 4 versus 6 mo of age. Steers averaged 527 ± 12 kg at harvest and weights were 23 ± 10 kg less (P < 0.01) for ES than LW. Differences in production of steers among calving and weaning strategies may be related to differences in harvest weights and time on feed as affected by weaning weights.

Three studies were conducted to evaluate late summer protein supplementation for growing steers on Northern Great Plains rangeland. In Experiment 1, crossbred yearling steers (N = 80 per year, mean initial live-weight = 275 kg) were allotted to 1 of 2 treatments replicated in 3 pastures in each of 3 years. Treatments were summer-long grazing with or without protein supplementation in late summer. Protein supplement (26% crude protein) was fed at a rate of 1.68 kg (dry matter basis) every third day. In 1995, a third treatment was added to additional pastures consisting of 1.62 kg (dry matter basis) of a 40% crude protein supplement fed every third day. There was no weight gain response to protein supplementation. In Experiment 2, yearling steers grazing rangeland from May to September were fed either no supplement, 1.5 kg of a 22% crude protein safflower meal-based supplement, 1.2 kg of 26% soybean meal-based supplement or 1.2 kg of a 26% safflower and soybean meal-based supplement every third day in late summer. Live-weight gain, forage intake, and digestibility were not affected by supplementation. A third experiment using ruminally cannulated steers fed grass hay and the 3 protein supplements based on safflower and soybean meals showed an increase in ruminal ammonia concentrations but no other appreciable change in ruminal fermentation with protein supplementation. Supplementation with as much as 648 grams of protein every third day was not a viable means to increase gains of steers grazing Northern Great Plains rangelands during late summer under the conditions of this experiment.

A 3-year study was conducted to evaluate grazing strategies for production of growing cattle during summer on Northern Great Plains rangeland. Crossbred yearling steers (N = 123 per year, avg initial weight = 275 kg) were allotted to 1 of 2 treatments replicated in 3 pastures. Treatments were season-long grazing of pastures at recommended stocking rates assuming a 4-month grazing period or intensive-early grazing of pastures stocked at the same rate assuming only a 2-month grazing season. Precipitation in 1993 was 169% of normal resulting in greater forage quality than in other years and no differences were observed in weight gains between treatments during 1993. In 1994 and 1995, steers in the intensive-early stocked pastures gained less weight during the 2 months of grazing than did those in the season-long stocked pastures; however, gain per hectare was greater in the intensive-early stocked pastures. Intensive-early stocking with growing steers may be a viable means to overcome limited forage quality during late summer in the Northern Great Plains and to maximize forage utilization in years of abundant forage.

The objective was to test the efficacy of an intravaginal progesterone insert and injection of PGF2alpha for synchronizing estrus and shortening the interval to pregnancy in cattle. Cattle were assigned to one of three treatments before a 31-d breeding period that employed artificial insemination. Control cattle were not treated, and treated cattle were administered PGF2alpha or an intravaginal progesterone-releasing insert (CIDR) for 7 d and treated with PGF2alpha on d 6. The treatments were applied in one of three experiments that involved postpartum beef cows (Exp. 1; n = 851; 56+/-0.6 d postpartum), beef heifers (Exp. 2; n = 724; 442.5+/-2.8 d of age), and dairy heifers (Exp. 3; n = 260; 443.2+/-4.5 d of age). Luteal activity before treatment was determined for individual cattle based on blood progesterone concentrations. In Exp. 1, there was a greater incidence of estrus during the first 3 d of the breeding period in CIDR+PGF2alpha-treated cows compared with PGF2alpha-treated or control cows (15, 33, and 59% for control, PGF2alpha, and CIDR+PGF2alpha, respectively; P < 0.001). The improved estrous response led to an increase in pregnancy rate during the 3-d period (7, 22, and 36% for control, PGF2alpha, and CIDR+PGF2alpha, respectively; P < 0.001) and tended to improve pregnancy rate for the 31-d breeding period for cows treated with CIDR+PGF2alpha, (50, 55, and 58% for control, PGF2alpha, and CIDR+PGF2alpha, respectively, P = 0.10). Improvements in rates of estrus and pregnancy after CIDR+PGF2alpha, were also observed in beef heifers. Presence of luteal activity before the treatment period affected synchronization and pregnancy rates because anestrous cows (Exp. 1) or prepubertal heifers (Exp. 2) had lesser synchronization rates and pregnancy rates during the first 3 d of the breeding period as well as during the entire 31-d breeding period. The PGF2alpha, and CIDR+PGF2alpha but not the control treatments were evaluated in dairy heifers (Exp. 3). The CIDR+PGF2alpha-treated heifers had a greater incidence of estrus (84%) during the first 3 d of the breeding period compared with the PGF2alpha-treated heifers (57%), but pregnancy rates during the first 3 d or during the 31-d breeding period were not improved for CIDR+PGF2alpha compared with PGF2alpha-treated heifers. In summary, the concurrent treatment of CIDR and PGF2alpha improved synchronization rates relative to PGF2alpha alone or control. Improved estrus synchrony led to greater pregnancy rates for beef cows and beef heifers but failed to improve pregnancy rates for dairy heifers.

A 2-yr study was conducted to evaluate the interactions of castration, feeding length, and dietary CP on growth and carcass characteristics of male cattle (bulls and steers) that vary in expression of muscular hypertrophy. Crossbred cows were bred by AI to Hereford, Limousin, or Piedmontese bulls, which represented genotypes with normal, moderate, and hypermuscularity, respectively, but with similar mature weights. Male calves (131 in yr 1 and 120 in yr 2) were placed in pens with individual electronic feeding gates. Calves were fed growing diets until they reached 386 kg BW and then were individually switched to finishing diets for 90 or 132 d. Interactions were observed among sire breed, gender, and feeding length on carcass composition. Bulls were more efficient than steers in producing live weight gain. Length of finishing period accounted for a larger source of variation than gender for weight characteristics, whereas gender was the larger source of variation for carcass composition. Concentration or degradability of dietary CP influenced rate of gain from weaning to 386 kg. Interactions resulting from varying management on carcass characteristics among calves of different sire breeds indicate that unique strategies may be beneficial for the production of meat from these breeds.

The effects of animal age and sex on chemical and botanical composition of diets of cattle grazing native rangelands were evaluated in a 2-year study. Samples were collected monthly from June through October using esophageally cannulated suckling calves, yearling heifers, mature cows, and mature steers. Dietary crude protein and digestibility differed among animal classes, but these differences varied over time. These 2 diet quality indicators did not vary in the same manner over time for all animal classes, Dietary crude protein varied from a low of 7.2% for steers in August 1994 to a high of 14.3% for heifers in June 1993, In vitro digestibility varied from a low of 50.7% for cows in October 1993 to a high of 74.3% for calves in June 1993, Botanical composition of diets varied with animal class and sampling date with interactions among these. Cool-season grasses accounted for an average of 70% of the diet with a range of 33 to 90%, Shrubs varied from 1 to 61% of the diet. Differences in chemical composition among age and sex classes of cattle grazing native rangeland during the growing season may be partially related to differences in botanical composition of diets. Animals used to obtain diet samples should, therefore, be of similar physiological state and age as animals being monitored for performance.

Breeds of larger mature size tend to grow more rapidly and be older when attaining a given level of fatness. Hereford, Limousin, and Piedmontese are of approximately equal mature size and yet may vary in body composition at a given degree of maturity. However, direct comparisons among these three breeds were not found. Therefore, the objective of this research was to compare Hereford, Limousin, and Piedmontese progenies for economically important traits. Crossbred cows were bred to Hereford (n = 23), Limousin (n = 24), or Piedmontese (n = 24) sires. Male calves were either left intact or castrated at approximately 2 mo of age. Calves remained with their dams until weaning at an average age of 179 d. Male calves were then individually fed a growing ration until they reached 386 kg and then fed a finishing ration either 90 or 132 d. They were then slaughtered at a commercial abattoir and carcass data were collected. Female calves were group-fed and used to examine nutritional effects on age at puberty. Data were analyzed using REML and linear contrasts among the breed-of-sire effects evaluated. Herefordsired calves had shorter gestation periods and weighed less at birth than either Limousin- or Piedmontese-sired calves. Calving difficulty of Hereford- and Limousin-sired calves was less than that of Piedmontese-sired calves. Limousin-sired calves tended to grow more rapidly than Hereford-sired calves. By the finishing phase, Limousin- and Hereford-sired calves had greater average daily gains than Piedmontese-sired calves. Differences in dry matter intake among breeds of sire were relatively small. Differences in carcass weight, longissimus muscle area, fat depth, and percentage kidney, pelvic, and heart fat resulted in a clear stratification of USDA yield grade between breeds of sire. Differences in percentage primal cuts were similar to those for USDA yield grade. Hereford-sired calves had more marbling than progeny of Limousin or Piedmontese sires. However, the force necessary to shear cores from steaks of Piedmontese-sired calves was less than for progeny of Limousin or Hereford sires. Hereford- and Piedmontese-sired heifers were younger at puberty than Limousin-sired heifers. Within breeds of similar mature size and growth rate, ample variation exists in age at puberty and body composition at an approximately equal degree of maturity.

Prepubertal F1 heifers (n = 246; from crossbred dams bred to either Hereford [H], Limousin [L], or Piedmontese [P] sires) were fed 1.9% (LF) or 4.4% (HF) dietary fat from 254+/-4 d of age until they reached puberty or the breeding season started. Safflower seeds (37% oil with 79% linoleic acid) were the added fat source. Blood samples and backfat thickness measurements were obtained from 60 randomly selected heifers representing the sire breeds and diets studied. In addition, five H-sired heifers from both diets were serially bled at 28-d intervals. Total gain, ADG, body condition score, and backfat thickness were affected by sire breed (P < 0.001) but not diet. Backfat thickness was affected (P < 0.01) by the diet x time on feed interaction. Diet did not affect pubertal age (P > 0.10) but tended (P = 0.08) to affect the percentage of heifers pubertal by the beginning of breeding (June 4). Sire breed effects on puberty age at beginning of breeding, percentage pubertal at the beginning of breeding, and puberty age during the entire study were all highly significant. The effect of the diet x sire breed interaction on percentage of heifers pubertal at beginning of breeding (P < 0.05) was 74.4 vs 76.3% in H-sired, 69.8 vs 60.5% in L-sired, and 76.2 vs 97.6% in P-sired heifers (LF vs HF, respectively). Number of AI services per pregnancy and final pregnancy percentage were not affected by diet or the diet x sire breed interaction. Diet affected progesterone (P < 0.05) and cholesterol (P < 0.001) concentrations, and sire breed tended to affect (P = 0.06) cholesterol concentrations. The effect of the diet x time on feed interaction on cholesterol concentrations was highly significant. There were no effects of diet or sample period on insulin or growth hormone concentrations in serially collected blood samples. We conclude that effects of supplemental dietary fat may be breed-dependent and hypothesize that a feeding period of approximately 60 d duration may be more appropriate than the 162 d used in this study.

Effects of prepartum fat supplementation of the dam on cold tolerance of calves were determined in two studies. In Exp. 1, 22 F1, crossbred heifers gestating F2 calves received diets containing either 1.7 or 4.7% dietary fat starting at d 230+/-2d of gestation. Safflower seeds (Carthamus tinctorius) containing 37% oil with 79% linoleic acid were the supplemental fat source in isocaloric-isonitrogenous diets. Calves were separated from their dams at birth, fed pooled dairy-cow colostrum, muzzled to prevent sucking, and returned to their dams in a heated (22 degrees C) barn for 3.5 h. At 4 h of age, a jugular catheter was inserted. At 5 h of age, calves were placed in a 0 degrees C room for 140 min and rectal temperatures and blood samples were obtained at 10- and 20-min intervals. Blood was assayed for glucose, cortisol, and cholesterol. In Exp. 2, 18 multiparous, crossbred beef cows bred to Murray Grey sires were randomly assigned to receive diets containing either 1.7 or 3.1% dietary fat starting at 235+/-2 d gestation. Safflower seeds were used as the supplemental fat source in isocaloric-isonitrogenous diets. At d 260 of gestation, premature parturition was induced in one-half of the cows from each diet group by feeding Ponderosa pine (Pinus ponderosa) needles. Experimental protocols were the same as in Exp. 1, except that cold exposure was at 9 degrees C for 200 min. Rectal temperatures were affected in Exp. 1 by time and diet x time (both P < .01) and diet x calf sex (P < .05) and in Exp. 2 by calf age (P < .05), time, and calf age x time (both P < .01). Plasma cortisol concentrations were affected by time (P < .01) and calf sex x time (P < .05) in Exp. 1 and by time ( P < .01) in Exp. 2. Cholesterol concentrations in Exp. 1 were affected by diet x time (P < .05) and in Exp. 2 by time (P < .05). Plasma glucose concentrations were affected in Exp. 1 by diet (P < .05) and in Exp. 2 by calf age, time, and calf age x time (all P < .01). We conclude from Exp. 1 that feeding heifers supplemental fat during late gestation increased glucose concentrations in the newborn calf, resulting in favorable responses in body temperature in the cold-stressed newborns. This increase in substrate availability suggests a potential positive effect on heat generation in newborns during sustained periods of cold stress. In Exp. 2, premature calves had compromised cold tolerance possibly due to impaired shivering or brown adipose tissue thermogenesis.