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ANIMAL PRODUCTION |
USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionLiterature Cited |
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Key Words: Beef Cattle • Calving Season • Rangelands • Weaning
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionLiterature Cited |
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). As nutritional requirements for beef cows vary with physiological states such as gestation and lactation, the adequacy of Northern Great Plains forage quality for meeting beef cow nutrient requirements depends on season of calving. Optimal calving times may vary for beef operations in this region depending on the particular goals of an individual operator. Understanding the relative performance of cattle born during different seasons of the year is important to meet specific goals and optimize economic returns (Reisenauer et al., 2001). Choice of calving seasons also may influence the most appropriate time of weaning for optimal production. For example, in a simulation model developed using data from ranches in southeastern Montana, Julien and Tess (2002) suggested that the greatest profitability was realized when spring-born calves were older at weaning and the grazing season was extended. The objective of the current study was to evaluate the effect of calving during late winter, early spring, or late spring, as well as the effect of two weaning times within each season, on cow and calf performance in a rangeland-based beef operation in the Northern Great Plains. |
Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionLiterature Cited |
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). Regional topography ranges from rolling hills to broken badlands, with small intersecting streams that flow into large permanent rivers meandering through broad, nearly level valleys. Climate is continental and semi-arid. Average annual rainfall in this area is 338 mm, 60% of which is received during the 150-d, mid-April to mid-September growing season. Average daily temperatures range from –10°C in January to 24°C in July; daily maximum temperatures occasionally exceed 37°C during summer, and daily minimum temperatures occasionally fall below –40°C during winter.
Herd Management
Initially (1997), approximately 600 cows from an early spring calving herd were assigned randomly to calve in one of three seasons of calving. Cows remained with their calving season assignment throughout the study. Exogenous hormone treatments were used to aid in altering calving dates, and cows were bred by artificial insemination in 1997. No data were used from 1997 while breeding seasons were being altered. Beginning in 1998, cows were mated by natural service in a 32-d breeding season that included an injection of prostaglandin 7 d after bulls were turned in with cows. Eighteen to 25 bulls were used for breeding, and cow-to-bull ratios averaged 12:1 throughout the study. The same bulls were used in each of the three calving herds within a year. Different bulls were used for breeding in 1998 and 1999, whereas the same set of bulls was used in 2000 and 2001. Bulls were at least one-quarter composite breeding (one-half Red Angus, one-quarter Charolais, one-quarter Tarentaise) crossed primarily with Hereford; however, actual breed combinations varied by year. Breeding occurred from approximately April 6 to May 9, June 6 to July 9, and August 6 to September 9 (exact dates vary by year), resulting in seasons of calving occurring in late January to late February (Feb calving), mid-March to mid-April (Apr calving), and mid-May to mid-June (Jun calving). Because calving seasons had not yet completely shifted to those designed for the study, 1998 performance data were not included in the analysis. Age structure and cow numbers for treatments throughout the study are presented in Table 1.
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) at calving, but we were not always successful. Cows were maintained primarily on native rangeland, except for calving periods for the Feb and Apr calving herds. At calving, the Feb calving herd was housed in drylots and fed corn silage or corn silage and hay through calving. The Apr calving herd calved in small pastures with inadequate forage to support the herd, so hay was fed during the calving period. The Jun calving herd calved in native rangeland pastures. During the calving period, the Feb calving herd was checked approximately twice per hour over the 24-h period for signs of dystocia, and calves were moved inside for warming as needed. The Apr cows were checked slightly less frequently because of the need to patrol a larger area (76 ha). The Jun cows calved in a 167-ha pasture of native rangeland and were checked throughout the day but not at night. Individual cow-calf pairs were moved to pasture for grazing within 1 wk after parturition in all systems. Bull calves were castrated at approximately 6 to 8 wk of age, and all calves received a seven-way clostridial vaccination at this time. Weaning occurred at approximately 190 and 240 d of age for the Feb and Apr calves and at 140 and 190 d of age for the Jun calves. Calendar dates for weaning were approximately August 15 (Feb, 190 d), October 19 (Feb, 240 d; Apr, 190 d; Jun, 140 d), and December 9 (Apr, 240 d; Jun, 190 d). Weaning dates were assigned randomly to calves within each year. Calves received a seven-way clostridial vaccine and vaccination against Haemophilus somnus, bovine respiratory syncytial virus, infectious bovine rhinotracheitis, bovine viral diarrhea, and parainfluenza 3 approximately 3 wk before weaning and at weaning. Calves also received a pour-on treatment for internal parasites at weaning. No implants were used in calves during the preweaning period.
After weaning, calves were housed in drylots for approximately 3 wk, after which time they were sorted into postweaning treatments. Steers were placed on one of two diets at LARRL or were shipped to the USDA-ARS Grazingland Research Laboratory (El Reno, OK). Heifers were placed either in a drylot and fed a diet of 60% corn silage, 39% hay, and 1% protein and mineral supplement (as-fed basis) or were placed on pasture for grazing with hay fed as needed (Grings et al., 2002).
Yearling heifers were added to cow herds at the beginning of the breeding season. Heifers remained in the calving systems in which they were born. These heifers had been raised under various management strategies from birth to weaning as described above and as described in Grings et al. (2002). At the end of the post-weaning treatment period, some heifers were chosen randomly to provide a suitable number of replacements. Heifers that had been raised in drylots were moved to pasture at least 1 wk before the beginning of the breeding season. Heifers then remained with the cow herds throughout the year and were not separated during the winter feeding period. Heifers were placed in separate pens or pastures at calving to allow for increased monitoring for calving assistance as needed.
Animal Data Collection
Cows were weighed approximately 3 wk before the start of the calving season (average = 26 d pre calving), within 48 h after calving, at the beginning of the breeding season (average = 64 d after calving), and at each weaning time. Condition scores were assigned (scale of 1 to 9; Herd and Sprott, 1986) at each weighing by palpation over the back and ribs by two technicians. Calves were weighed at birth, during the breeding season (average = 69 d of age), and at each weaning time. Weight at the second weaning time for calves weaned at 190 d (Feb or Apr) or 140 d (Jun) included data from calves that had been placed into varied post-weaning management programs.
The number of cows 
2 yr of age that exhibited a functional corpus luteum (CL) at the beginning of the breeding season was determined from blood progesterone concentrations. Blood samples were collected by tail vessel venipuncture on d –7 and 0 relative to the beginning of the breeding season. Serum was collected from blood after centrifugation (3,000 x g for 30 min), frozen, and subsequently analyzed for progesterone by radioimmunoassay (Kit TKPGX; DPC, Los Angeles, CA). A cow was assumed to have a functioning CL if at least one serum sample had a progesterone concentration >1 ng/mL. Pregnancy was determined by transrectal ultrasonography in October.
Harvested feed inputs were measured by daily recording of the feed supplied to each calving herd. Silage was measured by a scale on the feed truck at time of delivery. Hay inputs were recorded by the number of bales supplied. Representative bales were weighed to obtain estimates of the actual weight of hay supplied. Hay bales were cored after harvest each year for estimates of quality. Supplement (1.9-cm pellet) delivery was estimated by the calibration of a range cake feeder mounted on a truck. Hay, silage, and supplement samples were sent to a commercial laboratory for analysis of DM, ash, CP, and ADF (AOAC, 1990).
Diet Quality
Diet quality during grazing periods was estimated from esophageal extrusa. Diet samples were collected monthly, and sampling times were scheduled to provide estimates of diet quality at calving, breeding, and weaning, with other months sampled as appropriate. A total of 41 time-points were sampled between April 1998 and December 2001. Diet samples were not collected in January and November 2000 or in January and February 2001 when snow cover prevented grazing. Extrusa samples were collected using three to six adult esophageally cannulated cows in each pasture. Cows were allowed approximately 4 d to adapt to each pasture, and different cows were used in each pasture. Before each diet sample collection, cows were penned overnight with access to water. Two 45-min collection periods were conducted within 1 wk on nonconsecutive days for each calving system. Extrusa samples were lyophilized, ground to pass a 1-mm screen, and stored until analysis for DM, OM (both AOAC, 1990), CP, and in vitro OM digestibility (IVOMD).
Samples for CP determinations were placed in a roller grinder for 12 h (Mortenson, 2003). Nitrogen was determined by combustion techniques in a C-N analyzer (CE Elantech, Inc., Lakewood, NJ). Nitrogen was multiplied by 6.25 to obtain CP, and these values were expressed on an OM basis. The IVOMD was determined by the method of Tilley and Terry (1963).
Forage Quantity and Quality, Standing Crop, and Grazing Pressure
Forage quantity and quality data were collected to describe the study environment. Stocking rate and grazing pressure were calculated to ensure that effects attributable to calving systems were not affected by differences in grazing management, other than those directly related to altered season of calving. The total of 41 forage sample dates occurred approximately monthly during the period from April 1998 through December 2001 during the same week as diet sampling.
Triplicate herbage sample sites were subjectively located in each sample pasture on each of three topographic positions (Upland, Hillside, and Bottomland). Before clipping, the three dominant plant species at each site were recorded as a plant community descriptive metric. Thereafter, the herbage in fifteen 0.1-m2 randomly located quadrats was harvested by herbage type (grass or forb) to ground level, dried at 60°C, for 48 h or until dry, and weighed. Forage from the 15 quadrats was composited and ground for analysis of DM and CP as described for diet quality samples, with CP expressed as percentage of DM.
Monthly stocking rates (animal unit month [AUM]/ha) were calculated by proportionally adjusting for temporal movements of herds among varying sized pastures within months. No adjustments were made to animal units (AU) for varying class and size of animals, as proportions of various size and class of animals were similar among calving systems. Thus, dry cows, cow-calf pairs, yearling heifers, and bulls were all considered 1.0 AU. Instantaneous grazing pressures (AU/kg of available herbage) were calculated for each herd on each of the 41 herbage sample dates by dividing number of AU by whole pasture herbage availability estimates.
Statistical Analyses
Diet quality data were analyzed using the MIXED procedure of SAS (SAS Inst., Cary, NC). To aid in describing and interpreting the dynamic nature of the interface between the nutritional environment and cow physiological state, each sample date was assigned to the appropriate month after calving for each of the three calving systems (Figure 2). The model evaluating dietary CP and IVOMD included calving system and month from calving within calving system as fixed effects. Year, the interaction of year and calving system, and the esophageal cow used within calving system x month after calving for the calving system x year interaction were random effects. The effect of month after calving within calving system was tested using the mean square for esophageal cow within calving system x month after calving for the calving system x year term. Data from 1998 were included in the diet quality analysis as this provided an estimate of the pre-calving environment for performance in 1999.
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Statistical analyses of cow BW and BCS data were conducted using the MIXED procedure in SAS. Weigh dates were assigned values of 1 to 5, with 1 = precalving, 2 = calving, 3 = prebreeding, 4 = first weaning, and 5 = second weaning. Cow BW and BCS data were analyzed in a repeated measures model that included weaning age nested within calving system, weigh date, their interaction, and cow age. Year and the year x weaning age within calving system interaction were considered random. The subject for repeated measures was cow within weaning age x calving system x year. An unstructured covariance structure was used.
The effect of previous year’s weaning (including 1998) time on cow BW and BCS was tested using the MIXED procedure of SAS on data for multiparous cows. Fixed effects included previous year’s weaning assignment nested within calving system, cow age, and their interaction. Random effects included year, the interaction of year x previous year’s weaning assignment within calving system, and year x cow age. Contrasts were used to compare weaning at 190 vs. 240 d after calving for the Feb and Apr systems and 140 vs. 190 d after calving for the Jun calving system (Wean 1 vs. Wean 2).
Proportions of cows exhibiting luteal activity by the beginning of the breeding season and pregnant in the fall were analyzed by the CATMOD procedure in SAS. The model included year, calving system, cow age, and the calving system x cow age interaction. The effect of previous year’s weaning strategy on current year pregnancy status was tested using a CATMOD model that included cow age, weaning age within calving system, year, and the interaction of cow age x previous weaning age within calving system.
Standing crop data were analyzed using SAS GLM procedures. Main effects for assessing the effects of topographic position on quantity and quality of available forage were location, year, and sample date within year. Total herbage standing crops were estimated for sampled pastures by proportionally multiplying estimated topographic site standing crop estimates by topographic composition of pasture (Table A1). These data were ultimately combined with the stocking rate data to estimate a pasture level grazing pressure index (forage demand per unit of forage available). Stocking rate and grazing pressure estimates were analyzed using the MIXED procedures in SAS; calving system was considered a fixed effect, and year and treatment x year interaction were random effects. Mean separation procedures follow Tukey Q procedures, with statistical significance set at P < 0.05.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionLiterature Cited |
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and A2 provide an indication of the forage conditions under which this study was conducted. Averaged across sample dates, average standing crops were 782, 647, and 1,155 kg/ha for the Upland, Hillside, and Bottomland sites, respectively. Averaged across pastures and sample dates, average CP (DM basis) was 7.7, 6.2, and 7.5% for the Upland, Hillside, and Bottomland sites, respectively. Variations among years and sample dates (Figures A1 and A2) were the result of varying annual patterns of herbage growth and senescence, arising largely from variations in annual patterns of precipitation (Figure 1
). These yearly patterns generally agree with previous findings from this location (Heitschmidt et al., 1995, 1999) and across the Northern Great Plains in general (Singh et al., 1983).
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for April 2001 when stocking rates averaged 0.97, 1.52, and 0.22 AUM/ha for the Feb, Apr, and Jun calving systems, respectively. This was because 1) the Feb calving herd rotated through several pastures and received supplement (2.7 kg·cow–1·d–1) and some alfalfa hay (5.6 kg·cow–1·d–1) from April 1 to April 22, 2) the Apr calving herd was in two medium-sized (<56 ha) crested wheatgrass pastures while being fed alfalfa hay and supplement (22.3 and 2.9 kg·cow–1·d–1, respectively) for the entire month, and 3) the Jun calving herd was receiving no supplementation while grazing native rangeland. Analyses of the instantaneous grazing pressure estimates derived on the herbage sample dates showed no significant effects including no calving system x year interaction. The data were quite variable, however, for the same reasons outlined above for the stocking rate data.
Finally, although there were no significant main calving system effects for stocking rate or grazing pressure, relative differences in average stocking rates were indicative of the management tactics required to maintain these treatments. For example, for the entire length of the study, the heaviest stocking rate was 0.66 AUM/ha for the Jun calving system, and the lightest was 0.31 AUM/ha for the Feb calving system, with an intermediate stocking rate of 0.40 AUM/ha for the Apr calving system. These differences were largely because of differences among herds in amount of time spent in a nongrazing environment (i.e., under full-feeding conditions). In such instances, stocking rates were considered zero, and thus, average annual stocking rates were lowered. Differences among systems in total quantities of harvested feeds provided show that annual stocking rates will have to be greater for Jun calving systems than for either the Feb or Apr calving system if equal numbers of animals are to be maintained on equivalent areas of rangeland.
We conclude that the three calving herds were managed such that only minor differences in animal performance and productivity can be attributed to differing grazing tactics. This conclusion is reflected by the general absence of any calving system effects relative to stocking rates and grazing pressures.
Diet quality, estimated by CP and IVOMD, was quite variable throughout the year and followed patterns typical of the Northern Great Plains (Adams and Short, 1988), with a peak in June followed by a rapid decrease throughout the growing season as temperatures increased and precipitation decreased. The nutritional environment of beef cows in the late pre- and early post-calving periods is considered critical to efficient reproductive performance (Houghton et al., 1990). Cows from the three calving systems experienced very different nutritional patterns relative to their physiological state, especially during the first 3 to 4 mo after calving, a period that included the breeding season (Figure 2, A and B). Additionally, Jun cows experienced greater forage quality 2 to 3 mo before calving compared with Feb and Apr cows. The data in Figure 2 include nutrient levels from range forage only and do not account for any supplementary or hay feeding that may occur during the winter months. Supplemental feed would effectively increase dietary nutrient concentrations before calving for approximately 3 mo for cows calving in April and for 1 mo for cows calving in Feb. On average, diets were of similar quality between Wean 1 and 2 for the Feb calving herd because of fall precipitation, whereas diet quality decreased between Wean 1 and 2 for the Apr and Jun herds.
Calf Performance
Calving system x year and weaning age within calving system x year interactions were significant (P < 0.001) for all calf traits. These yearly variations in calf BW and gain are expected as both cow milk production and quality of forage consumed by calves are affected by precipitation pattern and its effect on quantity and quality of available forage. Year effects and all interactions that included year were considered random effects in this study. Choice of calving season is a long-term decision that does not allow for adjustments associated with yearly variation. Weaning age, however, can be used to adjust production to yearly changes in the environment. This study was conducted over 3 yr that fell within 78 mm of the 338-mm long-term precipitation average and is thereby representative of expected responses for a majority of years.
Calf birth weight averaged 36 kg and did not differ among calving systems (Table 2). Our results tend to disagree with reports that birth weights of summer and fall calves are less than those of winter and spring calves (Donald et al., 1962). As the reported lighter birth weights of summer calves might be related to warmer temperatures, this seasonal effect may have been minimized for the Jun calves, which were born before temperatures became hot and while forage was close to its greatest quality. Calf mortality averaged 3.5% for the Feb calving system compared with 1.5% for both the Apr and Jun calving systems; these data were not analyzed statistically.
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). However, once diet quality from rangeland improved (Figure 2), gains by these calves increased, as evidenced by the increased rate of gain from 69 d of age until the first weaning. Overall rate of gain from birth to weaning was greater for earlier than for later weaned calves for all calving systems (P = 0.03). Differences in the timing and amount of preweaning growth may have minimal effects on mature BW of cows (Holloway and Totusek, 1973) or carcass quality of steers (Morgan, 1972; Patterson et al., 1995) but could potentially affect subsequent milk production in heifers (Johnsson and Obst, 1984).
Calf weaning weight differed by weaning age within calving system (P < 0.001; Table 2); younger calves were lighter than calves 50 d older at weaning. When weaned at 190 d of age, calves from the Jun calving system tended (P = 0.06) to be lighter than calves from the Feb or Apr calving systems. Average daily gain from 69 d of age to first weaning was not decreased for Jun calves compared with those from Feb and Apr calving systems, indicating that the decreased weaning weight in October for Jun calves is primarily an age effect (i.e., 140 d), whereas the decreased weaning weight observed in December (i.e., 190 d) is a seasonal effect. Declining forage quality resulted in decreases in calf gains with advancing season and was presumably related to both a decrease in milk intake and a decline in quality of forage consumed by the calf (Grings et al., 1996).
Adams et al. (2001) reported decreased weaning weights for June vs. March born-calves raised on Nebraska sandhills rangeland, and similar results were observed by Smith et al. (2001) for early vs. late spring-born calves on the short-grass prairie of southern Wyoming. Pang et al. (1998) reported decreased preweaning ADG by calves with an average birth date of April 14 compared with May 27 when grazing rangeland in east-central Alberta. The decrease in weaning weights for later-born calves is related to declines in forage quality and different environmental conditions (Figure 1; NOAA, 1998–2001) than those for calves born earlier in the year. Temperature and snow cover also may play a role in decreased weaning weights for calves born and weaned later in the year. For example, in our study there were 26 d of snow cover >2.54 cm in November 2000, which would affect forage availability to December-weaned calves. Average temperature across the 3 yr decreased from 24.3°C in August (Wean 1 for Feb calves) to 9.3°C for the October weanings, to –5.2°C in December (Wean 2 for the Apr and Jun calves).
No weaning age within calving system effects for calf ADG between Wean 1 and 2 were observed (Table 2), although ADG ranged from 0.41 to 0.75 kg/d for the various calving and weaning systems. Basarab et al. (1986) reported a substantial decrease in calf BW gain for 1 mo following weaning at approximately 150 to 160 d of age compared with calves kept with their dams on Northern Great Plains native rangeland during this period. The effect was much less in our study, which may be related to a longer period (50 d) between early and late weaning that allowed early weaned calves to recover from the stress of weaning.
Cow Performance
Cow BW change dynamics were affected by calving system. Cow BW exhibited a calving system x cow age x weigh day interaction (P < 0.001), as shown in Figure 3. The pattern of BW change was similar by calving system for 3-yr-old cows and those 4 yr of age. The Feb cows lost BW (P < 0.001) between calving and the beginning of the breeding season, whereas Apr cows were able to take advantage of high-quality forage in late spring to gain BW (P < 0.001) between calving and breeding. Two-year-old cows showed less difference in BW relative to calving system than older cows, and there was a notable difference in the BW response around calving for Jun cows of different ages. Older Jun cows exhibited a limited BW change throughout the year, whereas 2-yr-old cows lost BW (P = 0.006) between the precalving and calving BW measures and weighed less (P = 0.04) than Apr 2-yr-old cows at breeding. In contrast, older Jun and Apr cows were similar in BW and heavier (P < 0.001) than Feb cows at breeding.
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compared a March 1 to April 1 calving date for range cows in the Sandhills of Nebraska and found equivalent cow productivity when calves were weaned at equivalent ages. They reported that April-calving cows lost less BW between precalving and prebreeding than did March-calving cows, and that the April-calving cows were heavier at prebreeding, which would be consistent with the BW relationships we observed for Feb and Apr cows between precalving and prebreeding. Bellido et al. (1981) reported greater fluctuations in cow BW for early calving (comparable with the Feb and Apr cows calving early in the season in our study) than late-calving cows (comparable with Apr cows calving late in the season and Jun calving cows in our study) grazing New Mexico rangeland.
Body condition score followed the same trend as BW for cows in the three calving systems (Figure 3). Calving system x cow age x date interactions (P < 0.001) occurred, as were observed with BW (Figure 3). For older cows, BCS before (Feb, P < 0.001; Apr, P = 0.056) and at calving (Feb and Apr; both P < 0.001) were lower for the Feb and Apr cows than at the previous fall’s weaning (Figure 3; Table 3), whereas Jun cows had similar BCS at calving and at previous weaning. This finding indicates the increased resiliency of older cows in early gestation to winter conditions in the Northern Great Plains compared with cows in late- (Feb) and mid-gestation (Apr). Two-yr-old Jun cows, however, lost more (P < 0.001) condition than older cows between the precalving and calving condition scorings, which is consistent with their BW change patterns. Our intent was to have all cows at a condition score of approximately 5 at calving to compare the feed inputs required to carry cows through the winter on an equal condition basis; however, we underestimated this point for the Feb and Apr cows, primarily in response to rapid changes in BCS associated with severe winter storms. Older Jun cows were able to withstand winter conditions with less variation in BW and body condition.
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). Condition score showed a similar tendency (P = 0.08). Effects on condition score were moderated by the relatively short (50 d) period between the two weaning times. Decreasing temperatures, along with declining diet quality, affected the BW and BCS responses to weaning by cows from the different calving systems. Additionally, Jun cows were in an earlier stage of lactation under similar environmental conditions than the Apr suckled cows, resulting in greater BW loss.
The proportion of cows with a functional CL by the beginning of the breeding season differed (P < 0.001) by calving system, with a greater proportion of cows in the Jun calving system exhibiting luteal activity early in the breeding season. The proportion of cows exhibiting luteal activity by the beginning of the breeding season from the three calving systems was Feb, 0.68 ± 0.02; Apr, 0.72 ± 0.20; and Jun, 0.86 ± 0.02. Additionally, an effect of cow age was observed (P < 0.001), with a greater proportion of cows 4 yr of age having functional CL by the beginning of the breeding season (0.82 ± 0.01) than 2- (0.79 ± 0.03) or 3-yr-old (0.60 ± 0.02) cows. No interaction between calving system and cow age occurred for this response.
Date of birth within a season did not differ among calving systems, indicating that although there were more cows exhibiting luteal activity at the beginning of the breeding season, this was not associated with more cows becoming pregnant early in the season. This result may be because of the response to injection of prostaglandin at 7 d into the breeding season.
The proportion of cows pregnant in the fall averaged 0.86 and did not differ among calving systems. A tendency (P = 0.09) for a calving system x cow age interaction was observed (Table 4), which was primarily related to an increased proportion of 3-yr-old cows in the Jun calving system being pregnant compared with the Feb and Apr calving systems. Bellido et al. (1981) also observed no effect of calving season on pregnancy rates in New Mexico, except in a drought year, when late-calving cows had higher pregnancy rates than early calving cows.
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suggested that, for cows calving earlier in the year, feed levels before and after calving are inadequate for short postpartum, anestral periods. Our cow BW change data support this suggestion, in that Feb cows were at their lightest BW at breeding, whereas Apr and Jun cows were able to utilize high-quality, late-spring forage for BW gain and were heavier than Feb cows at breeding. Photoperiod and temperature effects also may influence the length of the postpartum interval; these effects are more evident under conditions of limited nutrition (Montgomery et al., 1985). Although the cow BW data support the theory that feed levels may be less than optimum when calving early in the year in the Northern Great Plains, pregnancy rates did not differ for the Feb cows compared with those in other calving systems. Precalving nutrient intake was modified by use of harvested feedstuffs such as hay and pelleted supplements, so that total available nutrients for Feb and Apr cows were greater than indicated by the extrusa quality graph. Increased reliance on harvested feedstuffs can increase total cost of production. Because of the short-term nature of the experiment, the location’s farming system was not adjusted to fit a particular calving system. With the change of an entire operation to a specific calving season, alterations in harvested feed management could be made to make greater changes in the feeding strategies for each system than we used in this experiment. Previous year’s weaning status had minimal effects on cow performance. Significant differences caused by previous year’s weaning within calving system for cow BCS through weaning were due to differences across calving systems but not within (data not shown). The proportion of cows pregnant in the fall also was unaffected by previous year’s weaning status and the interaction of cow age with previous weaning status within calving system.
Estimated feed inputs were less for Jun cows than for Feb and Apr cows (Table 5). In addition to a lesser quantity of feed, a different quality of feed was provided to these cows because of different nutritional demands during the winter feeding period. Both the quantity and type of feeds can affect feed costs. There was more variation in the amount of harvested feeds provided to Jun cows among years than for other herds. The winter of 1999 to 2000 was relatively mild, and rangeland was free of snow much of the winter. Additionally, fall precipitation stimulated growth of some of the annual and cool-season grasses, such that winter forage quality was increased, and no supplemental feed was provided to the Jun cows throughout the entire winter (Figure 4). The following winter, however, rangeland was covered in snow by November, and it remained so until April, resulting in a need to provide harvested feeds to the Jun cows throughout the entire winter. These results indicate some risk with the Jun cows, in that a full winter of feed should be planned for, yet may not be required.
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Footnotes |
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2 The authors express appreciation to Pharmacia & Upjohn, Kalamazoo, MI, for providing the prostaglandin used in this study.
5 Current address: USDA-ARS, University Park, PA 16802.
3 Correspondence: 243 Fort Keogh Road (phone: 406-874-8202; fax: 406-874-8289; e-mail: elaine{at}larrl.ars.usda.gov).
Received for publication November 10, 2004. Accepted for publication July 8, 2005.
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionLiterature Cited |
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