October 1, 2000
Forage Systems Update
Vol 9, No. 4
Making Calving Season Decisions
Introduction: Timing of calving in a beef herd is one of the most important
economic decisions a beef producer makes. Season of calf marketing and ease of
breeding are two considerations that quickly come to mind for most producers. If
calves are to be sold at weaning, fall born calves are usually sold on a higher
market than spring born calves. It is well documented and recognized by most
producers that fall calving cows are easier to rebreed than spring calving cows in
an endophyte-infected tall fescue based forage environment. There are a number of other,
more subtle factors that also come into play when making calving season decisions.
These include type of cattle involved, potential winter feed costs, base forage
environment, among others. While most producers recognize the big differences between
spring and fall calving, many do not recognize the potential impact of changing
calving season by just a month or two. In an earlier edition of Forage Systems Update
(April 1, 1996;v5:n2), we had reported on an 1987 to 1992 study comparing lactation
response and calf growth for late-winter and early-spring calving cows. The bottom
line of that study was that cows calving closer to initiation of pasture growth
reached peak lactation quicker, produced more milk, and weaned heavier calves at
comparable day of age than did cows calving earlier in the winter.
Lactation places the greatest nutritional demand on a beef cow during her annual
production cycle. Depending on milking ability, net energy demand can increase from
30 to 100% compared to maintenance requirements. Calving in the winter compounds the
lactation-induced nutrient surge with increased energy requirements for maintenance.
A basic tenet in the "matching cattle to forage environment" theory is to have peak
nutritional demand coincide with peak forage availability.
Figure 1 is a theoretical construct based on the lactation curve derived from the
earlier research project described above and a generalized forage growth rate curve
based on data collected at FSRC from 1990 through 1993. The origin of the lactation
curve was placed at the point where daily pasture growth rate would equal the forage
intake of a lactating cow stocked at one cow per acre to identify a theoretically
optimal calving date. Calving at this date should ensure that all the additional
nutrients being demanded by lactation were being provided by growing pasture. In
this scenario, peak lactation occurred in mid-June and breeding began in early July.
Figure 1. Theoretical overlap of beef cow lactation curve and pasture growth curve
Experimental Procedures: To test the value of this relationship, we conducted
a pilot study at FSRC from 1996 to 1999 to evaluate late-winter calving compared to
mid-spring calving. Two separate herds of 60 Hereford X Gelbvieh and Hereford
Gelbvieh X Angus cows bred to Angus bulls were calved in two 45-day calving seasons:
Feb 15 to April 1 (WC) and April 15-May 30 (SC). Corresponding breeding seasons were
May 5 to June 20 and July 10 to August 25. Cows were assigned to their respective
calving group based on breed, age, and date of calving. Cows for the later calving
group were held open an additional 60 days compared to earlier calving herd. During
the growing season, both herds rotationally grazed mixed cool-season grass-legume
pastures managed with flexibility for hay harvest and additional paddock subdivision
as necessary. During the fall and winter, cows sequentially grazed stockpiled tall
fescue-red clover pastures and were fed hay harvested from the same pastures earlier
in the season. Cows were managed over three calving seasons according to the
experimental protocol.
Results: Average conception rate for WC cows was 96% over the three years
while conception rate for SC cows declined each year. (92%, 68%, 48% for 1996, 1997,
and 1998, respectively) Loss of cows from the SC herd forced termination of the
project after the 1999 calving season. Prior to the 1998 breeding seasons, cows in
both herds were checked for cyclicity. In both WC and SC herds, >90% of the cows
were cycling at the beginning of breeding. Cows were observed to cycle throughout
both breeding seasons. Bulls were semen checked at the beginning of the breeding
season and biweekly to determine whether bulls were maintaining breeding soundness.
Bulls remained in good condition for the WC breeding season (May 10 - June 20) while
bulls in the SC season rapidly declined in semen quality and condition through the
breeding season (July 10-Aug 25) suggesting that the observed breeding failure was
more likely related to inadequacies on the male rather than female side.
Weaning occurred in mid-October of each year with calves from both herds weaned on
the same calendar date in each year. The WC calves were from 67 to 78 days older than
SC calves, on average. In each year actual weaning weight was significantly higher
for WC calves. Calves in 1997 from WC and SC herds had equal ADG and 205-day weights,
while in 1998 and 1999, SC calves exhibited significantly greater ADG and 205-day
adjusted weights than WC calves. Overall, the calf response in this study supports
the earlier conclusion that cows having a substantial part of their early lactation
demand on pasture produce more milk and their calves grow faster. Contrary to the
earlier study, later born calves actually had lighter birth weights than winter-born
calves.
Table 1. Although actual weaning weights were higher for early calving cows, 205-day
adjusted weights were higher for later calving cows.
Birth Date Birth Weight Weaning Weight ADG/DOA 205 Day Weight
27-Feb-97 87 589 2.12 522
16-May-97 86 435 2.10 517
24-Feb-98 88 566 2.01 500
04-May-98 82 448 2.16 525
27-Feb-99 86 525 2.00 496
05-May-99 79 404 2.14 518
Discussion: One of the main questions arising from this study regards
the breeding failure of the bulls used in the later calving season. Breeding
season began approximately July 10 and ended August 25 which corresponds with
what is typically the hottest weeks of the year at FSRC. Those readers who have
visited FSRC probably recall that there is virtually no shade available in the
research pastures. While the same bulls have historically performed well in a
May 5 to June 20 breeding season, the absence of shade during midsummer may have
been a major contributing factor in the observed breeding failure.
While the bull failure was clearly documented in the final year of the study, there
are also possible causes for reduced conception on the female side. High temperatures
have been shown to induce what is termed early embryonic loss. Cows may be cycling,
bulls may be virile, fertilization may occur, but pregnancy is not sustained. This
type of breeding failure is very hard to document and measure in field studies.
Reproductive failure from both the male and female sides need to be further studied
before conclusions can be drawn about appropriate breeding seasons for different
types of cattle in different environments.
This brings up another one of the basic tenets of sustainable beef production
systems, which is matching animal type to the production environment. The Black
Angus bulls used in this study were not likely to be very well adapted to the no
shade, hot pasture conditions of midsummer. When not in use, the bull herd at FSRC
has always been maintained in shaded pastures. They had no historic exposure to the
conditions they were asked to work in during this study. It is very unlikely that
the herds they came out of existed in no-shade environments. If the decision is made
to calve in mid-spring and breed during midsummer, selection of heat tolerant stock
becomes very important, both within the context of breed selection and source of breeding
stock within a breed. This study completely failed to consider adaptation of the
experimental animals to the experimental environment and was a major reason for the
observed results.
The concept of summer calving which is being promoted in some areas avoids the
potential breeding problems by moving the breeding season into the cooler early autumn
time frame. Cows and heifers tend to be most fertile near the equinoxes which suggests
September and early October as prime breeding months. Fall calving systems which place
breeding season in November and December usually produce fairly tight calving seasons
with a high percentage of calves coming in the first cycle. In either of these calving
scenarios, shortening days and cooling nights stimulate fertility and breeding activity.
We are currently developing a fall calving herd at FSRC. In the 1970's a fall herd was
included in the research program at FSRC when the entire station was N-fertilized,
endophyte-infected tall fescue. The fall herd was discontinued primarily because
reproduction problems were much greater in the spring calving herd and research
emphasis was shifted to that area. We believe the economic viability of fall calving
in north Missouri may be increasing as our understanding of stockpile and winter annual
management improves.
Winter feed cost remain a prime consideration in any calving season. If feed costs are
reduced significantly, some reduction in both weaning weight and conception rate may be
acceptable. Ideally, a calving season which maintains breeding efficiency and produces
rapid weight gains while maximizing the use of standing forage is likely to be most
profitable. The unique set of conditions existing on each individual farm or ranch will
dictate when optimal calving and breeding seasons occur. Two farms side by side in north
Missouri may have different optimal seasons depending on breed of cattle, fescue endophyte
level, presence or absence of shade, and marketing plans. Reducing winter feed costs is
the one commonality that affects all producers in the region.
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