One of the major constraints in the improvement of reproductive efficiency of beef cows is the duration of the post-calving anestrus period. If cows are to maintain a calving interval of 1 year they must conceive within 80 to 85 days after calving. Body condition at calving time determines to a great extent the re-breeding performance of beef cows in the subsequent breeding season. Based research of mature and young cows from several studies, cows that maintained body weight and therefore ample energy reserves before parturition exhibited estrus sooner than cows that lost considerable body weight and consequently had poor energy reserves. Body weight change during pregnancy is confounded with embryo and placenta growth. Therefore the estimation of body fat by use of body condition scores is more useful in quantifying the energy status of beef cows. The numeric system of body condition scoring is an excellent estimator of percentage body fat in beef cows. Body condition score accounted for 85% to 91% of the variation in stored body energy in cows.
The processes of fetal development, delivering a calf, milk production and repair of the reproductive tract are all physiological stresses. These stresses require the availability and utilization of large quantities of energy to enable cows to be rebred in the required 85 days. Add to these physiological stresses, the environmental stresses of cold, wet weather on spring calving cows, and often energy intake of range beef cows is below body maintenance needs. As the intake falls short of the energy utilized, then the cow compensates by mobilizing stored energy or adipose tissue and over a period of several weeks, a noticeable change in the outward appearance of the cow takes place. This is a change in the body condition and can be monitored by assigning body condition scores to cows and quantifying the degree of change. Cows that are in a thin body condition at calving return to estrus slowly. Postpartum increases in energy intake can modify the length of the postpartum interval. However increases in the quality and quantity of feed to increase postpartum body condition can be very expensive. Can the improvement in reproductive performance achieved by expensive postpartum feeding to thin cows be adequate to justify the cost of the additional nutrients?
Oklahoma scientists used eighty-one Hereford and AngusxHereford heifers
to study the effects of body condition score at calving and postpartum
nutrition on rebreeding rates at 90 and 120 days postpartum. Heifers
were divided into two groups in November and allowed to lose body condition
or maintain body condition until calving in February and March. Each
of those groups was then re-divided to either gain weight and body condition
postpartum or to maintain body condition postpartum. Figure 1 illustrates
the change in body weight of heifers that calved in a body condition score
Figure 1. Postpartum body weight of heifers with body condition
>5 or <5 at calving and fed to gain or maintain weight. Pregnancy
rates are indicated on the right side of the legend. Bell, et al.
1990
greater than 5 or those that calved in a body condition score less than
or equal to 4.9. The same pattern that has been illustrated in the other
experiments is manifest clearly with these heifers. Thin heifers
that were given ample opportunity to regain weight and body condition after
calving actually weighed more and had greater body condition by eight weeks
than heifers that had good body condition at calving and maintained their
weight through the breeding season. However, the rebreeding performance
(on the right side of the legend of the graph) was significantly lower
for those that were thin (67%) at parturition compared to heifers that
were in adequate body condition at calving and maintained condition through
the breeding season (91%). Again postpartum increases in energy and
therefore weight and body condition gave a modest improvement in rebreeding
performance, but the increased expense was not adequately rewarded.
The groups that were fed to "maintain" postpartum condition and weight
received 4 lb of cottonseed meal supplement (41% Crude Protein; $.13/lb)
per day. The supplement cost for the 69 day feeding period was approximately
$36 per cow. The cows in the "gain" groups were fed 28 lb of
a grain mix (12% CP; $.073/lb) at a total supplement cost of $141.
Both groups had free choice access to grass hay (personal communication).
The improvement in reproductive performance (67% pregnant vs 36% pregnant)
of the thin two-year-old heifers was not enough to offset the large investment
in feed costs.
Other data sets have shown conclusively that cows that calve in thin
body condition but regain weight and condition going into the breeding
season do not rebreed at the same rate as those that calve in good condition
and maintain that condition into the breeding season. The following
table from Missouri researchers illustrates the number of days between
calving to the return to heat cycles depending on body condition at calving
and body condition change after calving.
Table 1. Predicted number of days from calving to first heat
as affected by body condition score at calving and body condition score
change after calving in young beef cows. (Body condition score scale:
1 = emaciated; 9 = obese; Lalman, et al.1997)
Condition score
at calving
Condition score change after calving to day 90
-1 -0.5
0 0.5
1 1.5
2
3
189 173
160 150 143
139 139
4
161 145
131 121 115
111 111
5
133 116
103 93
86 83
82
5.5
118 102
89 79
72 69
66
This data clearly points out that young cows that calve in thin body condition (BCS=3 or 4) cannot gain enough body condition after calving to achieve the same rebreeding performance as cows that calve in moderate body condition (BCS = 5.5) and maintain or lose only a slight amount of condition. Cows must be rebred by 85 days after calving to calve again at the same time next year. Notice that none of the averages for cows that calved in thin body condition were recycling in time to maintain a 12 month calving interval.
