Why are two-year olds such a challenge to get re-bred?

By: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist

Two year old cows that calved this spring will present a challenge to producers to get them to rebreed and stay in synch with the rest of the cow herd.  The issue at hand is the number of days between calving and the return to heat cycles when the cow has a chance to be rebred.  There are several factors that influence the “post partum anestrus period” or the days between calving and the beginning of estrous cycles.

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.  The data was compiled using two-year old Angus first-calf heifers.  Remember the cow must not only return to heat cycles but conceive by day 85 in order to have a calf on the same calendar date the next year.

Table 1. Predicted number of days from calving to first heat cycle 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)

                                                Condition score change after calving to day 90

Condition score 
at calving (below)
-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

Adapted from Lalman,  et al. 1997. Journ. of Animal Science. 75:2003.

This data clearly points out that young cows that calve in thin body condition (BCS=3 or 4) will take a long time to return to heat cycles.  Thin heifers cannot gain enough body condition after calving to return to heat cycles as quickly as cows that calve in moderate body condition (BCS = 5.5) and maintain or lose only a slight amount of condition. Pay particular attention to the heifers that calved in a body condition score of 4 and then were fed enough of a high energy diet to gain 1.5 condition scores by day 90.  Compare them with heifers that calved in a body condition score of 5.5 but lost a half score and were 5.0 at 90 days.  The heifers that calved in poor body condition and were fed well did not return to estrus as quickly (111 days vs. 102 days) as the heifers that were in good body condition and lost a small amount of body condition after calving.   It is very difficult to add body condition on young lactating cows in most range situations.

Once again remember, 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 two-year old cows that calved in thin body condition were recycling in time to maintain a 12 month calving interval.   This illustrates why many ranches breed the yearling heifers 2 to 3 weeks ahead of the start of the breeding season for adult cows.  It gives these heifers extra days to return to heat cycles and therefore breed at about the same time as the other cows in the herd.    A hidden aspect of the nutritional challenges of two-year-old cows is noted when we remember that this is time when they transition from baby teeth to adult teeth.

Other factors that influence the length of the “post partum anestrus period” include difficult births and suckling intensity.  Heifers that suffer a prolonged stage 2 of delivery will take a few days longer to return to heat cycles due to the length of labor during calving.  In addition, strong suckling intensity may have a small impact on the length of time between calving a return to heat cycles.  Research many years ago suggested that bull calves may nurse more vigorously than heifers and therefore result in a slight delay in the return to heat cycles.  Certainly cows nursing twins will be affected by the increase in suckling activity.