Replacement heifers that have just reached puberty and started cycling may be vulnerable to any drastic change in feed intake. A small trial conducted at Oklahoma State University (White, et al., 2001) illustrates the impact that sudden severe reduction in energy intake can have on cycling activity in replacement heifers. Nineteen heifers were divided into two groups. Both groups were fed at 120% of the maintenance requirements needed for yearling heifers. By the use of hormone assay and ultrasonography, it was determined that all heifers were cycling when the treatments began. Nine of the heifers were continued on the 120% of maintenance diet. The other ten heifers were placed on a diet that was 40% of the requirement for maintenance. They remained on this diet for 14 days. At the conclusion of the 14 day treatment period, only 3 of the feed restricted heifers responded to estrous synchronization and ovulated, whereas all of the heifers receiving the 120% of maintenance responded and ovulated.

Table 1. Impact of sudden, severe reduction in feed intake on cycling activity of yearling heifers

 

Treatments

 

120% of Maintenance

40% of Maintenance

Day of treatment

Day 0

Day 14

Day 0

Day 14

# of Heifers

9

9

10

10

Weight

704

711

691

658

# cycling

9

9

10

3

This very small, but impressive, data set illustrates that we must be cautious about any disruption in the feed intake of replacement heifers at the start of their breeding season.  The winter of 2015-2016 has seen many Oklahoma heifers raised on wheat pasture. Movement from high quality cool season grass (in the spring) to dormant winter native range may cause such a weight loss in a short period of time.  In most operations, the heifers must be moved to a pasture or trap near the headquarters for adequate facilities to be used at breeding.  Therefore the supplementation program on the dormant grass should allow the heifers to continue to gain weight in to and through the breeding season. 

Making other changes in diet at the start of the breeding season should be done carefully and gradually to avoid any chance of digestive disorder and the possibility of the heifers going "off-feed".