Many areas of Oklahoma have grown some wheat pasture for use as winter feed. Some producers may have questions about the utilization of wheat pasture for growing replacement heifers, before, during, and after their first breeding season. Unsatisfactory breeding performance has occasionally been anecdotally reported when replacement heifers have been exposed to bulls or AI while grazing wheat forages. Therefore an Oklahoma State University study was conducted to compare reproductive performance of heifers grazing wheat pasture before, and during breeding, with heifers grazing wheat pasture until approximately 3 weeks before breeding.
In each of two years, 40 spring born Angus and Angus crossbred heifers were placed on wheat pasture in December and randomly assigned to one of two treatment groups in mid March. Group one (Wheat Pasture; n=20) remained on wheat pasture (mean crude protein = 26.6 %) through estrus synchronization and fixed-time AI. Group two (Dry Lot; n=20) was placed in drylot and had free choice access to a corn-based growing ration (11.1% crude protein) through estrus synchronization and fixed time AI. The heifers were inseminated on about April 5 both years. Heifers were exposed to fertile bulls starting 10 days after fixed time AI for 45 more days. Fixed time AI conception was determined at 32 days after AI by ultrasonography.
The percentage of heifers cycling at the start of estrous synchronization was 75% and 55% for Wheat Pasture and Dry Lot, respectively. Weights of Dry Lot heifers were slightly heavier than Wheat Pasture heifers (897 vs. 867 pounds) at the time of AI but were similar at ultrasound (917 vs. 910 pounds). Conception rate to Fixed time AI was similar for Wheat Pasture (53%) and Dry Lot (43%) and final pregnancy rate was similar for Wheat Pasture (95%) and Dry Lot (88%). Reproductive performance of heifers grazing wheat pasture during estrus synchronization and Fixed time AI was similar to heifers consuming a corn-based growing diet. (Bryant, et al. 2009 Oklahoma State University Animal Science Research Report.)
Some of the problems previously reported concerning heifers grown on wheat pasture may be caused by the sudden change in diet when heifers are removed from wheat pasture and moved to dry dormant pastures before breeding. Perhaps the heifers are removed from the high quality wheat because the first hollow stems were found and wheat was being saved for grain production. Also, in those situations where artificial insemination will be used, the heifers are moved to pastures with working facilities nearby. These dormant warm season pastures are likely much lower in quality in early spring than the wheat pasture the heifers have been recently grazing. This sudden reduction in protein and energy has been shown to reduce cycling activity in heifers that are just starting to reach puberty (White, et al., 2001 Oklahoma State University Animal Science Research Report.) Therefore the pregnancy percentages resulting from AI or natural breeding can be disappointing. The remedy is to keep the heifers growing as they leave wheat pasture and go into the breeding season.