Variation in Crude Protein and Initial In Vitro Dry Matter Digestibility of Wheat Forage.

Annually 6 to 7 million spring-born calves (Bos taurus L.) are received in the southern Great Plains and pastured on winter wheat (Triticum aestivum L.) before feedlot finishing.  Frothy bloat can be a serious problem for ruminant livestock grazing pastures of winter wheat.  Decreased ADG with non-lethal bloat episodes, lethal yearly average herd losses up to 2%, and the cost and uncertainties of active bloat intervention strategies take a toll on the profitability of raising stocker cattle.  Wheat pastures have high digestibility, crude protein (CP), and soluble N fractions; all traits associated with bloat-provoking forages.  We assessed the variation in CP concentration and initial in vitro dry matter digestibility (IVDMD) of released wheat varieties and experimental lines to provide information needed to identify varieties and evaluate the feasibility of developing genotypes that could offer a decreased frequency and severity of bloat.  Late fall forage from seven variety trials (34 varieties, 15 to 23 varieties per trial) and late fall and late winter forage samples from 221 diverse experimental breeding lines were analyzed.  Significant (P < 0.05) differences among varieties and breeding lines were found for CP and IVDMD traits; trait variation among breeding lines was often substantially greater than among varieties.  Differences in CP levels among breeding lines was as great as 47% and averaged 15% for the seven variety trials.  Differences in the percent IVDMD occurring in the first 8 h of incubation among breeding lines was as great as 97% and averaged 24% for the seven variety trials.   Correlations between CP and initial IVDMD traits were not significant or at best very weakly correlated (breeding line trial r = 0.11, P = 0.01, n = 448), thus the development of wheat varieties with both reduced CP and low initial rates of IVDMD will require simultaneous selection for both traits.