Herbage selection, intake and digestibility in grazing beef cattle

AbstractThe objectives of this study were to measure voluntary herbage intake in kg of dry matter (DM) per day and in proportions of plant species and components (leaf, stem, dead material) of nonlactating Angus cows under grazing conditions and compare DM herbage intakes to intakes of the same cows when they were nursing their calves. Twenty nonlactating Angus cows (50±12mo of age, 525±55kg weight) were selected from a larger herd to create 4 groups of 5 cows with average DM intakes that ranged from 11 to 15kg/d during lactation. The cows were allocated for 28d as a group on the pasture that contained 5540kg DM/ha as tall fescue (Festuca arundinacea), bermudagrass (Cynodon dactylon var. Tifton-85), red clover (Trifolium pratense) and other plants. Pasture composition was measured by visual appraisal and manual separation of pasture clippings. Daily allocations provided approximately 2.5kg DM/100kg BW. Each cow was individually fed 0.82kg supplement DM daily that contained 498mg of the n-alkane dotriacontane (C32) and 448mg hexatriacontane (C36) during the last 14d. Fecal grab samples were collected from each cow during the last 5d. Grazing intake (8.92±1.5kg DM/d) was calculated for each cow from C32 intake and ratios of tritriacontane (C33):C32 in feces and did not differ (P=0.97) among cow groups. Individual cow intakes during lactation and after weaning, during grazing, were not correlated. Measured sward and calculated intake proportions of tall fescue (0.58 and 0.65), bermudagrass (0.38 and 0.33), and red clover (0.02 and 0.01) indicated cows selected slightly more tall fescue and less bermudagrass and red clover than was on offer. Manual separations of sward and calculated intake proportions of dead material and stem (0.89 and 0.95), green leaf (0.10 and 0.02) and other material (0.01 and 0.04) were similar. N-alkanes provided credible calculations of intake by grazing cows. Intakes of lactating cows did not predict their intake after weaning