Morphological components of Thynopirum ponticum tillers and their contribution to the diet of cattle grazing at contrasting herbage allowances

Tall wheatgrass (Thynopirum ponticum, Podp.) is a hardy cool season grass, which can offer nutritional and sward structural constraints to herbage DMI by grazing cattle. The objective of this study was to describe sward structural changes and dietary composition, in terms of morphological components of the grass tiller, by cattle grazing tall wheatgrass swards at three levels of herbage allowance (HA), as well as to describe the residual effect of the referred grazing managements on herbage morphology and diet composition during a subsequent grazing period. Three grazing periods were conducted. During the first and second periods, the 36 paddocks (0.26 ha, residence time: 7 d per paddock) were allocated to three levels of HA (3, 6 and 10 kg green DM/d/100 kg BW; HA3, HA6, and HA10, respectively). A core group of Angus beef heifers (140.7±7.2 kg BW) was kept on each treatment (4, 6 and 12 heifers, respectively). The allowances were met by varying the total number of heifers on each paddock. During the third period, 24 (4 paddocks per treatment could not be measured) of the 36 paddocks (eight per treatment, residence time: 3 or 4 d per paddock) were grazed at one HA of 6 kg of green DM/d/100 kg BW. Thirteen new Angus heifers (156±23 kg BW) were randomly allocated per treatment. The desired herbage allowance was achieved by changing the grazeable area of each paddock. Reductions in HA led to greater pasture utilization in the first and second grazing periods, going from 21.8% in HA10 to 61.7% in HA3 during the first grazing period, and from 22.9% to 59.3% in the second. By the third grazing, the first and the second periods had induced contrasting sward structures, with the highest HA resulting in a greater proportion of pseudostem (43% vs. 34%), less green material (60 vs. 76%), less green leaf (53% vs. 59%) and a higher sward surface height (43.8 vs. 27.8 cm) compared with the lowest HA. HA and the sward structure resulting from the two previous grazing periods strongly affected the sward structure. However, the detailed measurements on marked tillers indicated that the proportion of each morphological component of the tiller in the diet remained remarkably independent of treatment, with about 70% represented by the growing and the fully expanded leaves (still 100% green), between 15% and 25% by the senescing leaves, between 2% and 8% by dead leaves and less than 5% by pseudostem. In conclusion, HA determines herbage utilization and affects the sward canopy structure for subsequent grazing events. However, under the conditions of this study, cattle achieved a similar quality of the diet.