Determinants of grazed forage voluntary intake in cattle

Typescript (photocopy).The relationship of voluntary intake of grazed forage with digestibility, gastro-intestinal fill, turnover of undigested residues and rates of degradation and passage of forage particles was studied in 1977 and 1978 by varying maturity and selectable plant parts of Coastal bermudagrass [Cynodon dactylon (L.) Pers.,] available to esophageal-rumen fistulated animals. Forage attributes were varied by grazing plots of Coastal bermudagrass at three stages of regrowth by first an A grazer, followed by a B and C grazer. The effect of maturity on plant and animal measurements was variable between years and apparently influenced by residual forage left in plots after each grazing in 1977. The grazing sequence resulted in the A grazer consistently having higher voluntary intakes as compared to the C grazer with the B grazer being somewhat intermediate between years. Increased intakes were invariably associated with high dry matter digestibilities, faster turnover rates of undigested residues and lower gastro-intestinal fills. However, increased fills of undigested residues were compensated by reduced turnover rates of undigested residues thus resulting in a constant daily excretion of fecal dry matter per unit of body weight for the three grazers at all maturities. This suggested that the major factor affecting intake of different quality of Coastal bermudagrass in these studies was forage digestibility. Increased fill associated with reduced forage digestibility and voluntary intake was though tot be an indication of a limited capacity of the GI tract to process undigested residues. The exact location of the fill-limiting portion of the tract was not evident in this study but it appeared that increases in total tract fill could not be entirely accounted for by change in rumen fill. Rates of degradation and passage of rumen particles were affected by maturity, grazer sequence, plant part and particle size..