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Agronomic practices that impact the digestibility of fiber by lactating cows
Ruminal bulk fill limits intake of forage-rich diets early in lactation with coarse fiber being the primary contributor to ruminal fill. This ceiling on feed and energy intake can be lifted either by reducing the dietary concentration of NDF or by increasing the fermentation rate of NDF (NDFD). Numerous genetic (BMR) and environmental factors can alter NDF content and NDFD of forages. With most grasses and legumes, NDF content increases and NDFD decreases as plants mature, largely due to a decreased leaf:stem ratio. Consequently, earlier harvest, more erect plant stature, harvesting plants at greater height, and minimizing leaf loss all can increase forage quality. With most forages, yield increases as temperature, light intensity, nitrogen fertility, and water supply increase. However, by accelerating maturation, higher temperatures increase NDF content and lignification; factors that retard plant development help maintain forage quality. Unlike other tropical plants, maize harvested as silage is immature and NDF digestibility will not decline if plant health is maintained during kernel development. Restricting supply of irrigation water increases NDFD of maize plants probably through increasing the leaf:stalk ratio. Changes in NDF and NDFD (48 h) digestibility of various plant parts was measured for two non-BMR hybrids harvested across a range in plant DM from 28 to 39%. At all DM contents, maize cobs and husks had the highest NDF content; as plant DM increased, NDF content only for husks increased. NDF content of maize stalks at various heights did not differ, but NDF digestibility always was less for the lower stalk portions. NDFD was greatest for husks and leaves; NDFD of cobs and leaves dropped as plant DM increased. Plant NDFD dropped from 43 to 42% as plant DM increased from 28 to 40%. As lignin content increased, NDFD dropped for stalks. For the other plant parts, NDFD was not altered by the fractional percentages of hemicellulose, cellulose, or lignin in NDF nor was it altered by ratios among these components. As maize kernels matured from half-milk line to black line, kernel weight increased by over 20%. Based on Milk 2006 equations and analysis of plant nutrients at harvest, milk yield per metric ton harvested or per hectare should peak at 33.2 and 34.3% DM, respectively, for unprocessed maize silage. Delaying harvest to 40.5 and 37.2% DM, respectively, should give peak milk yield per ton or per hectare for processed maize silage.
Keywords: Maize, maturity, NDF