Net effects of nitrogen fertilization on the nutritive value and digestibility of oat forages

Authors: Coblentz, Wayne; AKINS, MATTHEW - University Of Wisconsin; CAVADINI, JASON - University Of Wisconsin; Jokela, William

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2016
Publication Date: 2/17/2017
Publication URL: http://handle.nal.usda.gov/10113/5617378
Citation: Coblentz, W.K., Akins, M.S., Cavadini, J.S., Jokela, W.E. 2017. Net effects of nitrogen fertilization on the nutritive value and digestibility of oat forages. Journal of Dairy Science. 100:1739-1750.

Interpretive Summary: Applications of soil amendments or fertilizers containing nitrogen are a routine part of most (grass) forage management strategies, with the primary goal of improving forage yields. But an increase in yield is usually accompanied by a decrease in nutritive value. In order to better evaluate this tradeoff, we evaluated the effects of nitrogen fertilization on the nutritive value of a single cultivar (ForagePlus) of fall-grown oat fertilized at planting with six different rates of urea (nitrogen) or two rates of dairy slurry. Concentrations of fiber components increased consistently with nitrogen fertilization, while water-soluble carbohydrates (sugars) exhibited the opposite response. Overall, the forage nutritive value of fall-grown oat declined mildly in response to nitrogen fertilization, but these responses were not nearly strong enough to offset the advantages obtained by improved forage yields. This study provides forage growers with the knowledge that fertilizing for improved yields will not greatly reduce the nutritive value of the forage fed to dairy cattle.

Technical Abstract: Applications of soil amendments containing N are part of routine forage management strategies for grasses, with a primary goal of increasing forage yield. However, the effects of N fertilization on forage nutritive value, estimates of energy density, and in-vitro DM or NDF disappearance often have been inconsistent. Our objectives were to evaluate the effects of N fertilization on the nutritive value of a single cultivar (ForagePlus) of fall-grown oat fertilized at planting with 0, 20, 40, 60, 80, or 100 kg N/ha of urea or two rates of dairy slurry (42,300 or 84,600 L/ha). Nitrogen fertilization exhibited consistent effects on fiber components; forages fertilized with urea or dairy slurry had greater concentrations of fiber components compared to those harvested from unfertilized check plots, and fiber concentrations increased linearly with urea fertilization rate. In contrast, concentrations of water-soluble carbohydrates were greatest for unfertilized forages (21.2%), but declined linearly with urea fertilization, exhibiting a minimum of 13.5% at the 80 kg N/ha urea application rate. Similarly, non-fiber carbohydrate (NFC) also declined linearly from 34.8% for unfertilized check plots to a minimum of 24.6% at the 80 kg N/ha urea application rate. Fertilization with urea resulted in consistent linear increases in crude protein (CP), neutral-detergent soluble CP, neutral-detergent insoluble CP, and acid-detergent insoluble CP; however, this partitioning of CP on the basis of association with specific fiber fractions could not be related to N fertilization when concentrations were expressed on a percentage of CP basis. The summative calculation of energy, expressed as total digestible nutrients (TDN) was closely related to N fertilization rate during both the 2013 (Y = -0.038 x + 72.2; P = 0.001; r2 = 0.961) and 2014 (Y = -0.040 x + 69.2; P = 0.021; r2 = 0.771) production years. Following 30- or 48-h incubations in buffered rumen fluid, in-vitro DM disappearance was greater for unfertilized forages compared to those fertilized with either urea or dairy slurry, and disappearance declined linearly with urea fertilization rate; however, these responses were not detected for NDF disappearance. Overall, the forage nutritive value of fall-grown oat declined mildly in response to N fertilization, but these responses were not nearly strong enough to offset the advantages obtained by improved forage yields.