Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2007
Publication Date: 10/15/2007
Citation: Mcginley, B.C., Coffey, K.P., Coblentz, W.K., Galdamez-Cabrera, N.W., Turner, J.E., Daniels, M.B. 2007. In Situ Ruminal Nitrogen and Neutral-Detergent Insoluble Nitrogen Disappearance from Bermudagrass Fertilized with Different Nitrogen Rates and Harvested on Two Dates. Professional Animal Scientist. 23:556-564. Interpretive Summary: Recent nutritional models require in-depth knowledge of forage protein characteristics for proper use. Specifically, nutritionists require estimates of rumen protein degradability, as well as an understanding of how the total crude protein pool is partitioned within the cell solubles or the cell wall. In general, southern forages have been characterized incompletely in this respect, and the effects of simple management practices, such as nitrogen fertilization, on these protein fractions is unclear. In this study, nitrogen fertilization increased concentrations of crude protein, as well as crude protein bound or associated specifically with the cell wall. However, most of the nitrogen recovered from fertilization was partitioned into the highly-digestible cell-soluble pool. Overall, nitrogen fertilization appears to increase concentrations of crude protein in bermudagrass forages. In addition, the proportion of that crude protein degraded in the rumen also is increased; this potentially reduces the need for supplemental rumen degradable crude protein sources in the diet.
Technical Abstract: Fertilizing bermudagrass (Cynodon dactylon L. Pers.) with N can produce large quantities of forage and increase plant N concentrations. Bermudagrass growing on a caged-layer manure-amended site was fertilized with ammonium nitrate at four rates (0, 56, 112, and 168 kg N/ha) approximately one month before first and third harvests on May 30 and August 18, 2000 to determine in situ disappearance kinetics of N and neutral detergent insoluble N (NDIN). Five crossbred ruminally cannulated steers (BW = 422 ± 21.0 kg) were used to evaluate in situ disappearance kinetics in a randomized complete block design with a 2 4 factorial arrangement. Immediately soluble N (Fraction A) and rate of N disappearance increased linearly (P<0.05) with fertilization rate for both harvests, but N disappearing at a measurable rate (Fraction B) decreased linearly (P<0.01) with fertilization rate for both harvests. A linear N fertilization rate harvest date interaction (P<0.05) was detected for the unavailable N fraction (fraction C) and for potential extent of ruminal N disappearance. Effective ruminal N disappearance (% of N) increased quadratically (P<0.01) with N fertilization rate. The intercept of the regression of effective N disappearance on N fertilization rate was greater (P<0.05) for bermudagrass harvested in May compared with that harvested in August. For NDIN, quadratic N fertilization rate harvest date interactions (P<0.05) were detected for fractions A and C and the potential extent of NDIN disappearance; rate of NDIN disappearance did not differ (P=0.17) across harvest dates or fertilization rates. Effective NDIN disappearance (% of NDIN) increased linearly (P<0.01) with N fertilization rate. The pool of total N (% of DM) effectively disappearing was composed largely of neutral-detergent soluble N (NDSN; 75.1 and 66.6 %, respectively from May and August harvests). Effective NDSN and NDIN disappearance (% of DM) increased quadratically and linearly (P<0.01), respectively, with N fertilization rate, and these NDSN and NDIN pools were larger (P<0.01) from bermudagrass harvested in May. Nitrogen fertilization may enhance the ruminal disappearance of both total N and NDIN from bermudagrass. On a percentage of DM basis, later harvests may have less extensive ruminal disappearance from both NDIN and NDSN pools.