assuming a constant value of the particle turnover K=0.06/h. Genetic Variation for In situ Kinetics of Protein Degradation in Alfalfa.
F. Guines, B. Julier, and C. Huyghe
INRA, UGAPF, Lusignan, France
Alfalfa is a protein-rich forage, but the proteins are extensively degraded in the rumen of ruminants (Hristov, 1992). This extensive degradation leads to decrease protein efficiency and contributes to an excessive formation of ammonia and may cause bloat. As a consequence, excessive protein degradation may be the most limiting nutritional factor in high-quality temperate forages legumes (Broderick, 1995).
The aim of this experiment was to determine if the in situ protein degradation varied genetically among 16 cultivars chosen for their contrasted digestibility and protein content. Nylon bags containing each cultivar were placed in the rumen of three fistulated cows for 2 to 72 hours of incubation. Nitrogen content from the residue of the degradation was determined using the Kjeldhal method. The percentage of disappearance of dry matter and nitrogen were calculated at each time of incubation from the proportion remaining after incubation in the rumen. Nitrogen and dry matter disappearance curves were adjusted to an exponential model: p = a + b (1-exp-ct) (Orskov and Mc Donald, 1979). This model includes 3 fractions in the forage: one rapidly degraded fraction (a), one with slower degradation (b) at a rate reducing exponentially (exp-ct) and one non-degradable fraction (100-a-b). The effective degradability was calculated using the equation of Orskov and Mc Donald (1979): assuming a constant value of the particle turnover K=0.06/h.
Fig 1
: Nitrogen degradation curves for 15 alfalfa cultivars.Degraded nitrogen ranged from 83.16% to 89.49 % after 72 hours of incubation in the rumen.
The analysis of variance showed significant differences among cultivars. Cultivars with higher protein content had an higher protein degradation. Cultivars were also significatively different for all parameters of the degradation kinetics of nitrogen and dry matter. The effective nitrogen degradability ranged from 63.36% (cultivar 9) to 70.27% (cultivar 3). Degraded nitrogen was positively correlated to in sacco dry matter degradation after 14 hours of incubation (0.81***), meaning that reducing protein degradation would also reduce dry matter degradation.
References
Broderick, G.A. 1995. Desirable characteristics of forage legumes for improving protein utilization in ruminants. J. Anim. Sci. 73:2760-2773.
Hristov, A.N. 1992. Effect of sample pre-treatment on alfalfa silage dry matter and protein degradability in sacco. Anim. Feed. Sci. Technol. 71:351-355.
Orskov, E.R. and Mc Donald, I. 1979. The estimation of protein degradability in the rumen from incubation measurements weighted according to rate of passage. J. Agric. Sci., Camb. 92:499-503.