Nitrogen metabolism in beef steers fed fodder beet ad libitum

Fodder beet has low concentrations of crude protein, fibre and minerals however, high water soluble carbohydrates and metabolisable energy (Gibbs, 2011). Despite low crude protein concentrations in fodder beet compared to winter pasture, Prendergast and Gibbs, (2015) found that feeding fodder beet to beef steers ad libitum resulted in a significantly higher microbial protein production compared to beef steers fed winter pasture. Additionally, microbial protein efficiency (g N/kg DMI) was higher on the fodder-beet diet compared to the winter pasture diet (15.5 vs. 12.5 g N/kg DMI, respectively) (Prendergast and Gibbs, 2015).

The steers on the fodder beet had a twofold increase in urine volumes excreted which indicates an increased water loading of the rumen, due to the low dry matter content and high dry matter intake. This indicates a potential increase in passage rate of digesta through the rumen which has been shown to increase microbial protein reaching the small intestine due to a reduction in rumen retention time resulting in less exposure of microbes to rumen digestion and decreased recycling of microbial N within the rumen (Pathak, 2008). However, as the rumen fluid passage rates were not significant between the treatments, Prendergast and Gibbs, (2015) suggested that the increased dietary water leaving the rumen was being absorbed across the rumen wall. This is the first reported observation of this in winter-crop fed cattle and should this be repeatable in future experiments, it suggests that there may be physiological adaptations beyond the rumen to grazing fodder beet to maintain high DMI and subsequent production (Prendergast and Gibbs, 2015). 

The voluntary dry matter intake was 36.1% higher for the beef steers grazing fodder beet and the increased in the daily microbial protein production was 84.5% (Prendergast and Gibbs, 2015). Therefore, although the increased dry matter intake may have accounted for some of the increase in microbial protein production, the characteristics of the fodder beet diet may have accounted for some of the increased microbial protein supply (Prendergast and Gibbs, 2015). The high supply of fermentable metabolisable energy in both diets support high microbial growth rates and suggest that microbes were not limited by energy intake. This indicates that the increased daily microbial protein production may be associated with greater rumen efficiency of N use and changes in N recycling in the rumen. This is supported by the high urea concentrations in the rumen relative to the ammonia concentrations observed in the beef steers fed fodder beet (Prendergast and Gibbs, 2015).
This study demonstrated that ad libitum diets of fodder beet resulted in significantly higher microbial protein supply and efficiency compared to diets of winter pasture which could have benefits for high growth rates and/or body condition gain. However, future research is required to confirm the observation reported by Prendergast and Gibbs, (2015) that there are physiological adaptations beyond the rumen involved when cattle graze fodder beet ab libitum. Further investigation regarding N metabolism when feeding fodder beet is warranted (Prendergast and Gibbs, 2015). 

REFERENCES

Gibbs, S. J. (2011). Wintering dairy cows on fodder beet. Conference Proceedings of the South Island Dairy Event. Lincoln, E.d. Lincoln University.

Pathak, A. K. (2008). Various factors affecting microbial protein synthesis in the rumen. Veterinary World 1(6), 186-189.

Prendergast, S. L. and Gibbs, S. J. (2015). A comparison of microbial protein synthesis in beef steers fed ad libitum winter ryegrass or fodder beet. Proceedings of the New Zealand Society of Animal Production 75, 251-256.