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Title: Relationship between pasture nutritive measurements and plasma urea nitrogen in lambs grazing silvopasture or open pasture

item Neel, James
item BELESKY, DAVID - Retired ARS Employee

Submitted to: Journal of Animal Science
Publication Type: Proceedings
Publication Acceptance Date: 4/10/2014
Publication Date: 7/20/2014
Citation: Neel, J.P., Belesky, D.P. 2014. Relationship between pasture nutritive measurements and plasma urea nitrogen in lambs grazing silvopasture or open pasture. Journal of Animal Science. vol. 92,E-Suppl. 2.

Interpretive Summary: Abstract Only

Technical Abstract: The relationship of herbage energy content relative to crude protein (CP) is an important aspect in nitrogen use efficiency of grazing livestock. Plasma urea nitrogen (PUN) is an excellent indicator of animal nitrogen status, increasing when excessive nitrogen is available in the diet, and resulting in greater urinary N excretion. Most analytical procedures utilized to estimate herbage energy content are laborious and expensive. Our objective was to evaluate the relationship between herbage CP and two herbage energy content indicators of differing assessment cost (total non-structural carbohydrate – TNC; total digestible nutrients - TDN), with animal PUN. We utilized winter born lambs (n=187) grazing either open or silvopasture over 4 consecutive grazing seasons to study the relationships. Grazing began in mid-April each year on cool season mixed-pastures with an initial lamb weight of 28.7 ± 2.1 kg, and concluded mid-September with a final lamb weight of 41.4 ± 2.9. Prior to PUN sampling, lambs grazed fresh paddocks (minimum of 35 d regrowth after initial grazing) for 2 h and then were held an additional 1 h in drylot prior to blood draw. We then correlated (Pearson) the relationship of pasture nutritive measurements and PUN. After tallying across treatments and years, the number of correlation coefficients within the following categories: r > 0.5, > 0.6, > 0.7 or > 0.8, we evaluated the relationships. Except for TDN, all other nutritive components performed similarly using the r > 0.5 criteria. Within this grouping, the ratio of TDN:CP (a negative relationship, -) had the greatest total number of r values > 0.5 (16 of 22, or 73%), while TDN alone (-) had just 3 of 22 (14%). Using > 0.6 criteria, sampling date (+) and TNC:CP (-) were best, with 11 of 22 (50%) being greater. They were followed closely by TDN:CP (-) and TNC (-) having 10 of 22 (45%), and 9 of 22 (41%) respectively. Within the grouping r > 0.7, sampling date and TNC:CP remained highest at 9 of 22 (41%), while TDN:CP and TNC were 5 of 22 (23%) and 8 of 22 (36%), respectively. For the r > 0.8 group, both sampling date and TNC:CP still had approximately 25% of the correlation coefficients falling within this category. The use of TNC:CP appears to be a quick, economical, and useful tool to evaluate pasture energy status relative to crude protein.