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Relationship between pasture nutritive measurements and plasma urea nitrogen in lambs grazing silvopasture or open pasture
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 indicator of animal nitrogen status, increasing with excessive dietary nitrogen, resulting in greater urinary N excretion. Analytical procedures utilized to estimate herbage energy content can be laborious and expensive. Our objective was to evaluate the relationship between herbage CP, and 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; initial weight 28.7 ± 2.1 kg; final weight 41.4 ± 2.9 kg), grazing either open or silvopasture over 4 consecutive grazing seasons. Grazing began in mid-April each year on cool-season mixed pastures and concluded mid-September. Forage nutritive value was determined from clipped samples taken the day before grazing events. Herbage TNC was determined directly, while TDN was estimated from ME (NRC, 1996) via ADF (MAFF/ADAS, 1987). Lambs grazed fresh paddocks (minimum 35 d regrowth after initial grazing) for 2 h and held an additional 1 h in drylot prior to blood draw. We 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. All nutritive components except TDN 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 (-), 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 r > 0.8, 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.
Keywords: Silvopasture, plasma urea nitrogen, nutritive value, total non-structural carbohydrate