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Leaf traits (leaf dry matter content [LDMC], specific leaf area [SLA] and leaf life span [LLS]) previously proposed to predict plant strategies for resource use, were studied to test if they can be used to rank grasses for digestible organic matter (DOM). On 14 native grass species from natural meadows in the French Pyrenees, leaf blade chemical components (fiber, cellulose, hemicellulose and lignin) and DOM were estimated for two growing periods using two different methods (chemical-enzymatic and Near Infrared Reflectance Spectroscopy). The ranking of species based on LDMC, SLA and LLS was conserved. Fiber content and DOM were significantly correlated even though the data were obtained in different years (2001 and 2002), on different organs (youngest adult blades in 2001 and all the green blades of tillers in 2002) and by different analytical methods. LDMC seems to be the most suitable trait to rank native grasses according to their nutritive value because it ranks species as well as leaf traits and it is the easiest to measure. We suggest using LDMC as an indicator to rank grassland communities for herbage nutritive values. Resumen Tres atributos foliares, el contenido en materia seca (LDMC), el á rea foliar específica (SLA) y la longevidad foliar (LLS) fueron estudiados sobre 14 gramíneas nativas de praderas situadas en los Pirineos franceses para determinar si dichos atributos las clasifican en el mismo orden que variables de valor nutritivo. Para esto, la composició n química (fibra, celulosa, hemi-celulosa y lignina) y la digestibilidad de la materia seca (DOM) de las laminas foliares fueron estimados durante dos períodos de crecimiento utilizando métodos diferentes (químico-enzimá tico y espectroscopia o NIRS). LDMC, LLS y SLA clasifican las especies de igual manera en que lo hacen la proporció n de fibras y la DOM. La composició n de tejidos foliares y la DOM mostraron une correlació n significativa aú n cuando los datos fueron obtenidos con métodos analíticos diferentes, en añ os diferentes y sobre muestras de ó rganos diferentes (la lá mina de la hoja adulta mas joven en 2001 y todas las láminas verdes en 2002). La LDMC se presenta como el mejor atributo para clasificar las especies por su valor nutritivo pues las ordena como los otros y su medició n es má s simple. Este atributo puede ser usado para clasificar comunidades herbáceas naturales segú n su valor nutritivo.

Tannins are secondary plant compounds which have been extensively studied in order to improve the nitrogen use efficiency (NUE) of ruminants. A meta‐analysis was performed of 58 in vivo experiments comparing milk yield, composition and nitrogen metabolism of lactating dairy cows fed diets with or without tannins. The meta‐analysis shows that tannins have no impact on corrected milk yield, fat and protein content or NUE (p > .05). However, tannins reduce ruminal ammoniacal nitrogen (N) production by 16% (from 10.95 to 8.47 mg/dl on average), milk urea by 9% (from 15.82 to 14.03 mg/dl) and urinary N excretion (−11%; p < .05). This is compensated for by a lower apparent N digestibility (61.51% with dietary tannins compared to 66.17% without). The effect of tannin on N metabolism parameters increases with tannin dose (p < .05). The shift from urinary to faecal N may be beneficial for environment preservation, as urinary N induces more harmful emissions than faecal N. From a farmer's perspective, tannins seem unable to increase fat‐ and protein‐corrected milk yield or reduce feed protein requirements and thus have no direct economic benefit. Potentially less costly than tannin extracts, forage or by‐products naturally rich in tannins could still be useful to reduce the environmental impact of ruminant protein feeding.

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