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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.

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Simple Summary: For sustainable meat production, beef farmers must make the best use of grass and roughage while limiting the carbon footprint of their herds. The genetic improvement in feed efficiency and enteric methane production of replacement heifers is possible if the recorded phenotypes are available. Intuitively, the relationship between the two traits should be negative, i.e., favorable, since the energy lost with the methane is not available for heifer metabolism. The measurement of feed efficiency requires several weeks of feed intake recording. The enteric methane emission rate can also be recorded over several weeks. The two traits of 326 beef heifers from two experimental farms were measured simultaneously for 8 to 12 weeks. The correlations between roughage intake, daily gain, and methane were all positive. The enteric methane emission rate was positively related to body weight, daily gain, and dry matter intake. The relationship with feed efficiency was slightly positive, i.e., unfavorable. Therefore, the two traits should be recorded simultaneously to evidence low-emitting and efficient heifers. This study also showed that replacing the feed intake recording with the carbon dioxide emission rate appeared potentially beneficial for selecting these low-emitting and efficient heifers.Abstract: Reducing enteric methane production and improving the feed efficiency of heifers on roughage diets are important selection objectives for sustainable beef production. The objective of the current study was to assess the relationship between different methane production and feed efficiency criteria of beef heifers fed ad libitum roughage diets. A total of 326 Charolais heifers aged 22 months were controlled in two farms and fed either a grass silage (n = 252) or a natural meadow hay (n = 74) diet. Methane (CH 4 ) and carbon dioxide (CO 2 ) emission rates (g/day) were measured with GreenFeed systems. The dry matter intake (DMI), average daily gain (ADG), CH 4 and CO 2 were measured over 8 to 12 weeks. Positive correlations were observed among body weight, DMI, ADG, CH 4 and CO 2 . The residual feed intake (r wg DMI) was not related to CH 4 or residual methane (r wi CH 4 ). It was negatively correlated with methane yield (CH 4 /DMI): R p = −0.87 and −0.83. Residual gain (r w iADG) and ADG/DMI were weakly and positively related to residual methane (r wi CH 4 ): R p = 0.21 on average. The ratio ADG/CO 2 appeared to be a useful proxy of ADG/DMI (R p = 0.64 and 0.97) and CH 4 /CO 2 a proxy of methane yield (R p = 0.24 and 0.33) for selecting low-emitting and efficient heifers.

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.

Despite the high degradability of their proteins, grass and legume silages represent an important option to reach more sustainable livestock systems. To improve the nitrogen use efficiency of these crops, this study assessed the potential of several additives (chestnut tannins, oak tannins, zeolite, erythritol by‐product solution and wood molasses) to reduce proteolysis in the silo and in vitro nitrogen degradability. Ryegrass (Lolium multiflorum) and red clover (Trifolium pratense) were ensiled in varying proportions in laboratory‐scale silos made of vacuum‐packed plastic bags. Dry‐matter content, chemical composition, pH, ammonia and volatile fatty acids content were analysed after 34 days of ensiling. Ruminal nitrogen degradability was assessed in vitro (Aufrère & Cartailler, ). We observed that the proportion of ammonia in silage was reduced by the addition of oak tannin (−12%) and zeolite (−16%). The addition of zeolite lowered in vitro organic matter digestibility. Rapidly degradable nitrogen (1‐hr degradability) was reduced in vitro by both tannins (−6.8% for chestnut and −6.6% for oak) and zeolite (−5.8%), but total degradable nitrogen (24‐hr degradability) was only reduced by oak (−6.5%) and chestnut tannins (−7.3%). It suggests that tannins protected proteins from plant and bacterial enzymes by forming a complex that better resists silage fermentations and in vitro protease action. The reduction effects on proteolysis in the silo and on in vitro ruminal nitrogen degradability are limited individually but could be cumulative. Erythritol by‐product solution and wood molasses had no effect on silo or in vitro proteolysis.

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