Regression equations can be used to estimate the preharvest neutral detergent fiber (NDF) concentration of temperate grasses and alfalfa. Our objective was to ...
Preharvest Neutral Detergent Fiber Concentration of Temperate Grasses as Influenced by Stubble Height Keenan Introduction Regression equations can be used to estimate the preharvest neutral detergent fiber (NDF) concentration of temperate grasses and alfalfa. Our objective was to develop NDF equations for grasses that incorporate stubble height. Stands of first-cut orchardgrass, reed canarygrass, and timothy were sampled in producers’ fields in New York in 2008, and cut into 5-cm segments. Predictive NDF and dry matter (DM) equations were developed for each segment. Equations describing the change, with grass height, in the contribution of each segment to the overall DM were used to develop equations for predicting wholeplant NDF.
Materials and Methods
Stands of first-cut orchardgrass, reed canarygrass, tall fescue, timothy and alfalfa-timothy mixed stands were sampled in the spring at experimental sites and producers’ fields in Tompkins and Cortland Counties of New York State in May 2009. Stands were sampled when alfalfa or grass height reached or exceeded 14” and a range of plant measurements and environmental characteristics were recorded. A total of 113 field samples (43 mixed, 16 orchardgrass, 26 reed canarygrass, 100 timothy, and 40 tall fescue) were cut a field height of 2” above ground level. Mixed samples were separated into alfalfa and grass fractions. Samples were then chopped into four 2” segments and plant tops using a modified paper cutter. Subsamples were then oven dried at 60°C, then ground and analyzed for NDF concentration.
1 McRoberts ,
2 Parsons ,
1 Cherney
David and Jerry H. 1 2 Cornell University , University of Tasmania Results Table 1. For vertically-sectioned grass, DM regressions used maximum tiller height (GMAXHT) or canopy height (GCANOPY) as explanatory variables, next with the quadratic term added, and also with both terms in the model. Height is in cm, with r2 values shown. All Grass Species height 5 10 15 20 25+
2
2
n GMAXHT GMAXHT GCANOPY GCANOPY 0.77 70 0.72 0.76 0.75 70 0.78 0.80 0.82 0.78 0.81 70 0.81 0.81 0.81 70 0.81 0.71 0.78 0.71 0.83 70 0.83 0.83 0.83
GCANOPY GMAXHT 0.79 0.84 0.87 0.75 0.88
Table 2. For vertically-sectioned grass, NDF regressions used maximum tiller height (GMAXHT) or canopy height (GCANOPY) as explanatory variables, next with the quadratic term added, and also with both terms in the model. Height is in cm, with r2 values shown. Some species, like orchardgrass shown here, had GCANOPY reasonable results.
Height 5 10 15 20 25+
n GMAXHT GCANOPY GMAXHT^2 GCANOPY^2 16 0.67 0.71 0.74 0.74 16 0.76 0.82 0.78 0.82 16 0.75 0.76 0.86 0.86 16 0.77 0.77 0.85 0.85 16 0.71 0.74 0.89 0.90
GMAXHT 0.76 0.84 0.87 0.87 0.89
Discussion 2 r
The values for estimating DM ranged from 0.56 to 0.93, depending on the species, segment, and explanatory variables used. For DM the results were very similar for all species examined (Table 1), and r2 values for multi-species equations ranged from 0.72 to 0.88. The values for estimating NDF ranged from 0.67 to 0.90 for the individual segments, depending on the species, segment, and explanatory variables used. For NDF of sections, multi-species equations performed 2 relatively poorly with r values ranging from 0.49 to 0.62. Individual species, such as orchardgrass (Table 2) worked fine. 2 r
The r2 values for predicting whole-plant NDF ranged from 0.72 to 0.87 for orchardgrass, 0.61 to 0.69 for reed canarygrass, and 0.67 to 0.77 for timothy. It may be possible to generate simplified prediction equations using stubble height as one of the variables, but more testing is needed. It is also possible that predictive equations including stubble height as a variable will need to be grass species-specific. Development of improved alfalfa-grass predictive equations for NDF will depend on improvements in equations for the grass component of the mixed stand.
Conclusion With further model evaluation the equations could be used by New York producers to obtain a more accurate estimate of preharvest NDF concentration in grass stands, as well as mixed stands. A hoop was used to select the sampling area.
Modified paper cutter used for cutting samples into 2” sections
