Nitrogen Fertilization Rate and Application Timing Effects on the Nutritive Value and Digestibility of Crabgrass

Chris D. Teutsch, Assistant Professor, Southern Piedmont Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, Blackstone 23824; John H. Fike, Assistant Professor, Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061; and William M. Tilson, Research Associate, Southern Piedmont Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, Blackstone 23824

Abstract

Crabgrass (Digitaria species) could provide high quality grazing for ruminant livestock in the mid-Atlantic region of the United States. However, little is known about managing crabgrass as forage in this region. This study was designed to evaluate the effect of N fertilizer rate and application timing on the nutritive value of crabgrass. Nitrogen rates ranging from 0 to 400 lb/acre were applied as a single application at seeding or as a split application, one-half at seeding and one-half after the first cutting. Crude protein concentration increased with N fertilization. In contrast, the effect of N fertilization on crabgrass fiber digestibility was variable and appeared to be linked to weather patterns and other factors. Results of the current study clearly indicate that crabgrass is highly digestible, possessing IVTD values in the range of 75 to 90%. Crabgrass could provide excellent quality forage during the summer months when cool-season grass growth is restricted by high temperature and intermittent rainfall. This species would likely be best for livestock classes with high nutritional requirements such as weaned calves, stockers, or lactating dairy cattle.

Introduction

Fig. 1. A volunteer crabgrass stand that was being utilized for summer grazing near Crewe, VA.

Crabgrass species (Digitaria species) are warm-season annual grasses that are commonly considered weeds due to their prolific growth rates and spreading morphologies. However, these species possess significant potential for supplying high quality summer forage for grazing livestock in the transition zone between subtropical and temperate regions of the United States (Fig. 1). Limited research has shown that improved crabgrass may possess in vitro dry matter digestibility concentrations in the range of 75% and crude protein (CP) values of 15% (7). In addition, average daily gains for stocker cattle grazing medium quality crabgrass averaged 1.9 lb/day (8).

Although crabgrass is already a component of most cool- and warm-season pastures in the Upper South (4), no research has evaluated its potential as summer forage in that region. Information on the nutritive value and digestibility of crabgrass in this region is not available. This study was designed to evaluate the effect of N fertilization rate and application timing on the nutritive value and digestibility of improved crabgrass.

Trials Using Nine Nitrogen Rates and Two Application Timings over Three Years

‘Red River’ crabgrass [Digitaria ciliaris (Retz.) Koel] was established on 7 May 2001, 16 April 2002, and 15 April of 2003 near Blackstone, VA. The soil series for all three years was a Wedowee sandy loam (fine kaolinitic, thermic Typic Kanhapludults). Initial soil nutrient levels are shown in Table 1. A conventional seedbed was prepared and plots were seeded using a cultipacker-type seeder and a seeding rate of 6.0 lb/acre. Nitrogen was applied at 0, 50, 100, 150, 200, 250, 300, 350, and 400 lb/acre either as a single application of ammonium nitrate at seeding or as a split application, one-half at seeding and one-half after the first cutting. Nitrogen treatments applied at seeding were incorporated into the seedbed by disking once with a finishing disk. All plots also received 100 lb/acre of P₂O₅ and 300 lb/acre of K₂O before seeding.

Table 1. Soil nutrient levels (lb/acre) and pH for 2001-03.

Year	pH	P^a	K	Ca	Mg
2001	6.0	33 (High^b)	47 (Medium-)	288 (Low+)	79 (High-)
2002	6.3	29 (High)	78 (Medium+)	343 (Low+)	95 (High)
2003	6.0	33 (High)	73 (Medium+)	256 (Low+)	62 (Medium+)

^a Mehlich I extract was utilized.

^b Soil Test Recommendations for Virginia (1994).

Bentazon [3-(1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] herbicide was applied at a rate of 0.50 lb ai/acre on 31 May 2001 and 0.75 lb ai/acre on 11 June 2001 for the control of yellow nutsedge (Cyperus esculentus L.). Bentazon and carfentrazone-ethyl {Ethyl alpha, 2-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-l-yl]-4-fluorobenzenepropanoate} herbicides were applied at a rate of 0.75 and 0.23 lb ai/acre, respectively, on 30 May 2002 and 6 June 2003 for broadleaf weed control.

The experimental design was a randomized complete block with a two-factor (N rate and single versus split application) factorial treatment arrangement and four replications. Plot size was 9 × 20 ft. Plots were harvested on 5 July and 31 August 2001; 11 July, 15 August, and 3 October 2002; and 11 July, 14 August, and 22 October 2003 by clipping a 4-ft-wide strip through the center of each plot using a self-propelled sickle bar-type forage harvester (Fig. 2). Harvest was initiated when the forage reached the late boot stage, except for the last harvest in each year, which was harvested at seed maturity to simulate natural reseeding of volunteer stands. The clipping height was 4 inches above the soil surface. A subsample of fresh forage was collected from each plot for determination of nutritive value and dried in a forced-air oven for 5 days at 140°F. The forage was then ground to pass through a #10 (2 mm) and #18 (1 mm) screen using Wiley (Thomas Scientific, Swedesboro, NJ) and Cyclone sample mills (Udy Corporation, Fort Collins, Co), respectively.

Fig. 2. Plots being harvested by clipping a 4-ft strip through the center of each plot in July, 2001.

Samples were analyzed for neutral detergent fiber (NDF), in vitro true digestibility (IVTD), neutral detergent fiber digestibility, and crude protein (CP) using near infrared spectroscopy. WINISI II software was used to select a calibration data set for wet chemistry determination (Infrasoft International, Port Matilda, PA). Neutral detergent fiber and IVTD were conducted using the ANKOM filter bag system (1). Neutral detergent fiber digestibility was calculated using NDF and IVTD values. Total N was determined using a modified Kjeldahl procedure (Technicon Auto Analyzer II, Industrial Method 334-74W/B, Tarrytown, NY). Crude protein was calculated as total N × 6.25.

Data were analyzed using the general linear model procedure from SAS (SAS Institute, Cary, NC). Only the N rate effect was considered for the first harvest since plots did not receive the split N application until after the first harvest. Regression analysis was performed on raw data using Sigma Plot 9.0 (Systat, Point Richmond, CA). In 2001, only two harvests were made and the second harvest was similar in maturity to the third harvest in 2002 and 2003. Therefore, the final harvest from each year was grouped for analysis and discussion.

Rainfall and Temperature Data for 2001, 2002, and 2003 Growing Seasons

Severe drought in 2002 and excessive rainfall in 2003 resulted in the driest and wettest growing seasons, respectively, ever recorded at the Southern Piedmont Agricultural Research and Extension Center, Blackstone, VA. Precipitation for the crabgrass growing season (May to September) was near normal in 2001, 6.5 inches below normal in 2002, and 20 inches above normal in 2003 (Fig. 3). The drier than normal conditions in 2002 were magnified by the extremely dry winter of 2001-2002. For the period of August 2001 to April 2002, rainfall was more than 14 inches below normal resulting in dry soil conditions in the spring of 2002. Temperatures for the growing season were on average normal for 2001, 1.9°F above normal for 2002, and 2.4°F below normal for 2003.

Fig. 3. Rainfall for the 2001, 2002, and 2003 growing seasons.

Nitrogen Fertilizer Rate and Application Timing Effects on Nutritive Value

Analysis of variance combined over years indicated significant year × treatment interactions (P < 0.05) for nutritive value parameters; therefore, data are presented by year. Significant N rate × application timing interactions occurred for CP in all harvests and years (P < 0.05). Interactions also occurred for second harvest NDF, IVTD, and NDF digestibility, and final harvest NDF in 2003. However, these interactions were due to changes in magnitude; therefore main effects will be presented for all nutritive value attributes other than CP.

Neutral detergent fiber. First-harvest NDF increased with N fertilization in 2001 and 2003, but not in 2002 (Fig. 4). Lower NDF with increased N in 2002 may have resulted from lower-than-normal soil moisture (15). Similarly, NDF also decreased with increased N at the second harvest in 2002. In 2003, N rate had limited effect on second-harvest NDF (Fig. 4). Final-harvest NDF was negatively related to N rate in all three years (P < 0.03), but the magnitude of this decrease was relatively small in 2002 and 2003. In 2001, NDF at final harvest was approximately 5 percentage units lower for the highest N rate compared to the no-N control (Fig. 4). At final harvest (seed maturity), NDF averaged approximately 61%, which is considerably lower than for other commonly used summer annual grasses and bermudagrass harvested at similar maturities (2). Splitting the N application had variable effects on NDF concentrations, but these differences were relatively small (<2%) (data not shown).

Fig. 4. Effect of N fertilization rate averaged over application timings on the neutral detergent fiber concentration of crabgrass forage for the first, second, and final harvests.

In vitro true digestibility. First harvest IVTD was negatively related to N fertilization rate in 2001 and 2003 (Fig. 5). In 2002, first-harvest IVTD increased with N rate (Fig. 5). The differences in trends were similar to those observed for NDF and were also likely influenced by available soil moisture (15). Second-harvest IVTD concentrations increased with N fertilization rate (Fig. 5). Final-harvest IVTD increased over the range of N treatments in a linear manner in 2001 and quadratic manner in 2002 (Fig. 5). Response was linear and non-significant in 2003. Overall, N application timing had limited impact on IVTD (< 3%) (data not shown). Delaying harvest until seed maturity resulted in an IVTD of approximately 78% for the final harvest, which was 8 percentage units less than for the first and second harvests.

Fig. 5. Effect of N fertilization rate averaged over application timings on the in vitro true digestibility of crabgrass forage for the first, second, and final harvests.

Neutral detergent fiber digestibility. First-harvest fiber digestibility decreased as N fertilization rate increased in 2001 and 2003 (Fig. 6). In 2002, first-harvest NDF digestibility increased in a manner similar to IVTD (Fig. 5). With the exception of the final harvest in 2003, second- and final-harvest NDF digestibilities increased with N fertilization rate (Fig. 6). Second-harvest NDF digestibility was approximately 6 percentage units lower (2003 only) when N was applied as a split application (data not shown). In contrast, final-harvest values tended to be higher for the split N application (data not shown). However, the magnitude of differences associated with the application timing was generally small (< 3 percentage units). Compared with first and second harvests, fiber digestibility was approximately 12 percentage units less for the final harvest. This indicates that as the plants neared seed maturity, changes were taking place in the fiber portion of the plant which rendered it less digestible. Even so, more than 60% of the NDF was digestible at seed maturity.

Fig. 6. Effect of N fertilization rate averaged over application timings on the neutral detergent fiber digestibility of crabgrass forage for the first, second, and final harvests.

Relationship between NDF and NDF digestibility. The relationship between crabgrass NDF levels and NDF digestibility was mixed. Increasing levels of NDF were always negatively correlated with NDF (and total plant) digestibility (Fig. 7). However, these relationships were poor (low r²) at most harvests in years with adequate rainfall (2001 and 2003). The one exception was the final harvest in 2001 (Fig. 7), when plants had been subject to a period of decreasing moisture availability (Fig. 3). Similar to the final harvest in 2001, fiber concentration and fiber digestibility had a much stronger relationship (greater slope coefficient) for all harvests in 2002 when moisture was limiting (Fig. 7). For each harvest among years, the slopes of the regressions in 2002 were at least twice those of the slopes for 2001 and 2003. These data, together with the lower levels of NDF in 2002, fit with the idea that in dry years, leaf:stem ratios are higher in crabgrass. Further, as fiber increases with dry growing conditions, it is likely more lignified and thus less digestible. The stronger relationship (larger slope coefficient and higher r²) between NDF level and NDF digestibility suggests that as canopies are more vegetative in nature, they are also more sensitive to small changes in fiber chemistry.

Fig. 7. Relationship of NDF and NDF digestibility for the first, second, and final harvests in 2001, 2002, and 2003.

Crude protein. Crude protein concentrations for the first harvest ranged from 10 to 17%, 5.5 to 15%, and 5 to 10.0% for the 2001, 2002, and 2003 growing seasons, respectively and increased with N fertilization (Fig. 8). Second harvest CP also increased with N fertilization. The increasing trend in CP with N fertilization is similar to observations of other researchers (10,11). Although a significant rate × timing interaction occurred for the second harvest, overall differences between the single and split application were relatively small (Fig. 9). In 2002 and 2003, final harvest crude protein increased at a greater rate when the N rates were applied as a split application (Fig. 10). In 2001, this trend was reversed. A quadratic response in which the no-N control had a greater CP concentration than plots receiving moderate rates of N was observed for a number of harvest-year combinations (Figs. 6, 7, and 8). This response may have been due to the fact that plots receiving little or no N produced much less total growth, but that growth tended to be dominated by leaf material (14).

Fig. 8. Effect of N fertilization rate on the CP concentration of crabgrass forage for the first harvest.

Fig. 9. Effect of N fertilization rate and application timing on the CP concentration of crabgrass forage for the second harvest.

Fig. 10. Effect of N fertilization rate and application timing on the CP concentration of crabgrass forage for the final harvest.

Nitrogen Management for Crabgrass

Total dry matter production, averaged over years, ranged from 3000 to 9000 lb/acre and increased as N rate increased with maximum dry matter production occurring at 305 lb of N per acre (14). In contrast, the effect of N rate and application timing on forage digestibility and fiber concentration varied. In wetter years, higher N rates appeared to stimulate upright growth and alter the leaf-to-stem ratio in favor of less digestible stems at the first harvest (5,6) (Fig. 11). This tended to increase the NDF concentrations of the plant while decreasing both NDF and total plant digestibility. In contrast, higher N rates appeared to delay plant maturity for later harvests and in turn increase plant digestibility (6,13) (Fig. 12). In 2002, dry conditions appeared to reverse trends in digestibility and fiber concentrations for the first harvest; increasing N rates resulted in higher first-harvest digestibility. Decreased lignification of plant fibers with N fertilization has been reported (3) and may have been a factor in these results along with a delay in plant maturity (5).


Fig. 11. Early in the growing season higher rates of N stimulated upright growth altering the leaf to stem ratio of crabgrass as shown in this photograph.		Fig. 12. Higher N rates delayed plant maturity at the final harvest as indicated by the greener coloration of plots shown in this photograph.

Although splitting annual N applications for summer annual forages is widely advocated, little data are available to validate this practice (9). In the current study, applying one-half of the N at seeding and one-half after the first harvest resulted in differences in second and final harvest fiber concentration and digestibilities, but the magnitude of these differences was relatively small and in some cases splitting the N application negatively impacted forage quality. Overall, the results of the current study agree with past research that the effect of N fertilization on digestibility and fiber concentration are variable and the causes are complex (12,15).

Conclusions

The effect of N fertilization on crabgrass digestibility was inconsistent and appeared to be linked to weather patterns and other factors. However, the results of the current study clearly indicate that crabgrass is highly digestible and could provide excellent quality forage during the summer months when cool-season grass growth is restricted by high temperature and intermittent rainfall. The use of crabgrass is most likely best for livestock classes with high nutritional requirements such as weaned calves, stockers, or dairy animals. Since crabgrass has a relatively short growing season, more work is needed to evaluate the yield and nutritive value of potential crabgrass-winter annual double cropping systems for use in the mid-Atlantic region of the United States.

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