Journal of the NACAA
ISSN 2158-9429
Volume 13, Issue 1 - June, 2020


Evaluating Planting Date for Interseeding Cover Crops Into Corn for Silage

Hogge, J. , Area Extension Educator, University Of Idaho
Hines. S, Extension Educator, University of Idaho
Packham, J , Extension Educator, University of Idaho
Willmore, C, Extension Educator, University of Idaho


Interseeding cover crop by aircraft into standing corn to provide for fall/winter grazing is a practice gaining interest in southcentral Idaho based on grower input. University of Idaho Extension faculty cooperating with a dairy producer studied the efficacy and best timing to plant a cover crop into corn. The questions guiding the research were: 1) what is the correct crop stage to interseed cover crop into standing corn to provide late-season grazing; and 2) will interseeding cover crop reduce corn silage yield? The study was replicated in 2017, and 2018 with the only researcher-controlled variable being planting date. Results indicate there was no statistical difference in yield between planting date treatments in 2017 (P=0.43), but there was a statistical difference in 2018 (P=0.03). The cover crop was clipped and weighed in October of each year to determine available forage. The results show that planting the cover crop at approximately corn stage V6 gives the best opportunity for establishment before the corn closes the canopy.


Cover crops can provide growers with many benefits including, erosion control, reduced offsite movement of nutrients, support for the development of soil organic matter and, in some cases, additional revenue such as forage or oilseeds (Wells et al., 2018). In the United States, only 5.6% of the total available cropland is planted to cover crops. In Idaho, the percentage is even less, 4.4% (LaRose & Myers, 2019). It is essential that Extension faculty work with growers to find ways to increase the number of cover crop acres in the US. There are many reasons why producers aren’t using cover crops, but one of the most obvious is a reduced growing season, which creates many challenges when trying to integrate cover crops (Wells et al., 2018). Interseeding cover crops during the corn growing season can address some of the potential challenges associated with shorter growing seasons (Wells et al., 2018).

Much of the available research is conducted in the corn/soybean growing regions of the United States, where the crop is rainfed, and the corn is harvested for grain. A quick internet search reveals large numbers of farm trials, on-farm demonstrations, and research trials from those regions. There is very little research data from the Western part of the country where corn is irrigated, and much of it harvested for silage. Farmers in a six-county region of Idaho, locally known as the Magic Valley, grow 212,500 acres of corn, and 73% of that is harvested for silage (USDA NAS, 2019). Interest in interseeding cover crop into standing corn is growing in the region; however, there is some apprehension by growers as cover cropping, in general, is a relatively new practice in the area. The topography and associated climate of the region is a significant concern when applying recommendations for specific methods from the other agricultural regions of the country. The elevation of the farming area in the Magic Valley is 2,800 feet elevation on the western edge, to 5,200 feet elevation on the eastern border. Planting cover crops after harvest of silage corn, potatoes, or sugar beets can be very risky, due to short water, shortening days, and dropping temperatures. Currently, no producers are planting after late harvests due to these factors.


University of Idaho Extension faculty partnered with a dairy producer to research the ideal time for interseeding cover crops into standing corn. Most studies show minimal corn yield impact with interseeding cover crops during the V5-V7 growth stages (Rees, et al., 2019). The producer has both dairy and beef cattle and was looking for a way to use cover crops to extend the grazing season after silage corn was harvested off several pivots on the farm. The farm used in the study lies at high elevation (4,800’) in a treeless area where the wind is significant during the winter and spring. By the time corn silage is harvested in September, the days are shortening, and heat unit accumulation decreases significantly. Planting a cover crop for late-season grazing under such conditions is not a protocol for success. For this reason, a two-year study was designed and conducted on the diary farm located near Raft River, Idaho.



The research team established the study on a 20-acre half-pivot field (Figure 1). The study design was a randomized complete block with five treatments and four replications in 2017 and reduced to four treatments in 2018. Treatment 2 was not replicated in 2018 because the treatment was planted before the glyphosate herbicide was applied, and the weed pressure was unacceptable. The plots were 22 feet wide (12 rows of corn on 22-inch centers) in 2017 and 18.33 feet wide (10 rows of corn on 22” centers) in 2018 and ranged in length from 355-392 feet both years. The variation in length was due to the curvature of the pivot tracks, which delineated the study area (Figure 1). The plots ranged from 0.15-0.16 acre. The cutting head on the forage harvester was 20 feet wide, so that established the harvested area from each plot. Ten rows of each 12-row plot were harvested for data in 2017. The forage harvester, a John Deere 8600, and heavy ten-wheel semis had to travel back over the study area to clean up the unharvested rows. The wheel damage to the study area was excessive and likely affected the amount of forage available for clipping in October of 2017. In 2018, 10-row plots were used, so the harvester took all the material in one pass, thus reducing additional wheel traffic.


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Figure 1. Approximate plot layout on 20-acre ½ pivot field near Raft River, Idaho.


The researchers controlled only the planting date of the cover crop. The field was prepared, fertilized, and planted by the producer. The plots were marked out after planting. Herbicide treatments and irrigation schedules were controlled by the producer using standard agronomic practices. The producer chose the corn variety and the cover crop mix, which he had flown on to other pivots on the farm. The producer also chose the harvest dates. Table 1 shows treatments and planting dates.


Table 1. Treatments and Planting Dates.


Planting Date 2017

Planting Date 2018

1-No cover crop check



2-Plant same day as corn planted

May 18


3-Plant ~ 30 days after planting (V6)

June 15

June 20

4-Plant ~ 60 days after planting (V12+)

July 14

July 17

5-Plant ~ 90 days after planting (~R3)

August 7

August 15


The seeding rate was 24 lbs/acre for the cover crop treatments. Aerial seeding is the most practical method for interseeding and was simulated using the Earthway® Model 2750 hand spreaders. The spreaders were calibrated by determining distance walked in 30 seconds and then determining the weight of captured seed by manipulating the spreader for 30 seconds. It is likely more seed reached the ground using hand spreaders over aerial applications due to spreading below leaf whorls, especially in treatments 4 and 5. The producer wanted to use a lower rate, but it was impractical with the chosen spreader as the seed mix would bridge in the opening stopping the flow. The cover crop mixes were changed from 2017 to 2018 because the producer and his consultants provided the seed mixes. The mixes were commercially applied to other corn fields on the farm, and the producer wanted the data to reflect what he was actually doing each year on his farm..The seed mixes used in the study are shown in Tables 2 and 3.


Table 2. Seed mix used in 2017.

CoverGraze Mix #2 (Barenbrug®)-2017

Cover Crop Species


Cayuse Oat


Barextra Annual Ryegrass


Barsudan Sorghum-Sudan grass


Berseem Clover




Montezuma Oats


Barprisma Italian Ryegrass


White grain Sorghum


Daikon Radish


Barmultra II Annual Ryegrass


Other crop, inter matter, weeds


Yellow Jacket Coating



Table 3. Seed mix used in 2018.

NRCS Mix (Agrisource, Inc®)-2018

Cover Crop Species


Green Spirt Italian Ryegrass


Medium Red Clover


Nitro Radish


Barkant Turnip


T-Raptor Hybrid Brassica



The mixture used in 2017 contained 40.80% of Yellow Jacket Coating®. The coating is a clay-like material to add weight to the seed for aerial application and, more specifically, to aide the seed in absorbing and holding moisture. The actual seed rate applied with the coating was approximately 14 lbs., much closer to the 12 lb. rate the producer wanted.




The study was harvested by chopping the entire plot into a single 10-wheel truck using a John Deere 8600 forage harvester with an 18’ all-crop head. The trucks drove to the silage piling area to be weighed, so each plot was considered individually. Harvest dates were September 27th in 2017 and September 17th in 2018. The yield and quality data were evaluated using SAS 9.4 PROC GLM. A sample of each truckload of the material was collected and assessed for quality. Yield data are shown in Figure 2, and the quality data are shown in Figures 3 and 4.


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Figure 2. Silage yield for 2017and 2018.


There was no significant silage yield difference between treatments in 2017 (P=0.43). There was a statistical difference in silage yield between treatments in 2018 (P=0.03). The difference in 2018 silage yield was seen between treatment 4 and treatment 5. Treatment 5 had the lowest yield at 29.2 tons/acre, while treatment 4 had the highest yield at 30.7 tons/acre. Replication block 4 contained the lowest yielding rep for 3 of the 4 treatments indicating a possible agronomic issue in that part of the field.



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Figure 3. Silage quality data (CP,TDN)-2017. 


Figure-4-2.png (400×248)

Figure 4. Silage quality data (CP,TDN)-2018.


The silage quality data from 2017 (Figure 3) show significant differences in both Crude Protein (CP; P=0.13) and Total Digestible Nutrients (TDN; P=0.05). The lower TDN value in 2017 treatment 2 is attributed to the vast amount of kochia, redroot pigweed, and lambsquarters that grew to maturity due to no herbicide application in that set of plots. Inversely, treatment 2 had the highest CP values. The weed contribution outweighed any increase of CP feed quality, however. There was no significant difference in either CP (P=0.82) or TDN (P=0.56) in 2018. A goal of the project was to determine if the cover crop significantly affects the quality of corn silage. These data indicate there is no significant difference between the check (treatment 1) and other treatments in either year.  


The final part of the study was to take sample clippings of the cover crop regrowth to estimate yield. In 2017 cuttings were taken October 26th, and in 2018 October 12th, mimicking the approximate time, the producer would turn cows onto the field. Samples were collected by taking three clippings in each plot using a 1ft2 clipping ring. In 2017, treament 2 was planted two days after the corn. This situation made it impoossible to apply glyphosate to the plots within the treatment and the weeds completely took over the plots. Between the weeds and the corn, the cover crop mix had no chance to establish. Please note that treatment 2 was not repeated in 2018. Treatments 3 and 4 had enough material to clip. In 2018, only treatment 3 had enough material to clip, and the results are shown in Figure 5. The mixes contained several species (Tables 2 and 3), but only the ryegrass varieties produced enough material to make up any usable feed. The researchers did not conduct a species analysis of the clippings, but it is estimated that over 95% of the material was ryegrass in both 2017 and 2018 growing seasons.


Figure-5-2.png (360×211)

Figure 5. Yield results of clipped cover crop on dry matter basis-2017 and 2018.



The purpose of the study was to determine the best time to interseed cover crop into corn grown for silage and evaluate if cover crop interseeding would affect the corn yield. The results indicate seeding at approximately V6 (treatment 3) after herbicide treatment, but before the canopy closes, offers the best opportunity for the cover crop to establish and grow enough before the canopy shades the plants for the summer. Once the corn is removed from the field, the V6 planting was large enough to take advantage of the sunlight and remaining growing days to offer feed for the cattle. The findings of interseeding the mixes form years 1 and 2 at the V6 stage worked well in our study. The results could vary with other seed mixes and would need to be considered carefully before being used in a cover crop mix. 

The yield of one treatment was significantly affected in a single year of the study (treatment 5, 2018). In the real world of agriculture, a producer would have to decide if 1.5 tons is significant in their operation. The producer indicated that yields for both years of the study plots mirrored his yield results from the entire field. A definitive conclusion cannot be drawn, but data indicate corn silage yield was not affected overall due to cover crop competition.

Interseeding cover crops into corn grown for silage is a viable method for establishing additional forage for fall feeding of cattle with the added benefit of providing ground cover to help armor the soil. More research needs to be conducted to strengthen the findings of this study. The producer continues to interseed several pivots on his farm with success.



Literature Cited


J. LaRose, and R. Myers (2019). Progress Report: Adoption of Soil Health Systems Based on Data from the 2017 U.S. Census of Agriculture, Soil Health Institute. Accessed March 12, 2020.


Rees, J., R. Elmore, C. Proctor, S. Melvin, and M. Sindelar (2019). Interseeding Crop into Corn or Soybean, University of Nebraska Extension, Crop Watch, Accessed March 12, 2020.


United States Department of Agriculture-National Agricultural Statistics Service. (2019).

Idaho Annual Statistical Bulletin. Boise, ID: USDA-NASS Accessed March 3, 2020.


Wells, M.S., A. Hard, E. Ristau, and D. Nicolai (2018). Early Season Cover Crop Interseeding into Corn, University of Minnesota Extension, Accessed March 12, 2020.