Evaluating the benefits of adding rapeseed to oat pasture
By Mary Drewnoski and Devin A. Jakub
Cover crops planted after corn silage harvest, spring wheat or hybrid seed corn harvest provide an opportunity for grazing calves or cows in late fall and into winter. Winter-sensitive annuals such as oats with or without brassicas such as turnips or rapeseed are commonly used in the Midwest. These species are highly digestible and thus considered high energy with only slight decreases in digestibility throughout the winter1.
Similarly, crude protein (CP) content of late-summer planted oats and brassicas remains relatively constant throughout the winter grazing period1. The fact that these forages have high nutritive value, and the value is retained in to the winter makes them an idea fit for grazing calves post weaning in the fall.
Fall grazing an oat monoculture2 and oat-brassica mixes3 planted in late summer after corn silage harvest has resulted in average daily gain (ADG) of 2.25 to 2.35 pounds/day for growing calves. However, direct comparisons of oat monocultures to oat-brassica mix have not been conducted.
The low fiber and high sulfur content of brassicas suggests they should not be grazed in monoculture but can be a good compliment to mix with late-summer-planted oats1 and would be expected to increase the energy and protein content of the diet compared to oats alone.
Therefore, inclusion of rapeseed, a low-cost brassica, into a late fall and winter grazing system may improve growing calf gain compared to grazing oats, while also decreasing cost of gain.
The objective of this study was to evaluate the inclusion of a brassica (rapeseed) with late summer planted oats and the effect on forage yield, forage quality, and calf gain. It was hypothesized that forage yield of the oats-rapeseed mix would be similar to the oats monoculture and that growing calf performance and cost of gain would be improved by inclusion of rapeseed.
Experimental procedure This three-year study was conducted at the U.S. Meat Animal Research Center near Clay Center, Nebraska. Following corn silage harvest (n = 3) or alfalfa termination (n =2), five irrigated pivots were divided into four quarters and planted.
Two quarters from each pivot were planted with 100 lb/ac of oat seed (Avena sativa; OAT) while the other two quarters were planted with 50 lb/ac of oat seed and 3 lb/ac of rapeseed (Brassica napus; MIX).
For the pivots that were planted after corn silage harvest an average of 24.4 lbs of N/ac was applied via pivot. Nitrogen was not applied to pivots that followed alfalfa termination.
Spring born cross-bred steers (n = 120, 240 and 240 in years 1, 2 and 3, respectively) were weighed and stratified by initial BW (583 ± 4.9, 637 ± 11.7, and 516 ± 9.5 lbs in years 1, 2 and 3, respectively) and assigned to treatment and replicate (30 steers per 30 acre quarter).
Steers were turned out to graze in early to mid-November. Grazing continued until forage appeared to be limiting in one quarter, with approximately 3 inches of growth remaining, upon which grazing ceased for all steers. Steers grazed for 99, 71 and 71 days in years 1 through 3, respectively.
Forage quality and biomass samples were taken prior to grazing. Biomass samples were also taken post grazing and forage disappearance was determined. Oat and rapeseed were clipped to ground level and immediately put on ice and frozen for at least 24 hours before drying in a 60 degrees Celsius oven.
Digestible organic matter (DOM, % of DM) was used as an estimate of total digestible nutrients (TDN). Calculation for DOM was made by multiplying the organic matter, % of the forage by the invitro organic matter digestibility (IVOMD, %). CP analysis was conducted using the combustion method.
A partial budget analysis was conducted to evaluate the establishment costs of each forage treatment. Average seed costs for OAT across years was $20.33/ac while cost of seed for MIX was $14.83/ac. Average seeding and fencing costs for all pivots across years were $13.88/ac and $5/ac, respectively.
Fertilizer amounts were different among pivots and were charged using N cost of $0.41 /lb N. Irrigation costs and amounts were also different among pivots and were charged $8.92/acre-inch in years 1 and 2 and $9.39/acre-inch in year 3.
Amount of water applied was 3.8, 2.6 and 4.6 inches/acre in years 1, 2 and 3, respectively. Total costs per acre were estimated to be $76.16/ac for OAT and $70.66/ac for MIX.
Results Except for post-grazing biomass, no interactions between treatment and year were observed. Initial yield prior to the start of grazing did not differ (P = 0.86; Table 1) among the OAT and MIX. In the MIX, rapeseed represented 25.4% of initial biomass on a dry matter (DM) basis.
There was a tendency for an interaction between treatment and year (P = 0.07) for post grazing which is due to there being no differences among treatment in year 1 and 2, but a tendency (P = 0.06) for MIX to be greater than OAT in year 3, with yields of 1518 and 1123 lbs/ac, respectively.
No differences (P = 0.83) were observed between MIX and OAT for forage disappearance, at approximately 28 lbs of DM/hd/d. This disappearance rate would equate to 4.8% of BW, suggesting that significant trampling losses occurred as it is unlikely that cattle consumed more than 50 to 60% of what disappeared.
The initial TDN and CP of MIX was greater (P < 0.01) than OAT. Suggesting that the mix could offer the potential for greater energy and protein intake.
ADG was greater (P < 0.01) for MIX than OAT, with MIX steers gaining 0.17 lb/d greater than OAT, although both would be considered moderate to high gains (Table 2).
A year effect (P < 0.01) was observed for ADG, with the low being 1.86 lb/d and the high being 2.2 lb/d. Cost of gain decreased significantly (P < 0.01) for MIX steers, being $0.08 lower per pound of gain. Again, this is due to not only a greater gain for MIX, but also to a lower seed cost.
Implications Initial forage yield was not affected by the inclusion of rapeseed with fall oat and seed cost of the oat monoculture ($20/ac) was greater than the mix ($15/ac). The higher digestibility of rapeseed results in a more energy-dense cover crop when planted with oats in late summer.
Calf gain was greater and cost of gain was decreased when rapeseed was included due to the combination of greater gains and lesser seed cost.
Including rapeseed in late summer planted oats may be beneficial for producers who want to graze growing calves in the fall.
References 1 Lenz, M.E., J. L. Cox-O’Neill, K. E. Hales, and M. E. Drewnoski. 2019. Nutritive Value Change During the Fall of Late-summer Planted Oats, Turnips and Radish. Crop Forage and Turfgrass Management. doi:10.2134/cftm2018.12.0097
2Brinton, M. M., B. H. Hansen, K. M. Ulmer, Z. E. Carlson, F. H. Hilscher, M. E. Drewnoski and J. C. MacDonald. 2020. Forage Production and Calf Gains When Grazing Oats Following Corn Harvest. MP107: 35-37. 3 Cox, J. L., K.E. Hales, K. M. Ulmer, R. Rasby, S. D. Shackleford, C. Engle, J. Rieckman, H. C. Freetly and M. E. Drewnoski. 2016. Utilizing Corn Residue or Fall Double Cropped Forages for Winter Backgrounding of Calves. Nebraska Beef Cattle Report. MP 103: 55-57.
Table 1. Forage yield, initial forage quality and for disappearance of an oat monoculture (OAT) or an oat-rapeseed mix (MIX) planted in late-summer and grazed from early November into January.
1TDN was estimated using digestible organic matter a proxy and calculated by multiplying IVOMD by the OM content of the forage. 2MIX contained 25.4% rapeseed on DM basis. a,bMeans with different superscripts differ (P ≤ 0.05).
Table 2. Growing steer performance when grazing a late-summer planted oat monoculture (OAT) or an oat-rapeseed mix (MIX) from early November into January.
1Abbreviations: IBW = initial body weight, EBW = ending body weight, ADG = average daily gain. 2Cost of gain includes seed costs at $20.33 /ac for oats or $14.83/ac for mix, plus seeding costs at $13.88/ac, fertilizer $6.80/ac, irrigation $33.40/ac and fencing at $5.00/ac. a,bMeans with different superscripts differ (P ≤ 0.05).
Drewnoski is a beef systems specialist and Jakub is a former master's student with the University of Nebraska.