Nitrogen Use Efficiency of Cool-Season Perennial Forage Grasses Planted With and Without Alfalfa Under Irrigation for Hay Production

‘Bozoisky’ RWR, ‘Hycrest’ CWG and ‘NewHy’ HWG produced the most hay with no addition of fertilizer N in 2010, whereas ‘Oahe’ IWG produced the most in 2011, and it and ‘NewHy’ produced the most for the two years (Figure 1). Although the grasses were harvested nine days later in 2011 compared to 2010, they produced less hay in 2011 by an average of 0.7 T/ac, except ‘Paiute’ OG which had similar yields both years, but still had the lowest among the grasses by an average of about a T/ac. Why ‘Bozoisky’, one of the top hay producing grasses without applied N in 2010, produced less than a third the amount of hay in 2011 is not known.

Although ‘Hycrest’ CWG and ‘NewHy’ HWG produced a similar amount of hay as ‘Bozoisky’ RWR in 2010 with no applied N, they apparently did not produce more dry matter forage per pound of soil N in the form of nitrate-N (NO3-N) as ‘Bozoisky’ (Table 1). Soil NO3-N under ‘Bozoisky’ averaged 7.9 lb/ac on April 15, whereas under ‘Hycrest’ and ‘NewHy’ it was 16.6 and 18.1 lb/ac, respectively (Table 2). By July 16, soil NO3-N content under ‘Bozoisky’ and ‘NewHy’ averaged 1.2 lb/ac, and under ‘Hycrest’ it was 1.9 lb/ac, indicating ‘Bozoisky’ used 6.6 lb of soil NO3-N/ac, ‘NewHy’ 17.0 lb and ‘Hycrest’ 14.7 lb. Why soil NO3-N content under ‘Hycrest’ and ‘NewHy’ on April 15 was 7 to 9 lb/ac more compared to the other six grasses is not known, especially when it was similar among the grasses on May 5, 2011 (Table 3). In 2008 and 2009, yellow sweet clover had a significant presence in the entire grass hay field including the plot area, which may explain the fairly high soil NO3-N contents under all the grasses, but there is no reason to suspect that it had a greater density within the ‘Hycrest’ and ‘NewHy’ plots.

‘Hycrest’ CWG produced more hay than the other grasses in 2010 with 50 and 100 lb N/ac by an average of 0.8 T/ac, whereas ‘Oahe’ IWG produced an average of 1.1 T/ac more hay than the other grasses at the 150 and 200 lb N/ac rates (Figure 2). In addition, ‘Oahe’ produced the most hay at each N rate in 2011 by this same amount (Figure 3).

Except for ‘Manchar’ SBG and ‘Luna’ and ‘Oahe’ IWG, the grasses produced less hay in 2011 with higher N rates than they yielded in 2010 (Figures 2 and 3). ‘Bozoisky’ RWR and ‘Paddock’ MBG had the greatest discrepancy, producing an average of 1.0 and 0.7 T/ac less, respectively. ‘Manchar’ produced the same amount of hay both years, and ‘Luna’ and ‘Oahe’ produced an average of 0.2 T/ac more in 2011 compared to 2010. Why hay yields of the grasses were generally not greater in 2011 as compared to 2010 may have been due to the cooler May and drier mid- to late-June conditions in 2011 as compared to in 2010. Had the grasses been irrigated in mid-June 2011, they might have produced higher hay yields.

The grasses, except ‘Paiute’ OG, ‘NewHy’ HWG and ‘Hycrest’ CWG, appeared to possibly not have reached their maximum forage yield in 2010 with 200 lb N/ac rate (Figure 2), and thus, as noted in the methods section, the reason for the higher N rates in 2011. However, even with 250 lb N/ac the grasses, except possibly ‘Paddock’ MBG, ‘Manchar’ SBG and ‘Oahe’ IWG, did not appear to have reached their maximum forage yield in 2011 (Figure 3) but, as will be shown below, maximum forage yield does not necessarily result in the highest net income per acre.

'Bozoisky’ RWR may have been the most efficient in its use of available soil NO3-N in 2010 for aboveground growth compared to the other grasses, but it was the least efficient in its use of applied fertilizer N for additional forage growth both years (Table 4). Because ‘Paiute’ OG generally produced more additional pounds of hay per pound of applied N in 2010 compared to the other grasses but had the lowest hay yields, using NUE to select a grass for hay production may not be advisable. Using net income per acre is probably a better way to select which grass or grasses would be the best to use for hay production.

At $80/T for grass hay and $0.77/lb for N plus $5/ac for application, the highest net incomes in 2010 would have been obtained with ‘Hycrest’ CWG at 50 and 100 lb N/ac and with ‘Oahe’ IWG at 150 and 200 lb N/ac (Table 5). In 2011, the highest net income at each N rate was obtained with ‘Oahe’, with the best returns at the 125 and 187.5 lb N/ac rates. ‘NewHy’ HWG and ‘Bozoisky’ RWR did not produce enough additional hay with applied N to return a greater net income then what would have been received for hay with no N fertilizer in 2010 and 2011, and likewise for ‘Paiute’ OG in 2011. 'Bozoisky’ may have had the highest net income with no applied N in 2010 and ‘Oahe’ the highest in 2011, but ‘NewHy’ HWG, along with ‘Oahe’, had the highest total net income for the two years.

Although soil NO3-N and NH4-N levels on July 17, 2010 in the 200 lb N/ac rate subplots were about twice the level found in the other four N rate subplots (Table 2), these two soil N sources were only slightly higher in the 200 lb N/ac subplots compared to the 0 lb N/ac subplots on May 5, 2011 (Table 3). This would indicate that the grasses probably used this soil N during the remainder of the 2010 growing season. Unfortunately, late summer/early fall 2010 forage yields were not determined to help veriy this.

Applied N appeared to have little to no effect on the Net Energy maintenance (NEm) content of the grasses both years, though it declined in ‘Paiute’ OG and ‘Hycrest’ CWG but increased in ‘Bozoisky’ RWR as N rate increased (Table 6). ‘Paiute’ contained the highest average amount of NEm among the grasses both years, followed by ‘Bozoisky’. ‘Paddock’ MBG contained the least amount of NEm in 2010, but in 2011 it contained an average of 0.09 Mcal/lb more, and thus met the needs of a lactating beef cow (0.60 Mcal/lb). The NEm content of ‘Manchar’ SBG and ‘NewHy’ HWG were borderline sufficient in meeting the above cow’s needs in 2010, but in 2011 it was sufficient in ‘Manchar’ but again borderline sufficient in ‘NewHy’. Both ‘Luna’ and ‘Oahe’ IWG contained a sufficient amount of NEm in 2010 to meet the above cow’s needs, but in 2011 they did not, especially ‘Oahe’.

Unlike NEm, Non-Fibrous Carbohydrate (NFC - primarily starch and sugars) content appeared to decline in the grasses as applied N increased (Table 7). Apparently as the grasses produced more biomass as a result of more available N starch, sugars were being replaced with cellulotic tissue. This would explain why NEm remained fairly constant acrossed N rates. As with NEm, ‘Paiute’ OG contained the highest average level of NFC, with ‘Hycrest’ CWG, ‘Manchar’ SBG and ‘NewHy’ HWG the lowest amounts.

Crude protein content in the grasses increased at a nearly linear rate both years as applied N rate increased, except in ‘Hycrest’ CWG, 'Bozoisky’ RWR, and possibly ‘NewHy’ HWG in 2010, where it appeared to level off between 150 and 200 lb N/ac (Table 8). ‘Hycrest’ and ‘Bozoisky’ mature earlier than the other grasses, and it is possible that the higher levels of N may have resulted in them reaching maturity sooner. At 0 lb N/ac only ‘Paiute’ OG, ‘Luna’ and ‘Oahe’ IWG and ‘NewHy’ contained an adequate level of crude protein to meet a lactating beef cows’ needs (10%) in 2010, but in 2011 all the grasses, except ‘Oahe’, ‘Luna’ and ‘NewHy’ contained an adequate amount. In addition, ‘Oahe’ and ‘Luna’ also contained the least amount of crude protein over all applied N rates in 2011, whereas in 2010 they contained similar levels as the other grasses. ‘Paddock’ MBG contained significantly less crude protein compared to the other grasses in 2010 and not enough to meet the needs of a nursing cow at applied N rates at less than 100 lb/ac, but in 2011 its content was similar to the other grasses, and at the 187.5 and 250 lb N/ac rates, only ‘Bozoisky’ RWR and ‘Paiute’ OG contained more.

Why ‘Paddock’ MBG contained more NEm and crude protein in 2011 compared to 2010 and ‘Luna’ and ‘Oahe’ IWG contained less is not clear. As grasses mature, their crude protein content declines, and to some degree NEm does as well. It is possible that ‘Paddock’ was not as mature on July 1, 2011 as it was on June 22, 2010, and this may have been true for ‘Paiute’ OG and ‘Bozoisky’ RWR as well, as their crude protein content in 2011 was higher than in 2010.

However, ‘Luna’ and ‘Oahe’, and to some degree ‘NewHy’ HWG, were the main reason for the later harvest in 2011, due to them still in the boot stage the third week of June, thus their lower crude protein content is somewhat perplexing. These three grasses did grow rapidly (seedhead extension) during the last ten days of June 2011, and this may be the reason for their lower forage quality. Thus, timing of harvest of these three grasses with regard to forage quality may be more critical than for the other grasses in this project.

Besides an increase in forage due to N fertilization, increase in crude protein content of the grass hay should also be taken into account when determining the economic benefit of applying N fertilizer. As indicated above, hay of ‘Paddock’ MBG, ‘Manchar’ SBG and ‘Bozoisky’ RWR in 2010, of ‘Oahe’ IWG and ‘NewHy’ HWG in 2011, and of ‘Hycrest’ CWG both years from the 0 lb N/ac subplots did not contain a sufficient amount of crude protein to meet the needs of a lactating beef cow. Thus a protein supplement would need to be provided to make up for the deficient. If this reference cow consumed 30 lb of dry matter forage each day, she would require 3.0 lb of crude protein. Based on the crude protein content of these grasses, they would only provide between 2.25 and 2.73 lb of protein per day. If the provided protein supplement cost $0.50 per pound of protein, then the weekly cost for the supplement when hay from these grasses was the forage source would range from a high of $2.63 for the 2010 ‘Paddock’ hay to a low of $0.95 for the 2010 ‘Hycrest’ and 2011 ‘Oahe’ hay. In addition, due to 2010 hay of ‘Paddock’ from the 50 lb N/ac rate containing only 8.8% crude protein, 0.35 lb/day of protein would need to be provided the cow at a cost of $1.22 per week.

Nitrate-N (NO3-N) content in the grasses, as with crude protein, increased as applied N rate increased. At rates above 50 lb N/ac in 2010 (Figure 4) and 62.5 lb N/ac in 2011 (Figure 5), hay from the grasses contained NO3-N levels greater than what is considered safe for pregnant and breeding livestock (350 ppm) if it is their only source of forage. In addition, at 200 lb N/ac in 2010 the NO3-N content of hay from ‘Manchar’ SBG, ‘NewHy’ HWG, ‘Oahe’ IWG and ‘Paiute’ OG were at levels greater than what is considered safe for non-breeding livestock (1130 ppm) if their only source of forage. For ‘Manchar’ this dangerous level of NO3-N occurred at 150 lb N/ac in 2010. Why the NO3-N content of the grasses in 2011 averaged nearly 300 ppm less than it did in 2010 at N rates of 100 lb/ac and above is not known. However, due to the high 2010 NO3-N levels in the grasses, it is advised that grass hay should be tested for its NO3-N content if N had been applied, especially at rates exceeding 100 lb/ac and if the hay is to be fed to pregnant and breeding livestock. It is also advised to swath grasses in the afternoon instead of in the morning if they have been N fertilized, as NO3-N levels tend to decline as the day progresses. Because ‘Paiute’ and ‘Manchar’ contained the highest average levels of NO3-N at N rates at 100 lb/ac and above, it may be wise to not fertilize them with more than 100 lb N/ac.

Applied N appeared to have little effect on the phosphorus (P) content of the grasses (Table 9). In addition, P content was similar among the grasses and sufficient to meet the needs of a lactating beef cow (0.20%). However, calcium (Ca) content increased in the grasses with an increase in applied N, except in ‘Paiute’ OG where it declined. The grasses generally contained enough Ca to meet the needs of a lactating beef cow (0.30%), with ‘NewHy’ HWG and ‘Bozoisky’ RWR containing the highest levels and ‘Manchar’ SBG the least. The Ca:P ratio was generally sufficient (1.5: 1) in hay from these grasses, except for ‘Luna’ IWG and ‘Manchar’ SBG at all N rates, ‘Oahe’ IWG at rates less than 150 lb/ac and ‘Hycrest’ CWG at rates below 100 lb/ac. Thus a Ca supplement might be needed when feeding hay from these grasses to the reference cow.

Potassium (K) content increased linearly with an increase in applied N in all the grasses (Table 10). ‘Paiute’ OG and ‘Bozoisky’ RWR contained the highest levels of K and ‘Hycrest’ CWG the least. However, even at 0 lb N/ac all grasses contained more K than what a lactating beef cow needs (0.70%). With respect to magnesium (Mg),‘Paiute’ OG was the only grass that contained a sufficient amount to meet a lactating beef cow’s needs (0.20%), regardless of N rate. ‘Hycrest’ CWG contained the least amount of Mg compared to the other grasses, and Mg content appeared to increase slightly in all the grasses with an increase in applied N. ‘Paiute’ OG and ‘NewHy’ HWG contained an adequate amount of sulfur (S) to meet the needs of a beef cow (0.15%) at all N rates, whereas the other grasses may not have contained a sufficient amount at 0 lb N/ac and ‘Paddock’ MBG not enough at 50 lb N/ac.

‘Bozoisky’ RWR contained the highest average level of iron (Fe) and ‘Manchar’ SBG the least (Table 11). Application of N fertilizer resulted in a slight decrease in grass Fe content, but the grasses from all N rates contained an adequate amount of Fe for a beef cow (50 ppm). However, with regard to manganese (Mn), the more N that was applied the higher its content in the grasses, but none of them contained a sufficient amount to meet a beef cow’s requirement (40 ppm) regardless of N rate, except for ‘Paiute’ OG. In addition, none of the grasses contained an adequate amount of zinc (Zn) to meet the requirements of a beef cow (30 ppm), and N fertilizer appeared to have little effect on grass Zn content.

Applied N may have increased the copper (Cu) content in the grasses slightly over no applied N (Table 12). ‘Luna’ IWG contained the highest average amount of Cu but still did not meet the needs of a beef cow (10 ppm), except at the 200 lb N/ac rate. Fortunately the grasses contained very low levels of molybdenum (Mo), and as N fertilizer rate increased its levels declined, thus interference with Cu absorption should be minimal. ‘Paiute’ OG contained the highest average amount of Mo among the grasses, but even at its highest level at 0 lb N/ac it was less than 1 ppm.

• Figure 3. Grass hay yields on 1 July 2011 at each nitrogen rate.
• Table 12. Grass hay copper and molybdenum contents (ppm) at each nitrogen rate on 22 June 2010.
• Figure 1. Grass hay yields from the 0 pound nitrogen per acre rate on 22 June 2010 and 1 July 2011.
• Table 1. Grass dry matter yields (lb/ac) on 22 June 2010 for each pound of soil nitrate-nitrogen (see Table 2) and applied nitrogen.
• Table 2. Top soil nitrate-nitrogen and ammonium-nitrogen contents (lb/ac) on 15 April 2010 for each grass and on 16 July 2010 for each grass by nitrogen rate treatment.
• Table 5. Net income ($/ac) for each grass by nitrogen rate in 2010 and 2011; hay at $80 per ton, nitrogen at $0.77 per pound and application cost $5 per acre.
• Table 6. Grass hay net energy maintenance content (Mcal/lb) at each nitrogen rate on 22 June 2010 and 1 July 2011.
• Table 8. Grass hay crude protein content (%) at each nitrogen rate on 22 June 2010 and 1 July 2011.
• Table 9. Grass hay phosphorus and calcium contents (%) and ratios at each nitrogen rate on 22 June 2010.
• Table 7. Grass hay non-fibrous carbohydrate content (%) at each nitrogen rate on 22 June 2010.
• Figure 5. Grass hay nitrate-nitrogen content at each nitrogen rate on 1 July 2011. Below yellow line (350 ppm) safe for all livestock; between yellow and red lines (1130 ppm) generally safe for non-pregnant livestock but limit feed to pregnant and breeding livestock; and above red line not safe for pregnant livestock and limit feed to non-pregnant animals.
• Table 11. Grass hay iron, manganese, and zinc contents (ppm) at each nitrogen rate on 22 June 2010.
• Table 3. Top soil nitrate-nitrogen and ammonium-nitrogen contents (lb/ac) on 5 May 2011 for each grass from the April 2010 0 and 200 lb N/ac treatments.
• Figure 2. Grass hay yields on 22 June 2010 at each nitrogen rate.
• Table 4. Pounds per acre of additional dry matter forage over 0 lb N/ac rate yields for each grass per pound of applied nitrogen on 22 June 2010 and 1 July 2011.
• Figure 4. Grass hay nitrate-nitrogen content at each nitrogen rate on 22 June 2010. Below yellow line (350 ppm) safe for all livestock; between yellow and red lines (1130 ppm) generally safe for non-pregnant livestock but limit feed to pregnant and breeding livestock; and above red line not safe for pregnant livestock and limit feed to non-pregnant animals.
• Table 10. Grass hay potassium, magnesium, and sulfur contents (%) at each nitrogen rate on 22 June 2010.