Effects of Adding Clovers to Tall Fescue Pastures on the Nitrogen Status of Animals, Forages and Soils.

摘要

Rotational grazing studies with Boer-cross and Kiko-cross meat goats (Capra hircus) were conducted during spring (March -- May) 2012, 2013 and 2014 and fall (September -- November) 2012. The objectives included evaluating animal performance, forage production, forage nutritive value, biological nitrogen fixation, and soil N status in tall fescue (Lolium arundinaceum Shreb.; TF) and TF pastures mixed with red clover (Trifolium pratense L.; RC) or white clover (Trifolium repens L.; WC). The experiment was a split block design with 4 main nitrogen (N) treatments: TF mixed with red clover (TFRC), TF mixed with white clover (TFWC), TF fertilized with 112 kg N ha -1 annually (TFPOS) and TF with no additional N (TFNEG). Within each main treatment plot, there were grazed (G) and mowed (M) subplots. Nursing does and their twin kids grazed for 56 d each spring and were moved to new forage every 2 d. Eight month old wethers were grazed for 39 d only in fall 2012. 15N Natural Abundance technique was used to calculate %N derived from the atmosphere (%Ndfa) of clovers. Soil samples were taken before and after grazing/mowing and weekly thereafter and analyzed for nitrate-N (NO3-N) and ammonium-N (NH4-N). The proportion of forage dry matter (DM) as WC was similar in spring 2012 and 2013 but declined in 2014 (40.4, 43.2 and 18.7%, respectively). The proportion of forage DM as RC also declined as the trial progressed from 2012 to 2014 (30, 11.2 and 2.9%, respectively). There was less clover growth in the fall as compared to the spring (P < 0.01). Mowed plots had a greater percentage of clover in 2012 and 2013 compared to grazed plots. Forage DM yield was affected by N treatment with TFNEG producing less (P < 0.01) in spring and TFWC producing less (P < 0.01) in fall than all other treatments. TFWC produced the greatest amount of forage N during spring (P < 0.01), but TFPOS produced the greatest amount of forage N during the fall (P < 0.01). The % Ndfa of TFWC declined from 82% to 42% from spring to fall and was correlated to an increase in soil NO3-N levels (P < 0.05). TFWC had greater soil NH4-N compared to the TF-only treatments in two of the three years and greater soil NO3-N in each of the three years. Grazed treatments had greater inorganic soil N than mowed plots. Swards containing clover had lower C:N ratios which likely led to greater N mineralization and soil N levels even in the mowed treatments. TFWC fixed a total of 121 and 76 kg N ha-1 in 2012 and 2013, respectively. TFRC fixed 62 kg N ha-1 in 2012 but only 15 kg N ha-1 in 2013. Average daily gain of the kids and wethers did not differ in spring (avg = 135.3 g/d; P = 0.19) or fall (avg = 71.2 g/d; P = 0.20). Animal output (kg gain ha-1) was greater (P < 0.01) in the TFWC treatment compared to TFPOS and TFNEG during each of the three spring grazing seasons. Addition of clovers to the TF swards led to increased serum urea N and fecal N concentration of the does during the first 2 years. Inclusion of WC or RC in TF swards resulted in similar forage yields as in TFPOS and in greater animal output when the percentage of clover was ≥ 30%. Although careful grazing management of TF-clover stands can reduce or eliminate the need for N fertilization, reseeding clover every 2 to 3 years may be necessary to maintain optimal levels of clover DM in the stand.