SEED METERING APPARATUS FOR A SEEDER This invention is in the field of agricultural implements and in particular a seed metering apparatus for a seeder that provides improved placement of seeds such as canola and 5 corn. BACKGROUND In the agricultural industry, considerable research has been done and is ongoing to 10 determine optimum plant spacing for various crops to provide maximum yields. This desired spacing in turn gives a desired plant population of plants per unit of area, such as a square foot or acre. In order to translate this into a usable form for the farmer, the average number of seeds in a unit of weight, such as pounds, is determined for an average sample of representative seeds. The number of desired plants per acre is then translated 15 into a seeding rate of pounds per acre. Seed sizes and weights as well as desired plant populations vary dramatically between different crops. A typical corn variety has large sized seeds and may have about 1200 seeds per pound, while a typical wheat variety has smaller medium sized seeds and may 20 have about 12,000 seeds per pound, and a typical canola has very much smaller sized seeds and may have about 120,000 seeds per pound. Similarly desired plant populations also vary dramatically. A typical population for corn may be about 0.5 plants per square foot (22,000 plants per acre), for canola about 5 plants 25 per square foot (220,000 plants per acre), and for wheat about 25 plants per square foot (1,100,000 plants per acre). Canola seed can cost up to $10 per pound, and so current research is showing that the traditional plant population of 10 plants per square foot can be reduced to 5 per square foot if fairly evenly distributed. 30 Ideally these plants are spaced equally in all directions in a grid like arrangement however in practice a given seeder has a fixed spacing between the furrow openers and so 1 the spacing between the rows of plants is fixed for any particular seeder, typically for a modem minimum tillage air seeder these row spacings are 10-12 inches. With the spacing between rows fixed, altering the seeding rate alters the spacing between seeds in a row. 5 With a wheat crop on a 12 inch row spacing, there should be about 25 seeds per foot of row, and the distance between seeds is then less than 12 inch. With canola there should be about 5 seeds per foot of row, and the distance between seeds is 212 inches. 10 With corn there should be about 0.5 seeds per foot of row, and the distance between seeds 24 inches. For maximum yields however, corn is seeded at a wider row spacing, typically 30 inches or more, with a row type planter. Corn is sometimes seeded with a 12 inch spacing air seeder by only using every other furrow opener so the row spacing is 24 inches and there are thus about 1.0 seeds per foot of row. 15 Thus it can be seen that to get an even plant spacing, the spacing is much more critical for corn than for either wheat or canola, while the spacing of canola seeds is more critical than for wheat. To achieve the desired even spacing, singulating metering devices are commonly used for planting corn. These typically comprise pockets or recesses on a 20 rotating disc and take the seeds from a seed tank one at a time and drop them into a furrow. In contrast, seed metering devices for seeding wheat and canola typically work on more of a bulk principal. One typical type of seed metering device includes a rotating feed 25 roller with recesses that can be in the form of grooves extending longitudinally along the length of the outer surface of roller parallel to the axis of rotation, or in the form of notches or pockets or the like spaced evenly along the outer surface of the roller. The feed roller is typically mounted below the feed opening at the bottom of a seed tank and configured such that the intake side of the roller is inside the feed opening exposed to the 30 seeds in the tank and the outer output side is above the furrow in a conventional seeder, or in an air seeder is above the air stream that carries the seeds to the furrow openers to be 2 deposited in the ground. As the roller rotates, seeds fill the recesses and are carried from the seed tank and dropped into the furrow in a conventional seeder, or are fed into the air stream of an air seeder. 5 The number of seeds dispensed is proportional to the rotational speed of the feed roller or auger, and the rate of metering seeds is adjusted by varying the rotational speed. The rotational speed is also coordinated with ground speed so that the same amount of seed is dispensed for each foot of distance travelled. 10 As a feed roller rotates, the seed is dispensed essentially in pulses. The recesses spill their contents into the furrow or air stream as they rotate, with one emptying before the next starts to spill. With wheat this is not particularly problematic, as the seed spacing is not so critical, and in addition the volumes of seed required to be dispensed for a typical seeding rate require a rotational speed that is relatively high so that the pulses blend 15 together and the result is a fairly constant flow of wheat seeds that is satisfactory for seeding wheat. With canola however, the number of seeds that are contained in any particular recess is about 10 times the number of wheat seeds that are contained in the same groove or recess. 20 In addition it is desired only to have 5 canola seeds per foot instead of 25 wheat seeds, so a feed roller rotating at a speed suitable to dispense the desired rate of wheat seeds will dispense canola seeds at about 50 times the desired rate. For seeding canola then, the speed of rotation must be reduced to 1/50 the rotational 25 speed for wheat, however reducing the speed that much becomes problematic as the pulsing nature of the flow of canola seeds is accentuated. The flow of canola seeds into the air stream is very uneven, and although the correct number of seeds per foot of row will be dispensed, the seeds will be deposited in the furrow in clumps instead of evenly spaced. In order to address this problem, different feed rollers are often used for wheat 30 and canola. The canola feed rollers will have shallower and smaller recesses such that less seed is dispensed with each revolution, and the rotational speed can be increased and 3 the pulsing effect reduced. The rotational speed of these canola feed rollers however is still very slow, in the order of 1 revolution per minute. At these slow speeds, and given the nature of a canola seed being very small and round, the movement and vibration caused by the seeder travelling along a field causes the canola seed to shake out of the 5 recesses, accentuating the uneven flow. SUMMARY OF THE INVENTION It is an object of the present invention to provide a seed metering apparatus for an air 10 seeder that overcomes problems in the prior art. In prior art feed rollers, the recesses that actually take the seeds from the tank and move them to the air stream are spread along the whole outer surface of the roller from one end to the other. In a typical configuration, the roller will be some inches long and exposed to 15 the seeds in the tank on an intake side and open to the air stream on the output side such that as the roller rotates seeds drop from the recesses into the air stream. In some types of air seeder distribution networks, chutes are provided at the output side to receive the seeds being dispensed and direct same into separate air streams. 20 Where each air stream is feeding the same number of furrow openers, these chutes will be of equal width so that each air stream receives the same amount of seed. It is also known, where the air streams are feeding different numbers of furrow openers, to vary the chute widths so that the width of the roller feeding each chute is proportional to the number of furrow openers being fed, and so each furrow opener receives the same amount of seed. 25 In a first embodiment the present invention provides a seed metering apparatus for a seeder. The apparatus comprises a seed container with a feed opening in a bottom thereof. A feed shaft is rotatably mounted in a substantially horizontal orientation extending substantially from a right side of the feed opening to a left side thereof. At 30 least one feed disc is mounted to the feed shaft such that the at least one feed disc and feed shaft rotate together, and a plurality of disc recesses are substantially equally spaced 4 along a periphery of the at least one feed disc. A shield plate extends from the right side of the feed opening to the left side thereof in an upright orientation from a bottom edge thereof to a top edge thereof, the shield plate substantially parallel to and adjacent to a front side of the feed shaft. The shield plate defines a disc slot having a width 5 corresponding to a thickness of the at least one feed disc and configured such that the at least one feed disc can extend through the disc slot with seeds substantially prevented from passing between edges of the disc slot and the at least one feed disc. The feed shaft, at least one feed disc, and shield plate are configured such that the shield plate substantially seals the feed opening, and such that an intake portion of the periphery of 10 the at least one feed disc is located in the feed opening exposed to seeds in the seed container, and such that an output portion of the at least one feed disc extends forward of a front face of the shield plate through the disc slot. Rotation of the feed shaft carries seeds out of the seed container in the disc recesses such that the seeds drop into a seed receiver below the front face of the shield plate. 15 In a second embodiment the present invention provides a seed metering apparatus for an air seeder. The apparatus comprises a seed container with a feed opening in a bottom thereof. A feed shaft is rotatably mounted in a substantially horizontal orientation extending substantially from a right side of the feed opening to a left side thereof. A 20 plurality of feed discs are mounted to the feed shaft such that the feed discs and feed shaft rotate together, and a plurality of disc recesses are substantially equally spaced along a periphery of each feed disc. A shield plate extends from the right side of the feed opening to the left side thereof in an upright orientation from a bottom edge thereof to a top edge thereof. The shield plate is substantially parallel to and adjacent to a front side 25 of the feed shaft. The shield plate defines a disc slot for each feed disc, the disc slots having a width corresponding to a thickness of the feed discs and configured such that the feed discs can extend through the corresponding disc slots with seeds substantially prevented from passing between edges of the disc slots and the feed discs. The feed shaft, feed discs, and shield plate are configured such that the shield plate substantially 30 seals the feed opening, and such that an intake portion of the periphery of each feed disc is located in the feed opening exposed to seeds in the seed container, and such that an 5 output portion of each feed disc extends forward of a front face of the shield plate through the corresponding disc slots. Rotation of the feed shaft carries seeds out of the seed container in the disc recesses such that the seeds drop into a seed receiver below the front face of the shield plate. 5 Instead of gathering and dispensing seeds across the entire width of the seed opening the feed disc of the present invention gathers and dispenses seeds from only an area equal to the thickness of the disc. The disc therefore rotates much faster than a conventional feed roller, and thus the flow of seeds from the metering apparatus is much more uniform, and 10 seed spacing in the furrow is thus more uniform. The higher speed also reduces the effects of field vibrations on the seed flow to a negligible amount. Assemblies with different disc thicknesses and/or disc recess configurations can be interchangeable in the feed opening to provide metering for different sized seeds and 15 different seeding rates. A roller assembly with a conventional feed roller can also be mounted in the feed opening if desired. DESCRIPTION OF THE DRAWINGS 20 While the invention is claimed in the concluding portions hereof, preferred embodiments are provided in the accompanying detailed description which may be best understood in conjunction with the accompanying diagrams where like parts in each of the several diagrams are labeled with like numbers, and where: 25 Fig. 1 is a perspective view of an embodiment of a seed metering apparatus of the present invention, the apparatus having a single feed disc; Fig. 2 is a front view of the embodiment of Fig. 1; 30 Fig. 3 is a schematic side view of the embodiment of Fig. 1; 6 Fig. 4 is a front view of the feed disc in the embodiment of Fig. 1; Fig. 5 is a perspective view of an embodiment of a seed metering apparatus of the present invention where the apparatus has a plurality of feed discs; 5 Fig. 6 is a perspective view of the disc assembly of the embodiment of Fig. 5; Fig. 7 is a perspective view of the seed container of the embodiment of Fig. 5; 10 Fig. 8 is a perspective view of the embodiment of Fig. 5 with the disc assembly of Fig. 6 removed and replaced with a conventional feed roller assembly, and with seed receiver removed; Fig. 9 is a perspective rear view of seed container and seed receiver of the embodiment 15 of Fig. 5 with seed receiver removed; Fig. 10 is a schematic front view of a feed disc and disc slot that are wider than that shown in the embodiment of Fig. 5; 20 Fig. 11 is a perspective view of a disc assembly with dividers between adjacent disc slots; Fig. 12 is a schematic front detail view of the disc slot and feed disc of the embodiment of Fig. 5; 25 Fig. 13 is a schematic sectional side detail view of the disc slot and feed disc of the embodiment of Fig. 5. 7 DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Figs. 1 - 3 illustrate an embodiment of a seed metering apparatus 1 of the present invention for a seeder. The apparatus 1 comprises a seed container 3 with a feed opening 5 5 in a bottom thereof. The illustrated seed container 3 is a hoppered container that is configured to be attached on the bottom of a large tank such as is used to carry agricultural seeds on an air seeder. A feed shaft 7 is rotatably mounted in bearings 9 in a substantially horizontal orientation 10 and extends across the feed opening 5 from a right side to a left side thereof. A feed disc 11 is mounted to the feed shaft 7 such that the feed disc 11 and feed shaft 7 rotate together. The illustrated feed shaft 7 has a hexagonal cross section as does the inner hole of the feed disc 11 such that both turn together. Other shapes or a keyway or the like could also be used cause both to rotate together. A plurality of disc recesses 13 are 15 equally spaced along a periphery of the feed disc 11. A shield plate 15 extends from the right side of the feed opening 5 to the left side thereof in a generally upright orientation from a bottom edge 15A thereof to a top edge 15B thereof. The shield plate 15 is parallel to and adjacent to a front side of the feed shaft 7 20 and defines a disc slot 17 having a width W corresponding to a thickness T of the feed disc 11 such that the feed disc 11 can extend through the disc slot 17 with seeds substantially prevented from passing between edges of the disc slot 17 and the feed disc 11. 25 The feed shaft 7, feed disc 11, and shield plate 15 are configured such that the shield plate 15 seals the feed opening 5, and such that an intake portion 11 A of the periphery of the feed disc 11 is located in the feed opening 5 exposed to seeds 19 in the seed container 3, and such that an output portion 1IB of the feed disc 11 extends forward of a front face 15F of the shield plate 15 through the disc slot 17. 30 8 Thus in the apparatus 1, rotation of the feed shaft 7 in the direction R carries seeds 19 out of the seed container 5 in the disc recesses 13 such that the seeds 19 drop into a seed receiver 21 below the front face 15F of the shield plate 15. The seed receiver 21 directs the seeds 19 eventually into one or more furrows made by furrow openers of the seeder, 5 Where the seeder is an air seeder, the seed receiver 21 directs the seeds 19 into an air stream for distribution to the furrow openers. In the illustrated apparatus 1, the feed disc 11 includes an expanded circular core portion 23 that is wider than the disc slot 17, with disc teeth 25 extending from the circular core 10 portion 23 to form the disc recesses 13 and extending through the disc slot 17. The feed disc 11 with circular core portion 23 and disc teeth 25 is illustrated in Fig. 4. Right and left seed plates 27 extend rearward from a rear face 15B of the shield plate 15 from corresponding right and left edges of the disc slot 17. The rear edges 27A of the 15 seed plates 27 have a circular shape corresponding to the circular core 23 and in proximity to the circular core 23. The seed plates 27 are in close proximity to the sides of the disc teeth 25 such that when the feed disc 11 rotates in direction R, seeds 19 are prevented from moving up between the teeth 25 and the side plates 27, thereby reducing seed damage. 20 In the illustrated apparatus 1, the seed plates 27 also extend forward from the front face 15F of the shield plate 15 from corresponding right and left edges of the disc slot 17 such that the teeth 25 rotate between the seed plates 27, and seeds 19 carried out of the seed container 5 in the disc recesses 13 are guided to fall downward between the seed plates 25 27. Compared to a conventional feed roller with recesses on the entire outer surface thereof and extending across the entire width of the feed opening 5, the illustrated feed disc 111 will turn at a much higher rotational speed to dispense the same rate of seeds from the 30 seed container 3 as the conventional feed roller. Thus a much more uniform flow of seed 9 is received by the seed receiver 21 as pulsations in the flow are much reduced because of the increased rotational speed. Figs. 5 - 9 illustrate an alternate embodiment of a seed metering apparatus 101 of the 5 present invention for use in an air seeder. The apparatus 101 comprises a seed container 103 with a feed opening 105 in a bottom thereof. A feed shaft 107 is rotatably mounted in a substantially horizontal orientation extending substantially from a right side of the feed opening 105 to a left side thereof. A plurality of feed discs 111 are mounted to the feed shaft 107 such that the feed discs 111 and feed shaft 107 rotate together. A plurality 10 of disc recesses 113 are equally spaced along a periphery of each feed disc 111. A shield plate 115 extends from the right side of the feed opening 105 to the left side thereof in an upright orientation from a bottom edge 115A thereof to a top edge 115B thereof. The shield plate 115 is parallel to and adjacent to a front side of the feed shaft 15 107. The shield plate 115 defines a disc slot 117 for each feed disc 111. The disc slots 117 have a width W corresponding to a thickness T of the feed discs 111 and are configured such that the feed discs 111 can extend through the corresponding disc slots 117 with seeds substantially prevented from passing between edges of the disc slots 117 and the feed discs 111. Thus seeds 119 are prevented from leaking out of the seed 20 container 103. The feed shaft 107, feed discs 111, and shield plate 115 are configured such that the shield plate 115 substantially seals the feed opening 105, and such that an intake portion 111A of the periphery of each feed disc 111 is located in the feed opening exposed to 25 seeds in the seed container 103, and such that an output portion 11 B of each feed disc 11I extends forward of a front face 115F of the shield plate 115 through the corresponding disc slots 117. Rotation of the feed shaft 7 carries seeds 119 out of the seed container 103 in the disc recesses 113 such that the seeds 119 drop into a seed receiver 121 below the front face 115F of the shield plate 115. 30 10 In the illustrated apparatus 101, as shown in Fig. 6, the feed shaft 107 is rotatably mounted in bearings 109 in end plates 131 of the shield plate 115 such that the feed shaft 107, feed discs 111, and shield plate 115 form a disc assembly 133 that is removably mounted in the feed opening 105. Clips 135 as shown in Fig. 7 can be released to 5 remove the disc assembly 133. Thus the illustrated disc assembly 133 with feed discs 111 having a thickness T and the illustrated configuration of disc recesses 113, can be replaced by a second disc assembly with feed discs 111', as schematically illustrated in Fig. 10, having a greater thickness T' (or a lesser thickness if the situation warrants) and a different configuration of disc recesses 113', and of course a shield plate 115' configured 10 accordingly. Thus a wide variety of crops can be seeded effectively. For example for seeding small seeds like canola, the thickness of the feed disc 111 will be small, and the disc recesses 113 will likewise be quite small, while for seeding large seeds like corn, a thicker feed 15 disc 111' can be provided with larger disc recesses 113' configured to be more suitable for the larger seeds. It is contemplated that it may be possible to size the recesses 113' to be about the same size as a corn seed, such that the corn seeds are carried out of the seed container 103 one at a time. 20 Further in the illustrated apparatus 101 a roller assembly 141, as illustrated in Fig. 8, can be provided to allow seeding still further different types of crops, such as wheat. The roller assembly 141 comprises a feed roller 143 with roller recesses 145 spaced evenly along an outer surface thereof. A bearing 147 is mounted on each end of the feed roller 141 and the roller assembly 141 is configured to removably mount in the feed opening of 25 the seed container 103 secured by the clips 135 in place of the disc assembly 133. The roller assembly 141 is configured such that when the feed roller 143 is stopped, seeds are prevented from moving out of the seed container 103, and such that when the feed roller 143 rotates seeds are moved out of the seed container 103 in the roller recesses 145. 30 In the illustrated apparatus 101 the seed receiver 121 comprises a chute 151 mounted under each disc slot 117. Each chute 151 is operative to direct received seeds 119 into an 11 air stream of an air seeder for delivery to at least one furrow opener. Figs. 8 and 9 show the apparatus 101 with the seed receiver 121 removed. A primary air stream is directed into the input port 153 of the seed receiver 121, and is divided into a plurality of separate secondary air streams that exit the seed receiver 121 through output ports 155. The 5 chutes 151 direct the dispensed seeds into the secondary air streams through venturis in the seed receiver, and conduits attached to the output ports carry the secondary air streams AS' and entrained seeds to furrow openers. Fig. 11 illustrates a disc assembly 233 that includes dividers 253 extending forward from 10 the front face 215F of the shield plate 215 between adjacent slots 215. The dividers 253 are configured to maintain a flow of seeds from one feed disc 211 into the seed receiver 221 separate from the flow of seeds from a second adjacent feed disc 211. Some seed types may be prone to scattering when exiting the disc recesses and such a divider 253 will ensured that the seeds drop into the proper location on the seed receiver 221. 15 The feed discs 111 can be configured, using a hex shaft such as illustrated in the apparatus 1 of Fig. 1 or using a keyed shaft as schematically illustrated in Fig. 13, or like means known in the art such that the feed discs 111 are slidable along the feed shaft 107. The feed discs 111 will be maintained in position on the feed shaft 107 by their extension 20 through the disc slots 117. Such an arrangement facilitates assembly of the disc assembly 133. Detailed schematic front and side views of the feed disc 111 and slot 117 are shown in Figs. 12 and 13. It is contemplated that in a typical application the feed shaft 107 and 25 discs 111 will be rotated in direction R such that rotation of the feed shaft 107 moves a bottom edge of the feed discs 111 forward through the slot 117. In order to reduce jamming of seeds between the edges of the disc slot 117 and the periphery of the feed disc 111, a portion 11 7A of each disc slot 117 adjacent to a bottom edge of each feed disc 111 is widened to reduce jamming of seeds between the edges of the disc slot 117 and the 30 periphery of the feed disc 111, and reduce seed damage. 12 As described above the present invention provides a seed metering apparatus for seeders that reduces pulsations in the flow of seeds dispensed, and also reduces the effects of seeder movement vibration on the flow of seeds from the metering device, thereby providing a more uniform flow of metered seeds and improved uniformity of seed 5 placement in furrows. The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous changes and modifications will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation 10 shown and described, and accordingly, all such suitable changes or modifications in structure or operation which may be resorted to are intended to fall within the scope of the claimed invention. 13