GRADING OF CERTIFIED SEED POTATOES.

New Zealand Journal of Agriculture, Volume XL, Issue 4, 22 April 1930, Page 264

GRADING OF CERTIFIED SEED POTATOES.

STANDARDS FOR THE CURRENT SEASON’S CROP. ' J. W. Hadfield, Agronomist, Plant Research Station, Palmerston North. The demand for certified' seed potatoes is increasing very rapidly, but it is evident that the merchant and the Department of Agriculture have different conceptions as to what should constitute the main features of “ certified seed.” * • <■ ■ ■ -- r . The merchant demands varietal purity and careful grading, so that the seed may be of good appearance and readily saleable. The Department is concerned also with varietal purity, but takes into account more particularly what is termed “ cropping-power,” and it has left the question of grading for arrangement between the vendor and purchaser. The Department aims at the distribution of seed capable of giving satisfactory yields, and this factor is not one that can be judged from the superficial appearance of the tubers. . . ■ It is, however, evident that both points of view require consideration, and the present object is to explain the difficulties encountered in attempting to set definite grading standards, and what steps are being taken to remedy the somewhat unsatisfactory position existing at the present time. Tags used in connection with certification have printed thereon a statement to the effect that grading is a matter left entirely between vendor and purchaser. Nevertheless, in the 1928-29 season a method was introduced by which the certifying officer could indicate on the

tag, by the use of certain figures, the grading of the seed within that sack. This method is explained later. No attempt was made to define standards for certified seed, but merely to indicate the grading of the line under inspection, so that the vendor could quote these grading figures when offering his seed; or, on the other hand, the purchaser could, in a very simple manner, stipulate a certain definite grade.

Generally speaking, growers understand the significance of these figures, but as a rule the merchants have either disregarded them or not understood their import, and an attempt is made here to explain in detail the scheme as intended it should apply to the produce of the 1929-30 season. It seems necessary to explain, in the first place, why definite standards have not been fixed for certified seed. -Such a proposal met with strong opposition from merchants in Canterbury who were handling most of the certified seed. They are called upon to fill orders from all over the Dominion, and the requirements of one merchant or one district may be very different from that of another in regard to the size of seed. Some purchasers require “ table ” size for cutting and planting, and, since this practice is one that the Department strongly recommends, it would be a retrograde step to refuse to certify to tubers

of this size. There can be no one standard to fill all requirements, and therefore it must fall to the purchaser to stipulate the grading .standard, and the method here described facilitates this to a very large degree. One requirement is probably universalthat the tubers should be reasonably uniform in size, irrespective of whether the demand is for large, medium, or small seed. A grading standard should therefore convey in simple terms (1) the average size of the tubers, and (2) the range in size— is, uniformity of grading. ' The average weight of tubers may be 3 oz. in a line ranging from 1 oz. to 8 oz., but will not be as well graded as a line having the same average weight but ranging in size from 2 oz. to 4 oz. The use of sieves to determine size is unreliable, giving varying results according to the way they are used and the tuber-shape of the variety. Moreover, growers would have to purchase a whole range of

sieves. On the other hand, probably every home has a set of moderately reliable scales, and taking tuber-weights is both accurate and convenient, and falls into line with the custom of referring to the size of tubers in terms of ounces. It has already been explained that the average weight gives no indication of the uniformity of grading, but the degree of uniformity

may be indicated by taking at random a definite number of tubers from each sack till 100 are obtained (see Figs. I, 2, and 3). If these are laid out on the ground they represent to the eye the average grading of the sacks sampled. To convey this grading in terms of figures the method adopted is to separate by eye measurement the sixteen largest and the sixteen smallest tubers. If these lots are weighed separately the weight will convey, after some little practice, a reasonably accurate idea of the standard of grading in that particular line. Sixteen tubers afford,- a convenient number, because if the weights are recorded in pounds they indicate also the average weight per tuber in ounces. Thus a line grading “ 6/3 ” means that the sixteen largest tubers weigh 6 lb. as against 3 lb. for the sixteen smallest. It also indicates that the

average . weight. of the sixteen largest tubers is 6 oz., and the average of the sixteen smallest 3 oz. The range is from round about 6 oz. to round about 3 oz. A few tubers will be over 6 oz. and a few below 3 oz. These grading figures are always to be found on the tags attached to the sacks by the grower. If the merchant regrades, the figures, of course,, fail to be of any value. . The final tuber-inspection for certification is undertaken when the seed is graded, in sacks, ready for sale. At least one sack in every six is opened up (with a minimum of eight sacks in any one line) and twenty-five tubers are taken at random from every sack opened. Each lot of 100 tubers-is weighed for the grading standard. Each tuber is inspected,. . and ; a certain proportion cut . to determine the percentage of disease present..

GRADING STANDARDS FOR THE 1929-30 CROP

In regard to grading, the following standards will be adopted for the 1929-30 seed-crop (1) If the sixteen largest tubers weigh under 2 lb. the line should be rejected. (2) If the sixteen smallest tubers weigh under if lb. the line should be rejected, but may be regraded and again inspected at the convenience of the certifying officer (Fig. 7).' . (3) If the sixteen smallest tubers weigh more than 4J lb. the grading figures should be indicated on the tag and the tag branded “ Table ” (Fig. 8). ■ ; (4) If the difference between the weight of the sixteen- smallest tubers and’ the weight of the sixteen largest tubers exceeds 2J lb. the line should be rejected for inefficient grading, but may be regraded and inspected at the convenience of the certifying officer (Figs. 4,5, and 6).

PURITY AND DISEASE STANDARD. —FINAL TUBER INSPECTION. In this connection the certifying officer will reject if there are present more than 2 per cent, of rogues (foreign varieties), 3 per cent, of late blight, 4 per cent, of powdery scab, 2 per cent, of bacterial rot, 4 per cent, of potato moth, 4 per cent, of eel-worm, or 7 per cent, of dry-rot, badly scabbed, or badly damaged tubers. ■ These standards are not. permanent. They represent a tightening of the previous season’s regulations, and it is probable that the follow-ing-season will, see the percentages of rogues, and \ disease allowed in certified seed still further restricted. . ‘ .4..

ACCEPTANCE OF CROPS PLANTED WITH CERTIFIED SEED NOT A MATTER OF COURSE.

It is obvious that a farmer who plants certified seed will not necessarily have his crop accepted; in. fact, it may be very much below the standard of the seed he purchased. Northern Star is the most common rogue in our white varieties, and increases so rapidly that 2 per cent, present in a certified line has been known to increase to 7 per cent, in one season. If the standard of purity were 100 per cent, the position would be different, but as matters stand at present growers must at all times be prepared to rogue their crops. The presence in a crop of such factors as. late blight, potato-moth, dry-rot, scab, &c.,

depends not so much upon the seed planted as upon climatic, soil, and management factors. It would be unwise to distribute seed containing an undue proportion of tubers affected with late. blight, but whether the subsequent crop is affected depends almost entirely upon the climatic conditions during the growing-period. It is obvious, therefore, that the planting of certified, seed carries no guarantee that the resultant crop will be passed. The use of certified seed is a wise and reasonable precaution which will ensure that under normal conditions satisfactory yields will be obtained, provided the variety is suited to the locality.

■ -Certificate-of-record and Official Herd-test. —During March, 1930, <846 cows, in the ownership of 267 breeders, were under C.O.R. test, as compared with 649 cows and 226 breeders in the corresponding month of 1929. The O.H.T. figures for March, 1930, 'were 1,341 cows and 128 breeders, as against 1,569 and 122 respectively for the same month • last year. . '