NORTH ISLAND ILL-THRIFT

Press, Volume XCVII, Issue 28606, 7 June 1958, Page 9

NORTH ISLAND ILL-THRIFT

Nitrate Factor In Grass A high nitrate content m rapidly-growing autumn pastures may be responsible for unthriftiness or ill-thrift in lambs, according to work which has been done in the North Island in studying the chemical composition of herbage. Dr. G. W. Butler, of the Plant Chemistry Division of the Department of Scientific and Industrial Research, told the farmers’ conference at Lincoln that on the Messey College sheep farm in the summer and autumn of last yeai lambs were grazed on two paddocks containing pure stands of perennial ryegrass and short rotation ryegrass. One paddock received three applications of lewt of sulphate of ammonia between November and April and the other received none.

On the paddock where no nitrogenous fertiliser was used, the rates of weight gains among the lambs were generally higher. On this paddock, the average overall gain in live-weight for six weeks was 4.91 b plus or minus 1.31 b, whereas on the fertilised pasture it was only 2.51 b. plus or minus 11b.

On the fertilised paddock, the lambs had satisfactory live-weight gains on only three out of eight weeks and on three occasions they had actually shown a net loss of weight. This trial, he said, had confirmed the poor thrift of hoggets feeding on rapidly-growing ryegrass growing under conditions of high fertility.

Dr. Butler said that work previously carried out at the Rukuhia Soil Research Station and by the Plant Chemistry Division had shown that the levels of nitrate in the soil under high fertility pastures in New Zealand could rise in the autumn to levels which in other countries would be regarded as typical of arable soils The herbage samples taken during the period of autumn flush growth were therefore screened to determine whether the hign nitrate levels in the soil gave rise to any nitrogenous chemical compounds not present at other times of the year Tenfold Increase It had been found, he said, that the nitrate content increased as much as tenfold in rapidly-grow-ing ryegrass. Grass growing in urine patches and stock camps and bordering bare patches of ground were more likely to contain herbage high in nitrate, but during the autumn flush the whole pasture contained “high nitrate” grass. Further, most nitrate was found 'in the lower parts of the ryegrass tillers—in and near the leaf sheaths and round the growing points of young leaves. Since the sheep was such a close grazer, it would be realised that the proportion of basal plant material which it ate was likely to be quite high.

Samples of autumn flush grass from the trial had a potassium nitrate equivalent content of 1 to 1.5 per cent., but in a few instances a content of up to 7 per cent, had been observed on areas which had been highly fertilised. “It is pertinent to consider, therefore. the possibility that the illthrift in this trial was due to the nitrate content of the herbage. ’ said Dr. Butler. It was well known that the ingestion of nitrate by ruminants could cause poisoning through conversion of the nitrate to nitrite by rumen micro-organisms. Normally nitrate would be reduced in the rumen by a stepwise process through nitrite to ammonia, but in the absence of sufficient sugar in the feed (when the ratio of carbohydrate to nitrate was low) appreciable amounts of nitrite might accumulate. The nitrate combined with the hemoglobin in the blood, and depending on the amount death might ensue.