The Agricultural and Related Industries Study Group of the Operational Research Society held a one-day meeting on ‘Modelling Agriculture under Pressure’ on 22 May 1991 at the Open University, Milton Keynes, UK. The group promotes the use of the scientific method in solving management problems. The aim of this meeting was to look at different ways of modelling the effect of changes on farm economics and profitability. Three papers discussed partial-equilibrium economic models and national policies. The other three papers concerned farm profitability. The speakers generated lively discussions. One of the most memorable replies was from Peter Midmore to the question of what he would use instead of input-output analysis: ‘1.2’.

Optimum applications of N fertilizer, Nopt have been related successfully to the amount of mineral N in the soil, Nmin in some parts of Europe but not always in the UK. If there is a body of mineral N, QN, that ultimately lessens the need for N fertilizer, it will not remain constant in its amount or its position. Mineralization will add to QN, while the nitrate component of QN will be leached downwards.Also, part of QN will be taken up into the crop where it will continue to lessen the need for fertilizer N but will be safe from leaching. A computer model was used to simulate these processes for 23 experiments, covering five sites and five years, in which N opt had been estimated. From these simulations we derived trial values of QN that took account of mineral N to a series of depths on a series of dates. For each date we used the trial values to find the depth for which Nopt was best correlated with QN andassumed that this was the depth, dL, of the lower boundary of QN on that date. Thus dL was a collective value for all 23 experiments. The value of dLincreased throughout the winter and the spring and was very closely related to the cumulative average drainage through 0·5 m soil at Rothamsted. By 15 April, dL, was 1·66 m, a depth that was compatible with observations by others that winter wheat can remove mineral N to a depth of at least 1·5 m. We inferred two likely reasons why Nmin may fail as a predictor of Nopt in the UK: insufficient depth of sampling, and too wide a spread of sampling dates. The values of Nopt were shown to be related satisfactorily to the values of QN computed, without any measurements of mineral N, for appropriate depths on single dates.

Three change-over experiments with yearling cattle examined the effects on voluntary intake and digestibility of treating barley, oat and wheat straws with NaOH. Increases in intake due to the NaOH treatment were: with barley straw 31 %, oat straw 9% and with wheat straw 1 %, and the corresponding improvements in digestibility were 36, 13 and 24%. A further three experimentswith yearling heifers and steers measured the effects of supplements on the responsein feeding value to NaOH treatment of wheat straw. Increasing the daily supplement of rolled barley from 10 to 3·4 kg led to a decrease in voluntary intake of treated straw from 4·6 to 3·6 kg organic matter (OM), while including urea, to increase the concentrationof nitrogenin a beet pulp supplement from 19 to 24 gN/kg dry matter (DM), gave an increasein intakeof straw from 3·4 to 4·8 kg OM. Providing more than c. 15 gN/kg dietary DM did not lead to a significant increase in voluntary intake or digestibility. In a further experiment, isonitrogenous supplements of rolled barley and soyabean meal, or rolled barley and fishmeal, led to similar intakes and digestibilities of treatedstraw. The variation in response to NaOH treatment is discussed.

Twenty-four Bos indicus cows of the Mashona/Nkone breeds, 36 dairy crossbreds (Mashona/ Nkone x Friesian), 48 Holsteins, 42 Friesians and 25 Jerseys were used to study the effect on milk production of sometribove (500 mg), a prolonged release formulation of bovine somatotropin (BST). BST was administered at 14-day intervals by subcutaneous injections into the ischio-rectal fossa which lies lateral to and on either side of the anus (tailhead). With the exception of the Bos indicus cows, which received seven injections of BST, all other treated cows received eight.

The administration of BST to Bos indicus cows, significantly (P < 0·05) increased milk yield from 0·45 to 1·75 kg/cow per day, and extended lactation. These effects combined to increase total milk production during the 14-week trial from 226 kg for the control group to 993 kg for the group treated with BST. With dairy crossbreds the use of BST significantly (P < 0·01) increased milk yield from 8·6 to 11·0 kg/cow per day. Milk composition for Bos indicus and dairy crossbreds wasunaffected by the administration of BST

BST significantly (P < 0·05) increased milk yield in Jersey (+ 2·9 kg/day), Friesian (+ 3·6 kg/day) and Holstein (+ 2·7 kg/day) cows. Bodycondition of treated cows tended to be slightly lower than that of the control cows at the end of treatment.

Cows remained in excellent health throughout all the trials. Farmers commented on how easy the injection was to give in the tailhead site, and on the complete absence of any adverse injection site reaction.

Breeding records over 12 years (1977–89), involving 455 West African Dwarf does derived from an original population of 133 breeding does, were used to examine the influence of dam age on the incidence of multiple births and perinatal reproductive wastage due to abortion and neonatal deaths. The animals were semi-intensively managed in two goat research units at the University of Ibadan Teaching and Research Farm.

The regression results showed that the variable component, multiple births, increased (P < 0·01) linearly with advancing age of the doe, while the incidence of abortionsand neonatal deaths increased (P < 0·01) exponentially with doe age. Graphs plotted from the regression results indicated a sharp increase in the incidence of abortion and neonatal deaths between the ages of 6 and 7 years. This suggests that the practice of keeping these animals for longer than necessary, especially in rural areas of the country where they are mostly reared and kept for prestigious reasons, should be re-evaluated. Therefore it is recommended that to maintain an economically viable production system, does should be culled between the ages of 6 and 7 years and the provision of replacements planned accordingly.

Maiden 18-month-old Cashmere does (JV=254) were inseminated laparoscopically with frozenthawed semen after treatment with Controlled Internal Drug Release devices and pregnant mare's serum gonadotrophin (PMSG) in the breeding season of 1989 in Tasmania. Kidding rates(does kidded/inseminated) were not statistically different after the insemination of 1, 5 or 25 × 106 motile spermatozoa (50·0, 55·8 and 56·4%), andfor semen frozen by the pellet or straw methods (56·8 and 50·8%). The fertility results suggest that the critical threshold may not have been reached at 1 × 106 frozen-thawed motile spermatozoa.