Sixteen Holstein-Friesian animals, comprising 8 multiparous cows and 8 first-lactation heifers, were divided into 2 equal groups which had access to either tree shade or no shade. All animals were in the first third of lactation and were subjected to treatment for 8 weeks in summer 1985. Average maximum monthly temperatures ranged from 26.2 to 28.22� over the experimental period. The provision of shade increased (P< 0.01) milk yield; however, there was a significant (P<0.05) interaction between response to shade and cow parity. Mean daily milk yields for multiparous cows were 19.2 and 17.2 kg/cow (P< 0.01) and 16.3 and 15.9 kg/cow (P> 0.05) for heifers in the shade and no shade groups respectively. Cows without shade incurred a significant decrease (P< 0.05) in solids-not fat yield, lactose percentage and lactose yield, and a significant increase ( P < 0.05) in somatic cell count. The composition of milk from heifers without shade was no different from that from heifers with shade. Rectal temperature was higher (P < 0.01) in animals without shade (mean 40.0�C) than in animals with shade (mean 39.4�C). The highest mean maximum rectal temperature of 41.6�C was recorded in the noshade group. It was concluded that shade is essential if milk yield and milk composition are to be maximised in this environment.

Field studies were conducted over 5 years on two dairy farms in southern Queensland to evaluate the impacts of zero-tillage, nitrogen (N) fertiliser and legumes on a winter-dominant forage system based on raingrown oats. Oats was able to be successfully established using zero-tillage methods, with no yield penalties and potential benefits in stubble retention over the summer fallow. N fertiliser, applied at above industry-standard rates (140 vs. 55 kg/ha.crop) in the first 3 years, increased forage N concentration significantly and had residual effects on soil nitrate-N at both sites. At one site, crop yield was increased by 10 kg DM/ha.kg fertiliser N applied above industry-standard rates. The difference between sites in fertiliser response reflected contrasting soil and fertiliser history. There was no evidence that modifications to oats cropping practices (zero-tillage and increased N fertiliser) increased surface soil organic carbon (0–10 cm) in the time frame of the present study. When oats was substituted with annual legumes, there were benefits in improved forage N content of the oat crop immediately following, but legume yield was significantly inferior to oats. In contrast, the perennial legume Medicago sativa was competitive in biomass production and forage quality with oats at both sites and increased soil nitrate-N levels following termination. However, its contribution to winter forage was low at 10% of total production, compared with 40% for oats, and soil water reserves were significantly reduced at one site, which had an impact on the following oat production. The study demonstrated that productive grazed oat crops can be grown using zero tillage and that increased N fertiliser is more consistent in its effect on N concentration than on forage yield. A lucerne ley provides a strategy for raising soil nitrate-N concentration and increasing overall forage productivity, although winter forage production is reduced.

SUMMARY The relationships between some environmental factors and reproductive rates of dairy cattle during summer in a tropical upland area were studied using records for Holstein‐Frieslan milking cows and non‐lactating growing helfers maintained at the Kalri Research Station on the Atherton Tableland in north Queensland. The Station is situated in a tropical upland environment with warm, wet summers and dry, cool winters. Data were collected over a 12‐year‐period (1977 to 1988) for cows grazing nitrogen (N)‐fertilised grass pastures and receiving either no supplement or about 3.5 kg molasses/cow/day. Cattle were mated over a 10‐week period starting from mid‐January to early February, with 32 to 36 cows and 5 to 42 heifers used in each year. There was considerable variation among years in the pregnancy rate after 3 Inseminations, with a low of 39% and a high of 94% pregnancy. This variation from year to year was reduced for cows grazing pastures receiving 300 kg N/ha/yr after molasses feeding commenced in 1984–85. Pregnancy rates after molasses Introduction were higher for cows grazing pastures receiving 300 compared with 100 kg N/ha/yr. Pregnancy rate in cows and heifers was reduced from 80 to 55% as mean daily maximum temperature increased from 26°C to 27.5°C. At mean maximum temperatures above 27°C for cows and 27.6°C for heifers pregnancy rates to 3 Inseminations were consistently below 60%. Much of the year to year variation in pregnancy rate was related to rainfall during the mating period. There was a positive association between pregnancy rate and rainfall up to 1000 mm, and this was associated with differences in growth rate of pasture. In years of very high rainfall, above 1500 mm, there was a depression in pregnancy rate, which may be related to the intense humidity in very wet years causing heat stress in cows and associated problems such as feet infections, or a deterioration in pasture quality. We conclude that much of the variation in pregnancy rate during summer may have been related to heat stress and the level of feed supply in the form of pasture and supplements.

Dairy farms located in the subtropical cereal belt of Australia rely on winter and summer cereal crops, rather than pastures, for their forage base. Crops are mostly established in tilled seedbeds and the system is vulnerable to fertility decline and water erosion, particularly over summer fallows. Field studies were conducted over 5 years on contrasting soil types, a Vertosol and Sodosol, in the 650-mm annual-rainfall zone to evaluate the benefits of a modified cropping program on forage productivity and the soil-resource base. Growing forage sorghum as a double-crop with oats increased total mean annual production over that of winter sole-crop systems by 40% and 100% on the Vertosol and Sodosol sites respectively. However, mean annual winter crop yield was halved and overall forage quality was lower. Ninety per cent of the variation in winter crop yield was attributable to fallow and in-crop rainfall. Replacing forage sorghum with the annual legume lablab reduced fertiliser nitrogen (N) requirements and increased forage N concentration, but reduced overall annual yield. Compared with sole-cropped oats, double-cropping reduced the risk of erosion by extending the duration of soil water deficits and increasing the time ground was under plant cover. When grown as a sole-crop, well fertilised forage sorghum achieved a mean annual cumulative yield of 9.64 and 6.05 t DM/ha on the Vertosol and Sodosol, respectively, being about twice that of sole-cropped oats. Forage sorghum established using zero-tillage practices and fertilised at 175 kg N/ha.crop achieved a significantly higher yield and forage N concentration than did the industry-standard forage sorghum (conventional tillage and 55 kg N/ha.crop) on the Vertosol but not on the Sodosol. On the Vertosol, mean annual yield increased from 5.65 to 9.64 t DM/ha (33 kg DM/kg N fertiliser applied above the base rate); the difference in the response between the two sites was attributed to soil type and fertiliser history. Changing both tillage practices and N-fertiliser rate had no affect on fallow water-storage efficiency but did improve fallow ground cover. When forage sorghum, grown as a sole crop, was replaced with lablab in 3 of the 5 years, overall forage N concentration increased significantly, and on the Vertosol, yield and soil nitrate-N reserves also increased significantly relative to industry-standard sorghum. All forage systems maintained or increased the concentration of soil nitrate-N (0–1.2-m soil layer) over the course of the study. Relative to sole-crop oats, alternative forage systems were generally beneficial to the concentration of surface-soil (0–0.1 m) organic carbon and systems that included sorghum showed most promise for increasing soil organic carbon concentration. We conclude that an emphasis on double- or summer sole-cropping rather than winter sole-cropping will advantage both farm productivity and the soil-resource base.

A group of 40 Holstein-Fresian cows grazing tropical pastures were supplemented from 14 days after calving with a grain-based concentrate containing 15% protein. Concentrate was given at nil, 2, 4, 6, and 8 kg/cow. day fed once daily after morning milking, and 4, 6, and 8 kg/cow.day in 2 equal feeds, after morning milking and before evening milking. The experimental period was 250 days for cows fed once daily and 150 days for those fed twice daily. Over 250 days of lactation, the milk yield of cows fed once daily was significantly increased with increasing grain feeding, from 12.8 kg/day for the nil group t o 20 kg/day for the 8 kg group. Milkfat percentage was lower (P<0.05) for cows given 8 kg grain concentrate daily than for those given 2-4 kg, and total fat yield increased (P<0.1) with up to 4kg/cow.day. Protein percentages were similar across treatments, and protein yield increased (P<0.05) with grain level. Lactose percentage was increased (P<0.05) with concentrate feeding, and lactose yield increased (P<0.05) with increasing concentrate level. Fat-corrected milk (FCM) increased (P<0.05) from 3046 kg at nil concentrate to 4465 kg at 8 kg/cow. day. Over the first 150 days of lactation, milk yield increased from 19.5 kg/day for cows fed once daily to 21.7kg/day for those fed twice daily. Milkfat and protein percentages were similar. Greatest responses to twice-daily feeding occurred at 6 kg concentrate/cow.day. It was concluded that total milk yield increased linearly to 8 kg concentrate/cow.day, but FCM yield was at a maximum at 6 kg concentrate1cow.day. There were advantages in feeding 6 kg concentrate twice daily.