Low live weight gain of cattle in the wet season of tropical areas was identified as a major limitation to achieving annual growth rates from tropical pasture systems sufficient to meet new market specifications of young animals of high carcass weight. Both protein and energy are limiting nutrients for growth. Net transfer of feed protein to the intestines is often not complete, and losses occur with grasses and legumes when CP content exceeds 210 g of CP/kg of digestible OM. This protein loss is important because a collation of experimental data indicated that cattle consuming low- and high-quality pasture and silage-based diets all responded to extra protein. The response was less for the higher-quality forage. The role of legumes in supplying this protein was investigated and, unless legumes can increase total DMI by at least 30%, they will not supply sufficient intestinal protein to increase live weight gain by about 300 g/d. The problem with legumes and some grasses is the loss of protein from the rumen, and increasing energy supply to the rumen, either through improved digestibility or energy supplements, is a strategy that could be used to reduce this. Strategies to increase the proportion of escape protein would be successful, but incorporation of lowly degradable protein fractions into legumes may be more difficult because of the level of expression of these protein fractions required for a significant live weight gain response. Cattle entering the wet season usually exhibit compensatory growth and are exposed to high ambient temperatures and often to high humidity. Intestinal protein above that stipulated in feeding standards may be beneficial in these circumstances, and more emphasis should be placed on the ability of legumes to supply protein postruminally. At present the protein delivery capacity of agronomically competitive legumes seems to be inadequate for the higher growth rates required in production systems, and supplements of energy and protein will be needed to achieve these higher targets until new cultivars appear.

The effects of heat stress on dairy production can be separated into 2 distinct causes: those effects that are mediated by the reduced voluntary feed intake associated with heat stress, and the direct physiological and metabolic effects of heat stress. To distinguish between these, and identify their effect on milk protein and casein concentration, mid-lactation Holstein-Friesian cows (n = 24) were housed in temperature-controlled chambers and either subjected to heat stress [HS; temperature-humidity index (THI) ~78] or kept in a THI<70 environment and pair-fed with heat-stressed cows (TN-R) for 7 d. A control group of cows was kept in a THI<70 environment with ad libitum feeding (TN-AL). A subsequent recovery period (7 d), with THI<70 and ad libitum feeding followed. Intake accounted for only part of the effects of heat stress. Heat stress reduced the milk protein concentration, casein number, and casein concentration and increased the urea concentration in milk beyond the effects of restriction of intake. Under HS, the proportion in total casein of αS1-casein increased and the proportion of αS2-casein decreased. Because no effect of HS on milk fat or lactose concentration was found, these effects appeared to be the result of specific downregulation of mammary protein synthesis, and not a general reduction in mammary activity. No residual effects were found of HS or TN-R on milk production or composition after THI<70 and ad libitum intake were restored. Heat-stressed cows had elevated blood concentrations of urea and Ca, compared with TN-R and TN-AL. Cows in TN-R had higher serum nonesterified fatty acid concentrations than cows in HS. It was proposed that HS and TN-R cows may mobilize different tissues as endogenous sources of energy.

In reproductive swards, stems appear to act as vertical or horizontal barriers to bite formation, influencing instantaneous intake rate (IIR). The hypothesis was tested that the stems' barrier effect is determined by the physical properties and density of stems. Artificial microswards, consisting of 20-cm leaves and 15-cm stems of Panicum maximum, were offered to three steers (362 kg) in a factorial combination of three stem densities (0, 100 and 400 stems m )2 ) and two levels of stem tensile resistance [low (LTRS) and high tensileresisting stems (HTRS)]. LTRS were not a barrier to defoliation and did not affect bite depth and bite mass. HTRS acted as both a horizontal barrier and a vertical barrier depressing bite depth (13AE4, 13AE6 and 5AE1 cm for 0, 100 and 400 stems m )2 , respectively), bite area (89AE3, 50AE8 and 47AE6 cm 2 for 0, 100 and 400 stems m )2 , respectively), bite mass (0AE51, 0AE29 and 0AE11 g for 0, 100 and 400 stems m )2 , respectively) and IIR (23AE8, 10AE5 and 3AE6 g sec )2 for 0, 100 and 400 stems m )2 , respectively). The results confirmed the importance of the density and physical properties of stems as determinants of the stems' barrier effect on bite dimensions and IIR.

This study assessed the use of pasture attributes to control daily intake and diet quality during progressive defoliation on pastures of Axonopus catarinensis. Three consecutive 12‐day grazing treatments of progressive defoliation were conducted with Brahman cross‐steers. Daily forage intake and defoliation dynamics were assessed using a pasture‐based method. The treatments differed in initial sward height (33, 44 and 61 cm) and herbage mass (1030, 1740 and 2240 kg ha−1). The post‐grazing residual sward height, at which forage intake decreased, appeared to increase with the initial sward height (12·3, 14·6 and 15·5 cm). Steers grazed up to four distinctive grazing strata in all treatments. The depth and herbage mass content of the top grazing stratum were at least five times higher than the lower grazing strata in all treatments. This explains why forage intake decreased when the top grazing stratum was removed in approximately 93% of the pasture area in all treatments, equivalent to approximately 7% of the pasture area remaining ungrazed. We conclude that the residual ungrazed area of the pasture, rather than residual sward height, can be used to develop grazing management strategies to control forage intake and diet quality in a wide range of pasture conditions.

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The effects of stem density of tropical swards and age of cattle on their foraging behaviour were evaluated using artificial microswards, consisting of leaves of 20 cm in height and high tensile-resisting stems of 25 cm in height of Panicum maximum. The treatments consisted of a factorial combination of four stem densities of swards (0, 100, 200 and 400 stems m )2 ) and two ages of cattle (1-and 3-year-old steers). There was a significant interaction between stem density of sward and age of cattle for bite area (BA), bite mass (BM) and instantaneous intake rate (IIR). Stem density had a significant negative effect on these variables describing ingestive behaviour which was particularly strong for older steers. In leaf-only swards, mature cattle achieved a much greater BA (106AE5 vs. 57AE9 cm 2 ), BM (0AE88 vs. 0AE47 g DM) and IIR (46AE9 vs. 17AE2 g DM min )1 ) than did young cattle. However, these variables were very similar across ages of cattle at the highest stem density of sward. These results show the importance of the high tensile-resisting stems as deterrents of the grazing process in tropical pastures, particularly in older cattle.

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