Dairy Cattle Nutrition

The following article is a special collaboration from AFMA (Animal Feed Manufacturers Association) www.afma.co.za
We thank their kind support.

Introduction

The objective of this paper is to address recent findings in the field of dairy cattle nutrition that are in the interest of on farm dairy producers in South Africa in order to optimise the efficiency of their dairy cattle operations.

Although dairy farmers have to a certain extent implemented dry cow feeding and management practises a lot more can still be done. It is in the financial interest of all dairy farmers to implement new research findings in order to optimise all aspects of importance in dry cow feeding and management. Recent research findings in this field are addressed to help dairy farmers increase the efficiency of their dry cow management practises and to ensure increased efficiency in production and reproduction during the next lactation. Recent research in the field of nutrition during lactation to optimise reproduction efficiency is also addressed. Although often discussed, forage quality is still probably the factor with the largest potential to increase efficiency of milk production, especially in high producing herds. Several aspects on forage quality for lactating dairy cows are also discussed.

Dry Cow Feeding and Management

The positive responses of effective dry cow management and nutrition have lead to increased interest in optimising this very important phase of dairy production.

Although South African dairy producers have to a certain extent implemented dry cow strategies a lot more remains to be done in order to maximise the financial benefits of a well planned and executed dry cow feeding and management program.

The aim of a well planned and executed dry cow feeding and management program is to mainly prevent or limit the incidence of fresh cow problems, thereby maximising the chances of the fresh cow to a "flying" start into the new lactation. The incidence and cost of fresh cow problems are still at very high levels and need to be addressed seriously.

Guard (1999) calculates the average cost per case of retained afterbirths in the U.S. at $206. The average incidence of retained placentas in US dairy herds is 15 % of all calvings bringing the total loss per 100 calvings per year to $3090.

The incidence and impact on reproduction and milk production of fresh cow problems as a percentage of cows that calve in US herds as reported by Hoards Dairyman is given in Table 1.



Table 1 The incidence and impact on reproduction and milk production of fresh cow problems (Hoards Dairyman, 1996)

* Includes cows with two or more related disorders listed

An effective dry cow feeding and management program will have to include the following components:

Dry matter intake of close-up cows should be maximised.

Dry matter intake (DMI) is one of the most impo rtant aspects of the transition period (3 weeks before calving until 3 weeks after calving) and must therefore be maximised at all cost during the last three weeks before calving. Grummer (1998) showed that the DMI of mature cows decrease by 31 % during the last three weeks before calving while Grummer (1995) demonstrated a clear positive correlation between DMI one day before calving and DMI 21 days after calving. Maximising DMI of close-up cows with a condition score of 3,25 – 3,50 is probably the most important management achievement for dairy producers to prevent fatty livers and to ensure high DMI’s in early lactation.

In order to allow the rumen micro-organisms to adapt to higher levels of non-structural carbohydrates (grains) and especially the type of grain (starch) that will be fed during early lactation, it is recommended that the close-up diet provides concentrate levels at 1 % of body weight and that the lactation grain type should constitute 70 % of the concentrate portion (Van Saun, 1991). Eastridge (1999) recommends that the close-up diet should contain a minimum of 25 % and a maximum of 45 % grain on a DM basis. These levels of concentrate and grain inclusion in close up diets will ensure micro-organism adaptation as well as the elongating of the rumen papillae from 0,5cm on the early dry period roughage diet to 1,2 cm on the lactation diet containing 50-60 % concentrates.

In practise dairymen, especially those feeding total mixed rations (TMR’s) during lactation, should also feed TMR’s to close-up dry cows containing 25 to 45 % grain. Close-up cows should be fed three to four times daily to encourage maximum DMI’s.






Ensuring that cows calf at a condition score of 3,25 to 3,5.

Thatcher et al (1999) recommend a condition score of 3,25 to 3,75 at calving while Eastridge (1999) recommends a condition score of 3,25 to 3,50. Most nutritionists are recommending a lower condition score (3,25 to 3,50) than in the past to prevent subacute and acute fatty liver syndrome or ketosis. Reid (1982) has shown that up to 66 % of high producing cows in the U.S. suffer from subacute fatty liver syndrome or ketosis in early lactation and that the main reason for this problem is overconditioned cows at calving resulting in a too high mobilisation of body fat and too low dry matter intakes during the close-up dry and early lactation periods.

Waltner et al (1993) showed that cows calving at a condition score of 3,5 produced more milk over the first 90 days of lactation when compared with cows with lower or higher condition scores. It is also well known that the most economical time to increase cow condition is the last 60 days of lactation as lactating cows are 15 % more efficient than dry cows (Moe, 1965). If necessary cow condition can also be increased during the dry period, however it is not recommended that cows should lose any condition during the dry period as this puts extra stress on the liver at a time when the cow is being prepared for the new lactation.

In practise dairymen are encouraged to utilise the tool of body condition scoring and ensure that the condition of cows is "corrected" during the last 60 days of lactation, to further ensure that no cows will lose condition during the dry period and that ultimately all cows will calve with a condition score of 3,25 to 3,50.

Ensuring that cows calf in an optimum calcium status.

This is probably the most important management tool that dairymen can use to prevent a very large number of problems at calving and in early lactation. South African dairy farmers have to a certain extent implemented some of the actions that will ensure optimum calcium status at calving. It seems as if dairymen are happy with their program for the optimisation of calcium status at calving when no acute symptoms of a calcium deficiency (milk fever etc.) at calving is experienced. It is of utmost importance for dairymen to understand that the economic benefits resulting from a full implementation of all the factors involved in maximising the calcium status of dry cows at calving is of such a magnitude that all possible actions should be implemented to the full to ensure the best possible optimum calcium status at calving and therefore a cow with the best possible chance of achieving top performance in that lactation.

It is important to realise that a non-optimum calcium status at calving could lead to several serious problems at calving and in early lactation (Table 2).

Table 2 Calving and early lactation problems related to a non-optimum calcium status at calving. (Stallings, 1998; Goff & Horst, 1997; Goff, 1999)

In order to prevent or minimise the occurrence of these problems dairymen should ensure that the close-up dry cow diet is well formulated in terms of macro-minerals, which will ensure cows calving in an optimum calcium status.

According to Goff (1999) a good macro-mineral profile for a close-up dry cow diet (lasts 3 weeks before calving) should be as follow (DM basis):

- 0,4 to 0,5 % when no anionic salts are added.

Goff (1999) recommends that the close-up diet should be formulated using forages with the lowest possible potassium content that can be found, while still being reasonably well digestible. Corn silage is excellent. From the given close-up diet macro-mineral specifications it should be clear that the biggest challenge for the on-farm situation would be to provide dry cows forages with the lowest possible potassium content. Table 3 provides potassium values of typical South African feedstuffs.

Table 3 The potassium contents of typical South-African feedstuffs

Maize silage or small grain hays will be the most ideal forages for close-up cows. Secondly anionic salts must be added to the diet to ensure the lower urine pH’s. Thirdly extra calcium must be added to the close-up diet when anionic salts are included to ensure a daily calcium intake of 120 to 200g for Holsteins and 90 to 150g for Jerseys. Very often anionic salts are added to close-up diets containing high potassium content forages resulting in a poor effect or the extra calcium that is recommended for close-up diets, containing anionic salts, are not added to the diet.

There is no doubt that formulating close-up diets in detail for macro-mineral balance will result in cows calving in an absolute optimum calcium status, thus ensuring the best possible chance for a cow to achieve optimum production for that specific lactation. It must be ensured that the sodium and potassium levels of the close-up diet are as low as possible, that an effective anionic salt mixture is added with increased calcium and magnesium levels and that the phosphorous and sulphur levels are on the recommended specifications.


Optimising the immune system.

The dairy cow’s immune system is depressed during the weeks before and after calving (Kehrli et al, 1989; Nagahata et al, 1988) with neutrophils exhibiting an impaired ability to ingest and kill bacteria. Increased supplementation of trace elements and vitamins, important in optimising the immunocompetence of the cow, will strengthen the immune system at this time.

According to Chew (2000) even though all essential micronutrients have long been identified and deficiency conditions well understood, modern day animal agriculture is mainly aimed at nutrient supplementation beyond correcting for deficiencies; it is aimed, rather, at minimising stress and optimising production efficiency. Therefore antioxidant supplements (manganese, zinc, selenium and copper as well as vitamins A and E) must be increased during the dry period. Goff (1999) recommends increasing NRC trace element requirements by 20 to 50 %.

In practise it is recommended that dry cows be supplemented with trace element and vitamins at 100 tot 150 % of NRC requirements. It is further recommended that cows be subcutaneously injected with chelated zinc, manganese, copper and selenium, and intramuscularly with vitamin A and E at 21 days before calving. This will ensure optimum levels of antioxidants at calving as well as optimum synergism between zinc and vitamin A (Cakalla et al, 1992) as well as selenium and vitamin E (Weiss et al, 1990). The importance of an injectable trace element source, 21 days before calving, was accentuated by the work of Harrison & Russell (1984) who showed that increasing the percentage calcium in diet DM above 1,2, decreases selenium absorption from the digestion track from an optimum of 50 % to below 20 %. Most close-up diets, containing anionic salts, will have calcium levels of above 1,2 % on a DM basis. Calcium is also well known to be a major antagonist for zinc and manganese (Underwood & Suttle, 1999) while Mills (1985) showed that increasing dietary calcium from 0,5 to 1,3 % of diet DM decreased copper retention from 10 mg/kg feed intake to –4 mg/kg feed intake in a diet containing 15,2 mg Cu/kg DM. These negative effects of increased calcium levels on the absorption of selenium, zinc, manganese and copper necessitates an injectable supplement three weeks before calving in an effort to optimise their status at calving for maximum natural resistance. Weiss (1997) is suggesting an intake as high as 4 000 IU of vitamin E/day for the two weeks before calving and 2 000 IU/day for the two weeks after calving.

The value of feed additives for dry cows. According to Stallings (1998) certain feed additives have the potential to be of assistance during the different phases of the transition period (three weeks before calving to three weeks after calving). These feed additives should however always be targeted to address a specific concern as many are expensive and will add significantly to the ration cost (Table 4).

The importance of anionic salts in the close-up diet has been discussed. Stallings (1998) also stressed the importance of anionic salts in close-up diets and even more important the fact that when anionic salts are used the total macro-mineral formulation must be done in accordance to ensure optimum calcium status at calving.

Table 4 Potential uses of feed additives for dry and early lactation cows
(Stallings, 1998)

One of the big challenges during the close-up period is to optimise DM-intake (energy intake) and to decrease or prevent the mobilisation of fat and thus prevent subacute ketosis or fatty liver syndrome. Tsang et al (1997) found that cows fed propionic salt three weeks before to three weeks after calving consumed more dry matter and mobilised less fat.

Fat is generally not added to dry cow diets. However fat in early lactation diets can have very positive effects on reproduction efficiency which will be discussed later in this paper.

Although niacin is used in diets for over conditioned lactating cows to assist in energy utilisation it is not clear if it is needed in close-up dry cow diets, especially in herds that don’t have over conditioned cows or problems with ketosis (Shaver, 1996).

Supplementing propylene glycol is also a means of preventing lipid mobilisation. According to Stallings (1998) there is no indication that it is effective if given before calving. It is recommended that propylene glycol be drenched once only just after calving at 350 to 600ml per cow. Drenching can be continued on a daily basis for up to 14 days after calving at 350ml/cow/day if cows are over-conditioned or if ketosis or fatty liver syndrome is suspected.

Christensen et al (1997) found that feeding propylene glycol as a oral drench or mixed with concentrate and fed separately to ensure total intake of the supplementary propylene glycol within 30 minutes was more effective than feeding it as part of the total diet.

Protected amino acids or high quality by-pass protein sources are expensive. Shaver (1996) indicated that it might be more economical if used during the transition phase (3 weeks before to 3 weeks after calving) rather than throughout the lactation.

Yeast has been a benefit in transition diets in some trials and not in others (Stallings, 1998). Mc Coy et al (1997) found that yeast culture fed during the transition period increased DM-intake and milk production and decreased weight loss. Especially the possibility of stimulating DM-intake of transition cows makes yeast a desirable compound to include in close-up dry cow diets.

Although trace elements and vitamins were not listed in Table 4 they are absolutely essential and are discussed as an important entity on its own.

The close-up dry period offers dairymen a unique opportunity to maximise production and reproduction in the next lactation.

Dairymen should use this opportunity and manage their close-up dry cow diets to ensure maximum DM-intake, a correct balanced intake of macro-minerals to ensure optimum calcium status at calving, and also to ensure optimum trace element and vitamin intake ("topped up" via injections at three weeks before calving) to ensure optimum natural resistance against any infections at calving.


Nutrition and reproduction efficiency.

In this section there will only be very shortly referred to the newer information on aspects influencing nutrition for effective reproductive efficiency. It is important to realise that most primary disorders will have a negative effect on reproduction efficiency. In Table 5 Britt (1992) emphasises the effect of primary disorders on secondary disorders. All the primary disorders lead to lower conception rates. According to Britt (1992) it is much more profitable to prevent these disorders by good dry cow management rather than treating them afterwards.

Table 5 Primary disorders that lead sequentially to secondary disorders in dairy cows (Britt, 1992)

It should be clear that all aspects discussed under dry cow nutrition are as important for optimum reproduction efficiency in the next lactation. Very important aspects and new information on nutrition and nutrition management in lactation affecting reproduction efficiency will be referred to in short in the following sections.

Condition score change and reproduction efficiency.

The goal should be not to loose more than one condition score after calving. Maximising DM-intake during the close-up period (Grummer, 1998) and especially continuing the process after calving by again maximising DM-intake during early lactation with well formulated diets for optimum rumen fermentation and that are high in energy should achieve this goal. Cows that calf with a condition score of 3,25 to 3,50 and consume acceptable levels of a well formulated diet within 10 days after calving should not lose more than 1 condition score after calving and should have a more than 50 % conception rate at first insemination. Any condition losses in excess of 1 condition score will decrease the conception rate.


Optimising the cow’s trace element and vitamin status for maximum conception at breeding.

One of the biggest challenges for dairymen is to ensure that the dairy cow is physiologically in an optimum trace element and vitamin status at time of breeding. Acceptable dietary concentrations of trace elements and vitamins in diets for lactating dairy cows differ from 100 % of NRC standards (Ferguson, 1991) to more than 200 % of NRC standards for certain trace elements (Chandler, 1988). It is of utmost importance that all seven essential trace elements should be supplemented minimally at recommended NRC levels. If higher than NRC levels are included it is recommended that all supplemented trace elements be increased by the same percentage to prevent antagonistic effects.

Because of the absolute importance of trace element and vitamin optimisation at time of breeding it is also recommended that cows are subcutaneously injected with limited amounts of chelated zinc, manganese, copper and selenium and intramuscularly with vitamin A and E three weeks before breeding. This second or alternative route of supplementation at three weeks before calving, which is a very critical stage in the annual cycle of the dairy cow, should enhance the effort for these four most important trace elements and these two most important vitamins for reproduction efficiency to be optimised physiologically.

The simultaneous injection of zinc and vitamin A and selenium and vitamin E should also enhance the very important synergistic effect between these trace elements and vitamins (Cakalla et al, 1992; Weiss et al, 1990).


Milk-urea-nitrogen (MUN) levels and optimum reproduction efficiency.

South African dairymen are often confronted with relatively high MUN levels with the associated fear of its very negative effect on reproduction efficiency. It must be acknowledged that high MUN levels can decrease conception rates significantly. Ferguson (1988) reported that cows with serum urea nitrogen (SUN) levels of higher than 20 mg/dL had conception rates of under 25 %. MUN levels are approximately 90 % of SUN levels. In another trial Ferguson et al (1991) reflected the negative results of high SUN levels in a graph (Figure 1). From these results it should be clear that too high levels of soluble and degradable protein in diets could have very negative effects on reproduction efficiency of dairy cattle.

However lactating dairy cow diets must always be formulated for adequate rumen degradable protein levels to ensure optimum rumen fermentation. Optimum rumen fermentation should never be sacrificed for lower SUN levels. Nutritionists should formulate diets in such a way that the relative high degradable protein content of diets, which is absolutely essential for optimum rumen fermentation, will be utilised to a large extent by the adequate inclusion of non structural carbohydrates (sugars, starches and pectin fibre sources) as well as the adequate inclusion of fats.

Thatcher et al (1999) showed that too high levels of MUN, originating from too high levels of rumen degradable protein in the diet, inhibited the production of normal levels of progesterone resulting in decreased conception rates.

However, Thatcher et al (1999) also showed that the negative effect of high MUN levels on progesterone production can be significantly reduced by increasing the fat content of the diet to 5 % (90 % DM basis) with the inclusion of whole cottonseed (Figure 2).

Therefore, although it is known that early lactation cows do not utilise dietary fats efficiently for milk production, the very positive effect of fat on progesterone production and conception, in diets causing higher than normal MUN levels, favours its inclusion.


The possible use of bovine somatotropin (bST) to increase reproduction efficiency.

Although pro-organic food production groups are still very much against the use of bST for increased milk production, bST has proved itself to be a very safe and very efficient tool for the increase of milk production in lactating dairy cows in South Africa, the USA and other non-European countries. It is however of utmost importance never to treat any cows with a condition score lower than 2,5.

For several years the question has been asked whether dairy cows physiologically need a minimum level of bST for efficient reproduction. Thatcher et al (1999) showed that a single bST injection at day 63 after calving (which was the beginning of the breeding period in a program involving Lutalyse®) increased conception percentage from 30 to 47 %. According to Thatcher et al (1999), although it is not yet clear exactly how bST influence reproduction physiologically, a single bST injection at time of breeding should increase conception significantly in cows with a condition score of 2,5 and higher.

Optimum reproduction efficiency is an absolute must for any successful dairy operation. Dairymen must ensure that they have nutrition programs in place which will result in not more than 1 condition score loss after calving, which will ensure optimum trace element and vitamin status at time of breeding and which will also ensure that MUN-values will not be in excess of 18 to 20 mg/dL. Furthermore bST can also be utilised to enhance conception in cows with a condition score of 2,5 and higher.



Quality roughage for dairy cattle in Southern Africa – the challenge with the largest potential.

It has been the worst summer (1999-2000) for making good quality roughage for dairy cattle in the summer rainfall areas of Southern Africa for the past 2 decades. The exceptionally high rainfall during the summer of 1999-2000 resulted in dairymen being unable to cut silage at the right stage with the effect that most silage was made in the hard dough stage or even later. All lucerne hay producing areas had the same problem resulting in the production of relative poor lucerne hay.

However, dairymen must realise that roughage quality in South Africa is by far the single most important factor in increasing efficiency of milk production. This fact should be taken a lot more serious and the planning for next season’s forage production should already be in place. The weather will probably be a lot friendlier during the 2000-2001 season and the possibility of producing top quality forages will probably be significantly better than during the past summer season.

To further emphasise the absolute importance of roughage quality for dairy cattle it is of great interest to know that when dairymen and nutritionists in the USA were asked to identify the most important factor preventing them from increasing the average milk production of their dairy herd, they singled out "roughage quality". When asked to identify the most important factor that will in future prevent them from increasing their average herd milk production to the target they have set, they again singled out "roughage quality". There should never be any doubt that "roughage quality" is without any doubt the single most important factor in increasing the efficiency of milk production (Hutjens, 2000.)

Although South African dairy farmers are very much aware of the importance of high quality forages it must be acknowledged that this aspect is not being approached or planned with the necessary commitment. There is very often the wrong perception that poor quality forages can be "corrected" by using the newest computer formulation programs or by feeding more of a good quality concentrate.

The forage portion of total diets comprises 30 to 40 % on a DM basis and is absolutely important in determining the milk production potential of the diet.

There are mainly two reasons for this. Firstly the forage portion contributes significantly to total diet energy, protein, calcium, magnesium, fibre and potash contents.

The energy and fibre contents, especially the digestibility of the fibre is of primary importance in this regard. High energy, moderate fibre should always be the ideal. Secondly, the digestibility of the forage type will directly determine the digestibility and flow rate and therefore the dry matter intake and milk production potential of the total diet. Following are a few examples stressing the importance of "cropping" forages at the optimum stage for milk production.

In the first graph, showing the difference of digestibility when small grain silage is cropped in the boot stage versus the milk stage (Figure 3; Staples, 1992), it can be seen that although cropping in the milk stage will produce 54 % more tonnage from a hectare the final product will be 17 % less digestible than silage cropped in the boot stage. Boot stage small grain silage will always be recommended as a forage for high producing dairy cattle while the regrowth can be utilised as pasture for heifers and dry cows to decrease the economic loss of the smaller yield.

The production potential of lucerne hay made in the pre-bloom stage was clearly demonstrated by Kavas (1983). In Table 6 it can be seen that lucerne hay made in the pre-bloom stage resulted in 10,2 kg milk per cow per day more than cows that were fed hay made in the mid-bloom stage.

Table 6 The effect of lucerne growth stage on the production of 4 % fat corrected milk (Kavas, 1983)

It has also been shown that maize silage cropped at the one-half to two-thirds milk line stage and chopped to 6-8 mm, to ensure adequate processing and compaction, will result in higher production than silage cropped at more mature stages (Chandler, 2000). Maize silage offers the greatest opportunity to dairy farmers in the summer rainfall areas of Southern Africa to produce internationally comparable top quality forage for dairy cows, especially when newer silage varieties with higher total plant digestibility and slower grain maturing ability is planted.



REFERENCES
· Britt, J.H., 1992. Reproductive performance as affected by nutrition and environment. Proceedings of the 3rd Annual Florida Ruminant Nutrition Symposium, Gainesville, Florida, January 23-24, 1992, p93.
· Cakalla, S., Kondracki, M. & Bednarek, D., 1992. The effects of oral administration of zinc sulphate to calves on the serum concentration of corotene, Vitamin A and gamaglobulins. Tierarztilche Umschay 47 (5), 342.
· Chandler, P.T., 1988. Vitamins, minerals and feed additives. In: Proceedings of the 1988 Dairy Herd Management Conference, Macom, Geogia, U.S.A., November 1 & 2, 1988, p43.
· Chandler, P., 2000. Establishing nutritive values for corn silage proves to be a challenge. Feedstuffs, April 10, 2000, p10.
· Chew, BP, 2000. Micronutrients play role in stress, production in dairy cattle. Feedstuffs, June 12, 11.
· Christensen, G.O., Grummer, R.R., Rasmussen, F.E. & Bertics, S.J., 1997. Effect of method of delivery of propylene glycol on plasma metabolites of feed restricted cattle. J. Dairy Sci 80, 563.
· Eastridge, M.L., 1999. Providing energy for the periparturient dairy cow. Feedstuffs, June 14, 1999, p10.
· Ferguson, J.D., 1991. Nutrition and reproduction in dairy cows. In: The Veterinary Clinics of North America – Dairy Nutrition Management, July 1991, p483.
· Ferguson, J.D., Blanchard, T., Galligan, D.T., 1988. Infertility in dairy cattle fed a high percentage of protein degradable in the rumen. J. Ani. Vet. Assoc. 192, 659.
· Goff, J.P. & Horst, R.L., 1997. Physiological changes at parturitian and their relationship to metabolic disorders. J. Dairy Sci. 80, 1260.
· Goff, J.P. 1999. Mastitis and Retained Placenta – Relationship to Bovine Immunology and Nutrition. Proceedings of the 1999 Western Canadian Seminar, University of Alberta.
· Grummer, R.R., 1995. Impact of changes in organic nutrient metabolism on feeding the transition dairy cow. J. Anim. Sci. 73,2820.
· Grummer, R.R., 1998. Transitional cow energy, protein nutrition examined. Feedstuffs, September 14, 1998, page 11.
· Guard, C., 1999. Retained Placenta: Causes and Treatments In: Proceedings of the 1999 Western Canadian Dairy Seminar, University of Alberta.
· Harrison, J.H. & Russell, C.H., 1984. Effect of dietary calcium on selenium absorption by the non-lactating dairy cow. J. Dairy Sci. 67, 1860 –1864.
· Hoard’s Dairyman – Dairy Cattle Fertility & Sterility, 1996. W.D. Hoard & Sons Company, 28 Milwaukee Avenue West, Fort Atkinson WI 53538 USA.
· Hutjens, M.F., 2000. Feeding management of the 40 000 pound dairy herds. J. Dairy Sci., Vol. 83, Suppl 1, Abstr 108, p26.
· Kawas, J.R., 1983. Significance of fibre level on nutritive value of alfalfa hay based-diets for ruminants. Ph.D. Diss., Univ. Wis., Madison.
· Kehrli, Jr., M.E., B.J. Nonnecke & J.A. Roth, 1989 Alterations in bovine neutrophil function during the periparturient period. Am. J. Vet. Res. 50, 207.
· Mc Coy, G.C., Drackley, J.K., Hutjens, M.F. & Garrett, J.E., 1997. Effect of yeast culture (Saccharomyces cerevisiae) on prepartum intake and postpartum intake and milk production of Jersey cows. J.Dairy Sci. Suppl. 1, 80, 262.
· Mills, C.F., 1985. Dietary interaction involving trace elements. Annu. Rev. Nutr. Vol. 5, 173.
· Moe, P.W., 1965. Effects of level of intake on the utilisation of diets by dairy cows. Ph.D. Dissertation, Cornell Univ., Ithaca, NY.
· Nagatha, H, Makino, S. Takeda, S., Takahashi, H. & Noda, H., 1988. Assessment of neutrophil function in the dairy cow during the perinatal period. J. Vet. Med. Ser. B. 35, 747.
· Reid, I.M., 1982. Fatty liver dairy cows – incidence, severity, pathology and functional consequences. Bov. Pract. 17, 149.
· Shaver, R., 1996. What we can do to help transition cows. Hoard’s Dairyman, 1996, p 637.
· Stallings, C.C., 1998. Feeding management during the transition phase. In: Proceedings of the Western Canadian Dairy Seminar, 1998, University of Alberta, Edmonton, Canada, p11.
· Staples, C.R., 1992. Forage selection, harvesting, storing and feeding. In: Proceedings of the large dairy herd management conference, American Dairy Science Association, 301 West Clark St., Champaign, IL, USA, p347.
· Thatcher, W.W., Staples, C.R., Van Horn, H.H. & Risco, C.A., 1999. Reproductive and energy status interrelationships that influence reproductive-nutritional management of the postpartum lactating dairy cow. Proceedings of the Southwest Nutrition & Management Conference, February 25 & 26, 1999, Phoenix, Arizona, p25.
· Tsang, D.S., Ballard, C.V., Sniffen, C.J., Allshouse, R.D., Miyoshi, S & Schlatter, L., 1997. The effects of Natrocal (fed pre- and post-partum) on health and performance of multiparons Holstein cattle. J. Dairy Sci. Suppl 1, 80, 209.
· Underwood, E.J. & Suttle, N.F., 1999. The Mineral Nutrition of Livestock, 3rd Ed. CABI Publishing, p 397 and 477.
· Van Saun, R.J., 1991. Dry cow nutrition – the key to improving fresh cow performance. In: The Veterinary Clinics of North America – Dairy Nutrition Management, July 1991, page 599.
· Waltner, S.S., Mc Namara, J.P. & Hillers, J.K., 1993. Relationship of body condition score to production variables in high producing Holstein dairy cattle. J. Dairy Sci 76, 3410.
· Weiss, W.P., Hogan, J.S., Smith, K.L. & Hoblet, K.H., 1990. Relationship among selenium, vitamin E, and mammary gland health in commercial dairy herds. J.Dairy Sci. 73, 381.
· Weiss, W.P., Hogan, J.S., Todhunter, D.A. & Smith, K.L. (1997). Effect of vitamin E supplementation in diets with low concentration of selenium on mammary gland health of dairy cows. J. Dairy Sci 80, 1728.

Willie Smith, Meadow Feeds, PO Box 2946, Randburg 2125